What is your trimmer doing? Pick the symptom that matches and follow that branch down. Each step is designed to confirm or rule out a component in the correct order — fuel first, then spark, then air, then mechanical drive. Do the tests in order; most string trimmer problems are found in the first three steps, and the majority are fuel-related.
Most string trimmer problems are diagnosed with fresh fuel, a spark test, and a visual inspection — not a credit card. Run through these free checks before ordering any replacement parts. They take 20 minutes and save you from buying the wrong component.
Use fresh 2-cycle fuel mix before replacing the carburetor
Stale fuel or an incorrect mix ratio is the number one cause of starting and running problems on string trimmers. Drain the tank, refill with fresh fuel mixed at the ratio specified for your engine (typically 50:1), and retest before buying any parts. Fuel older than 30 days has lost enough volatility to cause hard starting.
Clean the air filter before condemning the carburetor
A clogged air filter mimics carburetor problems — bogging, poor idle, and no-start. Remove it, clean it (foam filters can be washed and lightly oiled), or replace it. It costs a fraction of a carburetor and fixes a surprising number of running issues.
Test spark before buying fuel system parts
A free spark test — remove the plug, reattach the boot, hold the threads against the block, and pull the rope — rules out ignition problems before you spend money on a carburetor or fuel lines. A fouled spark plug is far cheaper to replace than a carburetor and is a common cause of no-start.
Clean the spark arrestor screen before replacing the carburetor
A clogged spark arrestor screen in the muffler causes power loss, bogging, and the need to run with the choke on — the same symptoms as a clogged carburetor. Remove the muffler and clean the screen with a wire brush before buying a carburetor rebuild kit. It costs nothing and takes five minutes.
Check the fuel cap vent before replacing the fuel filter
A clogged fuel cap vent causes the engine to die after running for a few minutes, mimicking a clogged fuel filter. Loosen the cap and run the trimmer — if it runs normally, the cap vent is the problem, not the filter. Replace or clean the cap; it costs less than a filter.
Replace fuel lines as a set, not one at a time
Fuel lines on a string trimmer harden and crack together from age and ethanol fuel. Replacing one cracked line while leaving the others guarantees a comeback leak or air leak. Replace all fuel lines as a set — they are inexpensive and the labor is the same whether you replace one or all.
The engine pulls over but never fires — no pop, no sputter, nothing.
A no-start on a 2-cycle string trimmer means one of three things is missing: the right fuel mix, spark, or compression. Work through them in that order — fuel is by far the most common cause, and a stale or incorrectly mixed fuel will also ruin the engine over time. Do not pull the rope repeatedly with the choke on; that floods the engine and makes diagnosis harder.
Fresh fuel mix check
Confirm the tank has fresh 2-cycle fuel mix — typically 50:1 gas to 2-stroke oil. Fuel older than 30 days loses volatility; fuel with no oil will seize the engine. Drain stale fuel into a labeled container and refill with fresh mix.
Fresh, correctly mixed fuel is in the tank — move to step 2.
Fuel is stale, straight gas, or older than a month — drain and refill with fresh 50:1 mix. If the engine ran on straight gas, stop and check compression before restarting.
Fuel delivery to carburetor
Press the primer bulb (if equipped) 5–6 times. It should feel spongy and pull fuel through the lines. If the bulb stays flat or fills with air, fuel is not reaching the carburetor.
Primer bulb pulls fuel and feels firm — move to step 3.
Bulb stays flat or pulls air — check the fuel filter, fuel lines for cracks, and the pickup tube in the tank. Replace any cracked lines as a set.
Spark test
Remove the spark plug, reattach the plug boot, and hold the plug threads against the engine block with insulated pliers while a helper pulls the rope. Look for a strong blue spark across the gap. Do this away from fuel vapors.
Strong blue spark — move to step 4.
Weak or no spark — replace the spark plug first. If still no spark, test the ignition coil and the kill switch wire for a short to ground. Replace the coil if it tests bad.
Air filter & choke check
Remove the air filter and inspect it. A fully clogged filter can prevent starting. Confirm the choke plate closes fully when on and opens when off. A stuck choke floods the engine.
Air filter is clean and choke operates — move to step 5.
Filter is clogged — clean or replace it. Choke is stuck — free and adjust the choke linkage or shutter.
Carburetor inspection
If fuel, spark, and air are all present, the carburetor is the most likely culprit. Stale fuel leaves varnish that clogs the tiny jets and passages inside a trimmer carburetor. Remove the carburetor and check the main jet and diaphragm for varnish and debris.
Carburetor passages are clean — move to step 6.
Varnish or debris found — clean the carburetor with carb cleaner and a soft wire, or install a carburetor rebuild kit with a new diaphragm and gaskets.
Compression check
If everything above checks out, do a compression test. On a 2-cycle, pull the rope slowly — you should feel strong resistance at the top of each stroke. A compression gauge should read 90–100+ PSI. Low compression points to worn rings, a scored cylinder, or bad crankcase seals.
Compression feels strong — recheck fuel and spark; the engine should run.
Compression is low or the rope pulls with little resistance — the engine has internal wear (rings, cylinder, or crank seals). This is a major repair; on an inexpensive trimmer, replacement is often more economical.
Stale or incorrectly mixed fuel, a clogged carburetor from varnish, or a fouled spark plug. Compression failure is the least common cause and usually only appears on older, heavily used trimmers.
The engine fires up, runs for a few seconds, then quits — every time.
If the trimmer starts but won’t keep running, the carburetor can deliver enough fuel for the initial firing but can’t sustain it. This is almost always a fuel starvation issue: a clogged idle circuit, a failing diaphragm, a restricted fuel filter, or an air leak that leans out the mixture after the initial fuel in the carburetor is consumed.
Fuel cap vent test
Loosen the fuel cap slightly and start the trimmer. If it runs normally with the cap loose, the cap’s one-way vent is clogged and a vacuum is building in the tank, starving the carburetor.
Runs fine with the cap loose — replace the fuel cap or clean the vent.
Still dies with the cap loose — move to step 2.
Fuel filter check
A clogged fuel filter starves the carburetor after the initial fuel in the bowl is consumed. Pull the fuel filter from the tank and inspect it. If it is dark, gummed, or restricted, replace it.
Fuel filter is clean — move to step 3.
Filter is clogged — replace it. They are inexpensive and should be changed annually.
Carburetor idle circuit
The idle circuit in a trimmer carburetor has tiny passages that clog easily with varnish from stale fuel. Remove the carburetor, remove the idle jet (if serviceable), and clean all passages with carburetor cleaner and a soft wire.
Idle passages are clear — move to step 4.
Idle passages are clogged — clean them thoroughly and reinstall. If they will not clear, replace the carburetor.
Carburetor diaphragm
The rubber diaphragm in the carburetor pumps fuel and meters the mixture. With age and ethanol fuel, it stiffens, tears, or loses elasticity. Remove the diaphragm cover and inspect the diaphragm for stiffness, tears, or deformation.
Diaphragm is flexible and intact — move to step 5.
Diaphragm is stiff, torn, or deformed — install a carburetor rebuild kit with a new diaphragm and gaskets.
Vacuum leak (gaskets)
An air leak between the carburetor and the engine leans out the mixture and causes the engine to die after starting. Check the mounting gaskets between the carburetor and the intake, and between the intake and the engine. Replace any dried or cracked gaskets.
Gaskets are intact — move to step 6.
Gaskets are dried or cracked — replace them and reassemble with even torque.
Fuel lines
Check all fuel lines for cracks, softness, or loose connections. A cracked line draws air instead of fuel, causing the engine to die after starting. Pay attention to the lines at the tank outlet and carburetor inlet.
Fuel lines are supple and sealed — recheck the carburetor; the issue is internal.
Lines are cracked or soft — replace all fuel lines as a set. They harden together and replacing one leaves cracked others.
A clogged carburetor idle circuit, a stiff or torn carburetor diaphragm, or a clogged fuel cap vent. The fuel filter and cracked fuel lines are the next most common causes.
The engine runs at full throttle but dies the moment you release the trigger.
If the trimmer runs fine at wide-open throttle but dies when you return to idle, the idle circuit in the carburetor is restricted or the mixture is too lean at low RPM. The engine can draw enough fuel through the main circuit but not through the idle circuit. This is a carburetor problem — not an ignition or compression problem.
Idle speed screw adjustment
Locate the idle speed screw (usually marked “T” or “LA” on the carburetor). Turn it clockwise in small increments to raise the idle speed. The throttle should hold the engine running without your finger on the trigger.
Engine idles after adjustment — the screw was simply set too low.
Engine still dies — move to step 2.
Carburetor idle circuit
The idle jet and passages inside the carburetor are very small and clog easily with varnish. Remove the carburetor, remove the idle jet if serviceable, and clean all idle passages with carburetor cleaner and a soft wire.
Idle passages are clear — move to step 3.
Idle passages are clogged — clean them and reinstall. If they will not clear, replace the carburetor.
Carburetor diaphragm
A stiff or torn diaphragm cannot meter fuel accurately at the low demand of idle. Remove the diaphragm cover and inspect it for flexibility, tears, and deformation.
Diaphragm is flexible and intact — move to step 4.
Diaphragm is stiff or torn — install a carburetor rebuild kit with a new diaphragm and gaskets.
Fuel filter
A partially clogged fuel filter can supply enough fuel at full throttle but restrict flow at idle. Pull the filter from the tank and inspect it.
Filter is clean — move to step 5.
Filter is restricted — replace it.
Air filter
A dirty air filter changes the air-fuel ratio and can make the idle mixture too rich or too lean. Remove the air filter and try idling without it (briefly, and away from debris).
Engine idles without the air filter — clean or replace the air filter.
Engine still won’t idle — move to step 6.
Vacuum leak
An air leak between the carburetor and engine leans out the idle mixture and causes the engine to die at idle. Check the mounting gaskets and the intake manifold for cracks. Replace any dried or cracked gaskets.
Gaskets are intact — the carburetor likely needs replacement.
Gaskets are cracked or dried — replace them and retest the idle.
A clogged carburetor idle circuit, a stiff carburetor diaphragm, or an air leak between the carburetor and engine. The idle speed screw being set too low is the simplest cause and the first thing to check.
If cleaning the idle circuit and adjusting the idle screw does not restore idle, the carburetor diaphragm is likely stiff or torn. Install a carburetor rebuild kit with a new diaphragm and gaskets. If the engine still won’t idle after a rebuild, check the crankcase seals for air leaks using a pressure and vacuum test — a bad seal leans out the mixture at idle and is a common failure on older 2-cycle trimmers.
The engine runs with the choke on but dies the moment you open it.
A trimmer that only runs with the choke on has a lean fuel condition — the carburetor’s main circuit is restricted and cannot supply enough fuel at full airflow. The choke compensates by reducing airflow, which richens the mixture. This is a carburetor problem: a partially clogged main jet, a stiff diaphragm, or restricted fuel flow. Do not keep running the trimmer with the choke on; it masks a problem that will worsen and eventually prevent starting.
Carburetor main jet
The main jet in the carburetor is partially clogged with varnish from stale fuel. Remove the carburetor, remove the main jet (if serviceable), and clean it and all main circuit passages with carburetor cleaner and a soft wire.
Main jet is clear — move to step 2.
Main jet is restricted — clean it thoroughly and reinstall. If it will not clear, replace the carburetor.
Carburetor diaphragm
A stiff diaphragm cannot lift enough fuel to match the airflow at full throttle. Remove the diaphragm cover and inspect it for flexibility, tears, and deformation.
Diaphragm is flexible and intact — move to step 3.
Diaphragm is stiff or torn — install a carburetor rebuild kit with a new diaphragm and gaskets.
Fuel filter
A clogged fuel filter restricts fuel flow at higher demand. Pull the filter from the tank and inspect it. If it is dark or gummed, replace it.
Filter is clean — move to step 4.
Filter is restricted — replace it.
Fuel cap vent
A clogged fuel cap vent builds a vacuum in the tank and restricts fuel flow. Loosen the cap and run the trimmer with the choke off.
Runs with the cap loose — replace or clean the fuel cap vent.
Still dies with the cap loose — move to step 5.
Air filter
A dirty air filter reduces airflow and can mask a lean condition by keeping the mixture richer. Remove the air filter and test with the choke off (briefly, away from debris).
Runs without the air filter — clean or replace the air filter.
Still dies without the air filter — move to step 6.
Carburetor rebuild or replacement
If the main jet is clean, the diaphragm is good, and the fuel filter and cap vent are clear, the carburetor body itself may be warped or the internal passages are too restricted to clean. Install a carburetor rebuild kit; if that does not fix it, replace the carburetor.
Engine runs with the choke off after rebuild — problem solved.
Engine still requires choke — replace the entire carburetor with a new or quality aftermarket unit matched to your model.
A partially clogged carburetor main jet from stale fuel varnish. A stiff or torn carburetor diaphragm is the second most common cause. A clogged fuel filter or fuel cap vent can also restrict fuel flow enough to require the choke.
A trimmer that runs only with the choke on has a lean fuel condition — the carburetor main circuit is restricted. Remove the carburetor, disassemble it, and clean the main jet and all passages with carburetor cleaner and a soft wire. If the main jet is clear, replace the diaphragm and gaskets with a carburetor rebuild kit. If the problem persists after a rebuild, the carburetor body may be warped or worn and should be replaced with a new unit matched to your trimmer model.
The engine starts and idles but bogs down or loses power when you open the throttle.
If the trimmer idles fine but bogs under load, the engine cannot get enough fuel or air at higher demand, or the exhaust is restricted. The most common causes are a clogged air filter, a clogged fuel filter, a partially clogged carburetor main jet, or a clogged spark arrestor screen in the muffler. Test them in order — the air filter and spark arrestor are free to check.
Air filter check
Remove the air filter and inspect it. A clogged filter chokes the engine at higher RPM. If it is saturated with oil and dirt, clean it (foam filters can be washed and oiled lightly) or replace it.
Air filter is clean — move to step 2.
Filter is clogged — clean or replace it. This is the most common and cheapest fix.
Fuel filter check
A clogged fuel filter restricts fuel flow at full throttle. Pull the filter from the tank and inspect it. If it is dark or gummed, replace it.
Fuel filter is clean — move to step 3.
Filter is restricted — replace it.
Fuel cap vent test
Loosen the fuel cap and run the trimmer at full throttle. If it runs normally with the cap loose, the cap vent is clogged and a vacuum is restricting fuel flow.
Runs fine with the cap loose — replace or clean the fuel cap vent.
Still bogs with the cap loose — move to step 4.
Carburetor main jet
The main jet supplies fuel at full throttle. A partial clog from varnish restricts fuel flow and causes bogging. Remove the carburetor and clean the main jet and all main circuit passages with carburetor cleaner and a soft wire.
Main jet is clear — move to step 5.
Main jet is restricted — clean it and reinstall. If it will not clear, install a carburetor rebuild kit or replace the carburetor.
Spark arrestor / muffler screen
Many string trimmers have a spark arrestor screen inside the muffler exhaust port. Over time, carbon and unburned oil build up on the screen and restrict exhaust flow, causing power loss and bogging. Remove the muffler and inspect the screen. Clean it with a wire brush and carb cleaner, or replace it if it is burned through.
Screen is clean — move to step 6.
Screen is clogged with carbon — clean or replace it. This is a very common cause of bogging on 2-cycle trimmers.
Carburetor diaphragm
If all of the above are clean, the carburetor diaphragm may be stiff and unable to supply fuel fast enough at full throttle. Remove the diaphragm cover and inspect it for flexibility and tears.
Diaphragm is flexible — check for an air leak at the intake gaskets.
Diaphragm is stiff or torn — install a carburetor rebuild kit with a new diaphragm and gaskets.
A clogged air filter, a clogged spark arrestor screen in the muffler, or a partially clogged carburetor main jet. A clogged fuel filter or fuel cap vent can also restrict fuel flow under load.
The primer bulb stays flat, pulls air, or fills but won’t hold — fuel runs back into the tank.
The primer bulb on a string trimmer pulls fuel from the tank, through the carburetor, and back to the tank — it primes the carburetor with liquid fuel so the engine starts without excessive pulling. If the bulb won’t fill, fuel isn’t reaching it. If it fills but won’t hold, a check valve or fuel line is failing. The fault is in the fuel lines, the primer bulb itself, or the carburetor inlet.
Fuel tank level
Confirm the tank has enough fuel to cover the pickup filter inside the tank. If the fuel level is below the pickup, the primer will pull air.
Tank has adequate fuel — move to step 2.
Tank is low — refill with fresh 2-cycle mix and retest the primer.
Fuel lines
Check all fuel lines for cracks, softness, or loose connections. A cracked line between the tank and the primer draws air instead of fuel. Pay attention to the lines at the tank outlet, primer bulb, and carburetor. Replace any cracked lines as a set.
Fuel lines are supple and sealed — move to step 3.
Lines are cracked or soft — replace all fuel lines as a set. They harden together and replacing one leaves cracked others.
Primer bulb check valves
The primer bulb has two one-way check valves — one lets fuel in, one lets fuel out. If the inlet valve fails, the bulb won’t fill. If the outlet valve fails, fuel runs back into the tank. Remove the bulb and test by blowing through each port (fuel should flow one way only). Replace the bulb if either valve fails.
Check valves are working — move to step 4.
A check valve has failed — replace the primer bulb. They are inexpensive and usually sold with new gaskets.
Carburetor inlet screen
A fine screen at the carburetor fuel inlet can clog with debris and prevent the primer from pulling fuel through. Remove the fuel line at the carburetor inlet and inspect the screen. Clean it with carb cleaner or replace it if damaged.
Inlet screen is clear — move to step 5.
Screen is clogged — clean or replace it and retest the primer.
Fuel filter
A clogged fuel filter in the tank restricts fuel flow enough to prevent the primer from working. Pull the filter from the tank and inspect it. If it is dark or gummed, replace it.
Filter is clean — the primer circuit is intact; recheck the bulb and lines.
Filter is restricted — replace it.
Cracked or hardened fuel lines drawing air, a failed primer bulb check valve, or a clogged fuel filter. The carburetor inlet screen is a less common cause but should be checked if the lines and bulb are good.
Fuel is visibly leaking from the trimmer — from the tank, lines, carburetor, or primer bulb.
Fuel leaks on a string trimmer are almost always from cracked fuel lines or a failed fuel cap gasket. The rubber and plastic components harden and crack with age and ethanol fuel. Never operate a leaking trimmer — fuel near a hot engine and spark is a fire hazard. Identify the source before replacing parts; do not guess.
Fuel cap gasket
Remove the fuel cap and inspect the O-ring or gasket. If it is cracked, flattened, or missing, fuel will seep from the cap when the trimmer is tilted. Replace the cap or just the gasket if available.
Cap gasket is intact — move to step 2.
Gasket is cracked or flattened — replace the fuel cap or gasket. This is a common leak source.
Fuel lines
Check all fuel lines for cracks, especially at the tank outlet and carburetor inlet where they are stretched over barbed fittings. Squeeze each line — if it is hard, brittle, or has visible cracks, it is leaking. Replace all fuel lines as a set.
Fuel lines are supple and sealed — move to step 3.
Lines are cracked or brittle — replace all fuel lines as a set. They harden together and must be replaced together.
Carburetor gasket and diaphragm
Fuel can leak from the carburetor body if the gasket between the carburetor and the engine is dried or cracked, or if the diaphragm cover gasket is failing. Remove the carburetor and inspect all gaskets. Replace any dried or cracked gaskets with a rebuild kit.
Carburetor gaskets are intact — move to step 4.
Gaskets are dried or cracked — install a carburetor rebuild kit with new gaskets and a new diaphragm.
Primer bulb
A cracked primer bulb can seep fuel, especially around the base where it meets the housing. Inspect the bulb for cracks and confirm it is seated properly. Replace the bulb if it is cracked or loose.
Primer bulb is intact and seated — move to step 5.
Bulb is cracked or loose — replace the primer bulb and its gasket.
Fuel tank
Inspect the fuel tank for cracks, especially at the seams, mounting points, and where the fuel lines enter the tank. A cracked tank will drip fuel continuously. Replace the tank if it is cracked — do not attempt to seal it with epoxy or tape, as fuel will dissolve most adhesives.
Tank is intact — recheck all fittings and connections above.
Tank is cracked — replace the fuel tank. Do not use a cracked tank; it is a fire hazard.
Cracked or hardened fuel lines, a failed fuel cap gasket, or a dried carburetor gasket. A cracked fuel tank is less common but requires immediate replacement.
Fuel leaks on string trimmers are almost always from cracked fuel lines or a failed fuel cap gasket. Replace all fuel lines as a set — they harden and crack together, so replacing one while leaving cracked others guarantees a comeback. If the lines are good, replace the fuel cap O-ring. If fuel leaks from the carburetor body, install a rebuild kit with new gaskets and a diaphragm. Never operate a leaking trimmer — fuel near a hot engine is a fire hazard. If the tank itself is cracked, replace it; do not attempt to seal it with epoxy or tape.
The engine starts and revs normally, but the trimmer head doesn’t rotate.
When the engine runs but the head doesn’t spin, the problem is in the mechanical drive path — the clutch, the drive shaft, the gearbox, or the head attachment. The engine is doing its job; the power isn’t reaching the cutting head. Always disconnect the spark plug boot before working on the drive system or trimmer head.
Trimmer head attachment
Confirm the trimmer head is properly seated and locked onto the drive shaft. Remove the head, clean the mounting surface, and reinstall it. If the head uses a bolt, confirm the bolt is tight. A loose or incorrectly mounted head can slip on the shaft.
Head is properly mounted — move to step 2.
Head was loose or incorrectly mounted — clean, reinstall, and tighten. Retest.
Drive shaft (flex shaft)
On straight-shaft trimmers, a flexible drive shaft inside the tube transmits power from the engine to the head. Remove the shaft from the engine end and inspect the flex shaft for breakage or stripped ends. A broken or stripped flex shaft is the most common cause of a non-spinning head.
Flex shaft is intact — move to step 3.
Flex shaft is broken or stripped — replace the flex shaft. On curved-shaft trimmers, the shaft and tube are often one assembly.
Clutch
String trimmers use a centrifugal clutch that engages at a certain RPM. If the clutch shoes or springs are worn or broken, the engine revs but the clutch doesn’t engage the drive shaft. Remove the clutch drum and inspect the shoes and springs. Replace the clutch if the shoes are worn below specification or the springs are broken.
Clutch shoes and springs are intact — move to step 4.
Clutch is worn or broken — replace the clutch assembly. Do not run the trimmer with a slipping clutch; it will damage the drum and shaft.
Gearbox
On straight-shaft trimmers, an angle gearbox at the head end transfers power from the flex shaft to the head. The gears can strip or the grease can dry out. Remove the gearbox cover and inspect the gears. If the gears are stripped, replace the gearbox or the gear set. If the grease is dry, repack with lithium grease.
Gears are intact and greased — move to step 5.
Gears are stripped or grease is dry — replace the gear set or gearbox, and repack with fresh lithium grease.
Shaft coupling
The coupling between the engine output shaft and the flex shaft can strip if the set screw loosens or the shaft rounds off. Remove the coupling and inspect both the coupling and the shaft ends for stripping. Tighten the set screw with Loctite or replace the coupling if it is stripped.
Coupling is intact and tight — recheck the clutch and gearbox.
Coupling is stripped or set screw is loose — replace the coupling and tighten the set screw with thread-locking compound.
A broken or stripped flexible drive shaft, a worn centrifugal clutch, or a stripped angle gearbox. The head attachment being loose is the simplest cause and should be checked first.
The trimmer line won’t advance when you bump the head, or it runs out and won’t reload.
A bump-feed trimmer head advances line by centrifugal force when you tap the head on the ground. If the line won’t feed, the problem is in the head: the line is tangled or wound wrong, the bump mechanism is jammed or worn, the line is melted together, or the exit holes are blocked. Always disconnect the spark plug boot before opening the trimmer head.
Line wound correctly
Open the spool and check that the line is wound in the correct direction (arrows on the spool show the way) and is not tangled or crossed. Line wound the wrong way will not feed. Rewind the line following the arrows on the spool.
Line is wound correctly — move to step 2.
Line is tangled or wound wrong — rewind it following the direction arrows on the spool. Do not overfill the spool; leave a small gap.
Bump head spring and plate
The bump knob at the bottom of the head pushes a spring-loaded plate that advances the line. If the spring is broken, the knob is worn, or the plate is jammed with grass, the head won’t feed. Disassemble the head, clean all parts, and inspect the spring and knob for wear. Replace the bump knob and spring if worn.
Spring and knob are intact — move to step 3.
Spring is broken or knob is worn — replace the bump knob and spring assembly. Clean all grass from inside the head.
Line melted or welded
Trimmer line can melt and weld together inside the spool from heat generated during use, especially if the line is too heavy or the head is run at low RPM. Pull the line from the spool — if it is stuck together or fused, cut the welded section off and rewind the remaining line.
Line is not fused — move to step 4.
Line is melted together — cut off the fused section and rewind. Use a lighter line weight if the problem recurs.
Eyelets and exit holes
The line exits through small eyelets in the head. Grass and debris can pack into these holes and block the line. Clean all debris from the eyelets with a pick or compressed air. Confirm the eyelets are not worn or enlarged — oversized eyelets let the line slip back in.
Eyelets are clear and intact — move to step 5.
Eyelets are blocked or worn — clean them. If they are worn or enlarged, replace the head.
Line size and type
Confirm the line diameter matches the specification for your head. Line that is too heavy won’t feed through the eyelets; line that is too light will break and weld. Check the head or manual for the correct diameter (commonly 0.065–0.095 inch).
Line size is correct — move to step 6.
Line is the wrong size — replace it with the diameter specified for your head.
Replace bump head
If the head is cracked, warped, or the internal tracks are worn, the line will not feed reliably even with new line and a clean spring. Replace the entire bump head with a new one matched to your shaft. Aftermarket universal heads are available if an OEM head is not found.
Head is in good condition — recheck the line and spring.
Head is worn or cracked — replace the entire bump head assembly.
Line wound in the wrong direction, a jammed or worn bump head spring, or line melted together from heat. Blocked eyelets and incorrect line size are the next most common causes.
The trimmer vibrates far more than normal during operation — your hands tingle or the whole unit shakes.
Some vibration is normal on a 2-cycle trimmer, but excessive vibration means something is unbalanced or loose. The most common causes are debris packed under the trimmer head, a damaged or unbalanced head, a bent or worn drive shaft, or a bent crankshaft from hitting a solid object. Stop the engine immediately and diagnose — running a vibrating trimmer can damage the engine and cause hand-arm injury.
Trimmer head debris
Stop the engine, disconnect the spark plug boot, and clean all grass and debris from under and around the trimmer head. Packed grass throws the head out of balance and is the most common cause of sudden vibration.
Head is clean and vibration is gone — the debris was the cause.
Head is clean but vibration persists — move to step 2.
Trimmer head balance and condition
Inspect the trimmer head for cracks, warping, or missing pieces. A damaged head is unbalanced and causes vibration. If the head is cracked or a piece is broken off, replace it. Confirm the head is mounted squarely on the shaft.
Head is intact and balanced — move to step 3.
Head is cracked or unbalanced — replace the trimmer head.
Drive shaft
A bent or worn drive shaft (flex shaft on straight-shaft models) causes vibration that increases with RPM. Remove the shaft and roll it on a flat surface — a straight shaft will roll true; a bent shaft will wobble. Replace a bent flex shaft.
Drive shaft is straight — move to step 4.
Drive shaft is bent — replace the flex shaft. Do not run the trimmer with a bent shaft; it will damage the clutch and bearings.
Clutch
A worn or broken clutch shoe can cause imbalance and vibration. Remove the clutch drum and inspect the shoes and springs. If a shoe is broken or a spring is missing, the clutch is unbalanced. Replace the clutch assembly.
Clutch is intact and balanced — move to step 5.
Clutch shoe or spring is broken — replace the clutch assembly.
Engine mount and hardware
Check all mounting bolts and screws on the engine, handle, and shaft. A loose engine mount or handle allows the whole unit to shake. Tighten all hardware to specification. Inspect the engine mounting isolators (rubber bushings) for deterioration and replace if cracked.
All hardware is tight and isolators are intact — move to step 6.
Hardware is loose or isolators are cracked — tighten all bolts and replace any worn isolators.
Crankshaft
If all of the above are good, the crankshaft itself may be bent — usually from hitting a fence, rock, or other solid object. A bent crankshaft causes severe vibration that increases with RPM and cannot be repaired economically. On an inexpensive trimmer, engine replacement is the only practical fix.
Crankshaft appears straight — recheck the head and shaft for subtle imbalance.
Crankshaft is bent — the engine must be replaced. A bent crankshaft is not repairable and running the engine will destroy the main bearings.
Debris packed under the trimmer head, a cracked or unbalanced trimmer head, or a bent drive shaft. A bent crankshaft from hitting a solid object is the most serious cause and usually requires engine replacement.
Excessive vibration is usually caused by a damaged trimmer head, debris packed under the head, or a bent drive shaft. Stop the engine, disconnect the spark plug boot, and clean all debris from under the head. If the head is cracked or unbalanced, replace it. If vibration persists with a clean, intact head, inspect the drive shaft for bends — roll it on a flat surface and look for a wobble. A bent crankshaft — often from hitting a fence or rock — causes severe vibration that increases with RPM and is not economical to repair; on most trimmers, engine replacement is the only practical fix.
This flowchart is part of Lil Engine's complete string trimmer troubleshooting cluster. Each branch above links to a full step-by-step diagnosis page when one exists — here are the rest:
Lil Engine · lil-engine.com/troubleshooting-flowcharts/string-trimmer-troubleshooting
What is your trimmer doing? Pick the symptom that matches and follow that branch down. Each step is designed to confirm or rule out a component in the correct order — fuel first, then spark, then air, then mechanical drive. Do the tests in order; most string trimmer problems are found in the first three steps, and the majority are fuel-related.
Stale fuel or an incorrect mix ratio is the number one cause of starting and running problems on string trimmers. Drain the tank, refill with fresh fuel mixed at the ratio specified for your engine (typically 50:1), and retest before buying any parts. Fuel older than 30 days has lost enough volatility to cause hard starting.
A clogged air filter mimics carburetor problems — bogging, poor idle, and no-start. Remove it, clean it (foam filters can be washed and lightly oiled), or replace it. It costs a fraction of a carburetor and fixes a surprising number of running issues.
A free spark test — remove the plug, reattach the boot, hold the threads against the block, and pull the rope — rules out ignition problems before you spend money on a carburetor or fuel lines. A fouled spark plug is far cheaper to replace than a carburetor and is a common cause of no-start.
A clogged spark arrestor screen in the muffler causes power loss, bogging, and the need to run with the choke on — the same symptoms as a clogged carburetor. Remove the muffler and clean the screen with a wire brush before buying a carburetor rebuild kit. It costs nothing and takes five minutes.
A clogged fuel cap vent causes the engine to die after running for a few minutes, mimicking a clogged fuel filter. Loosen the cap and run the trimmer — if it runs normally, the cap vent is the problem, not the filter. Replace or clean the cap; it costs less than a filter.
Fuel lines on a string trimmer harden and crack together from age and ethanol fuel. Replacing one cracked line while leaving the others guarantees a comeback leak or air leak. Replace all fuel lines as a set — they are inexpensive and the labor is the same whether you replace one or all.
The engine pulls over but never fires — no pop, no sputter, nothing.
Confirm the tank has fresh 2-cycle fuel mix — typically 50:1 gas to 2-stroke oil. Fuel older than 30 days loses volatility; fuel with no oil will seize the engine. Drain stale fuel into a labeled container and refill with fresh mix.
Pass: Fresh, correctly mixed fuel is in the tank — move to step 2.
Fail: Fuel is stale, straight gas, or older than a month — drain and refill with fresh 50:1 mix. If the engine ran on straight gas, stop and check compression before restarting.
Press the primer bulb (if equipped) 5–6 times. It should feel spongy and pull fuel through the lines. If the bulb stays flat or fills with air, fuel is not reaching the carburetor.
Pass: Primer bulb pulls fuel and feels firm — move to step 3.
Fail: Bulb stays flat or pulls air — check the fuel filter, fuel lines for cracks, and the pickup tube in the tank. Replace any cracked lines as a set.
Remove the spark plug, reattach the plug boot, and hold the plug threads against the engine block with insulated pliers while a helper pulls the rope. Look for a strong blue spark across the gap. Do this away from fuel vapors.
Pass: Strong blue spark — move to step 4.
Fail: Weak or no spark — replace the spark plug first. If still no spark, test the ignition coil and the kill switch wire for a short to ground. Replace the coil if it tests bad.
Remove the air filter and inspect it. A fully clogged filter can prevent starting. Confirm the choke plate closes fully when on and opens when off. A stuck choke floods the engine.
Pass: Air filter is clean and choke operates — move to step 5.
Fail: Filter is clogged — clean or replace it. Choke is stuck — free and adjust the choke linkage or shutter.
If fuel, spark, and air are all present, the carburetor is the most likely culprit. Stale fuel leaves varnish that clogs the tiny jets and passages inside a trimmer carburetor. Remove the carburetor and check the main jet and diaphragm for varnish and debris.
Pass: Carburetor passages are clean — move to step 6.
Fail: Varnish or debris found — clean the carburetor with carb cleaner and a soft wire, or install a carburetor rebuild kit with a new diaphragm and gaskets.
If everything above checks out, do a compression test. On a 2-cycle, pull the rope slowly — you should feel strong resistance at the top of each stroke. A compression gauge should read 90–100+ PSI. Low compression points to worn rings, a scored cylinder, or bad crankcase seals.
Pass: Compression feels strong — recheck fuel and spark; the engine should run.
Fail: Compression is low or the rope pulls with little resistance — the engine has internal wear (rings, cylinder, or crank seals). This is a major repair; on an inexpensive trimmer, replacement is often more economical.
Most likely cause: Stale or incorrectly mixed fuel, a clogged carburetor from varnish, or a fouled spark plug. Compression failure is the least common cause and usually only appears on older, heavily used trimmers.
Full guide: lil-engine.com/fix/weed-eater-wont-start
The engine fires up, runs for a few seconds, then quits — every time.
Loosen the fuel cap slightly and start the trimmer. If it runs normally with the cap loose, the cap’s one-way vent is clogged and a vacuum is building in the tank, starving the carburetor.
Pass: Runs fine with the cap loose — replace the fuel cap or clean the vent.
Fail: Still dies with the cap loose — move to step 2.
A clogged fuel filter starves the carburetor after the initial fuel in the bowl is consumed. Pull the fuel filter from the tank and inspect it. If it is dark, gummed, or restricted, replace it.
Pass: Fuel filter is clean — move to step 3.
Fail: Filter is clogged — replace it. They are inexpensive and should be changed annually.
The idle circuit in a trimmer carburetor has tiny passages that clog easily with varnish from stale fuel. Remove the carburetor, remove the idle jet (if serviceable), and clean all passages with carburetor cleaner and a soft wire.
Pass: Idle passages are clear — move to step 4.
Fail: Idle passages are clogged — clean them thoroughly and reinstall. If they will not clear, replace the carburetor.
The rubber diaphragm in the carburetor pumps fuel and meters the mixture. With age and ethanol fuel, it stiffens, tears, or loses elasticity. Remove the diaphragm cover and inspect the diaphragm for stiffness, tears, or deformation.
Pass: Diaphragm is flexible and intact — move to step 5.
Fail: Diaphragm is stiff, torn, or deformed — install a carburetor rebuild kit with a new diaphragm and gaskets.
An air leak between the carburetor and the engine leans out the mixture and causes the engine to die after starting. Check the mounting gaskets between the carburetor and the intake, and between the intake and the engine. Replace any dried or cracked gaskets.
Pass: Gaskets are intact — move to step 6.
Fail: Gaskets are dried or cracked — replace them and reassemble with even torque.
Check all fuel lines for cracks, softness, or loose connections. A cracked line draws air instead of fuel, causing the engine to die after starting. Pay attention to the lines at the tank outlet and carburetor inlet.
Pass: Fuel lines are supple and sealed — recheck the carburetor; the issue is internal.
Fail: Lines are cracked or soft — replace all fuel lines as a set. They harden together and replacing one leaves cracked others.
Most likely cause: A clogged carburetor idle circuit, a stiff or torn carburetor diaphragm, or a clogged fuel cap vent. The fuel filter and cracked fuel lines are the next most common causes.
Full guide: lil-engine.com/fix/weed-eater-starts-then-dies
The engine runs at full throttle but dies the moment you release the trigger.
Locate the idle speed screw (usually marked “T” or “LA” on the carburetor). Turn it clockwise in small increments to raise the idle speed. The throttle should hold the engine running without your finger on the trigger.
Pass: Engine idles after adjustment — the screw was simply set too low.
Fail: Engine still dies — move to step 2.
The idle jet and passages inside the carburetor are very small and clog easily with varnish. Remove the carburetor, remove the idle jet if serviceable, and clean all idle passages with carburetor cleaner and a soft wire.
Pass: Idle passages are clear — move to step 3.
Fail: Idle passages are clogged — clean them and reinstall. If they will not clear, replace the carburetor.
A stiff or torn diaphragm cannot meter fuel accurately at the low demand of idle. Remove the diaphragm cover and inspect it for flexibility, tears, and deformation.
Pass: Diaphragm is flexible and intact — move to step 4.
Fail: Diaphragm is stiff or torn — install a carburetor rebuild kit with a new diaphragm and gaskets.
A partially clogged fuel filter can supply enough fuel at full throttle but restrict flow at idle. Pull the filter from the tank and inspect it.
Pass: Filter is clean — move to step 5.
Fail: Filter is restricted — replace it.
A dirty air filter changes the air-fuel ratio and can make the idle mixture too rich or too lean. Remove the air filter and try idling without it (briefly, and away from debris).
Pass: Engine idles without the air filter — clean or replace the air filter.
Fail: Engine still won’t idle — move to step 6.
An air leak between the carburetor and engine leans out the idle mixture and causes the engine to die at idle. Check the mounting gaskets and the intake manifold for cracks. Replace any dried or cracked gaskets.
Pass: Gaskets are intact — the carburetor likely needs replacement.
Fail: Gaskets are cracked or dried — replace them and retest the idle.
Most likely cause: A clogged carburetor idle circuit, a stiff carburetor diaphragm, or an air leak between the carburetor and engine. The idle speed screw being set too low is the simplest cause and the first thing to check.
The engine runs with the choke on but dies the moment you open it.
The main jet in the carburetor is partially clogged with varnish from stale fuel. Remove the carburetor, remove the main jet (if serviceable), and clean it and all main circuit passages with carburetor cleaner and a soft wire.
Pass: Main jet is clear — move to step 2.
Fail: Main jet is restricted — clean it thoroughly and reinstall. If it will not clear, replace the carburetor.
A stiff diaphragm cannot lift enough fuel to match the airflow at full throttle. Remove the diaphragm cover and inspect it for flexibility, tears, and deformation.
Pass: Diaphragm is flexible and intact — move to step 3.
Fail: Diaphragm is stiff or torn — install a carburetor rebuild kit with a new diaphragm and gaskets.
A clogged fuel filter restricts fuel flow at higher demand. Pull the filter from the tank and inspect it. If it is dark or gummed, replace it.
Pass: Filter is clean — move to step 4.
Fail: Filter is restricted — replace it.
A clogged fuel cap vent builds a vacuum in the tank and restricts fuel flow. Loosen the cap and run the trimmer with the choke off.
Pass: Runs with the cap loose — replace or clean the fuel cap vent.
Fail: Still dies with the cap loose — move to step 5.
A dirty air filter reduces airflow and can mask a lean condition by keeping the mixture richer. Remove the air filter and test with the choke off (briefly, away from debris).
Pass: Runs without the air filter — clean or replace the air filter.
Fail: Still dies without the air filter — move to step 6.
If the main jet is clean, the diaphragm is good, and the fuel filter and cap vent are clear, the carburetor body itself may be warped or the internal passages are too restricted to clean. Install a carburetor rebuild kit; if that does not fix it, replace the carburetor.
Pass: Engine runs with the choke off after rebuild — problem solved.
Fail: Engine still requires choke — replace the entire carburetor with a new or quality aftermarket unit matched to your model.
Most likely cause: A partially clogged carburetor main jet from stale fuel varnish. A stiff or torn carburetor diaphragm is the second most common cause. A clogged fuel filter or fuel cap vent can also restrict fuel flow enough to require the choke.
The engine starts and idles but bogs down or loses power when you open the throttle.
Remove the air filter and inspect it. A clogged filter chokes the engine at higher RPM. If it is saturated with oil and dirt, clean it (foam filters can be washed and oiled lightly) or replace it.
Pass: Air filter is clean — move to step 2.
Fail: Filter is clogged — clean or replace it. This is the most common and cheapest fix.
A clogged fuel filter restricts fuel flow at full throttle. Pull the filter from the tank and inspect it. If it is dark or gummed, replace it.
Pass: Fuel filter is clean — move to step 3.
Fail: Filter is restricted — replace it.
Loosen the fuel cap and run the trimmer at full throttle. If it runs normally with the cap loose, the cap vent is clogged and a vacuum is restricting fuel flow.
Pass: Runs fine with the cap loose — replace or clean the fuel cap vent.
Fail: Still bogs with the cap loose — move to step 4.
The main jet supplies fuel at full throttle. A partial clog from varnish restricts fuel flow and causes bogging. Remove the carburetor and clean the main jet and all main circuit passages with carburetor cleaner and a soft wire.
Pass: Main jet is clear — move to step 5.
Fail: Main jet is restricted — clean it and reinstall. If it will not clear, install a carburetor rebuild kit or replace the carburetor.
Many string trimmers have a spark arrestor screen inside the muffler exhaust port. Over time, carbon and unburned oil build up on the screen and restrict exhaust flow, causing power loss and bogging. Remove the muffler and inspect the screen. Clean it with a wire brush and carb cleaner, or replace it if it is burned through.
Pass: Screen is clean — move to step 6.
Fail: Screen is clogged with carbon — clean or replace it. This is a very common cause of bogging on 2-cycle trimmers.
If all of the above are clean, the carburetor diaphragm may be stiff and unable to supply fuel fast enough at full throttle. Remove the diaphragm cover and inspect it for flexibility and tears.
Pass: Diaphragm is flexible — check for an air leak at the intake gaskets.
Fail: Diaphragm is stiff or torn — install a carburetor rebuild kit with a new diaphragm and gaskets.
Most likely cause: A clogged air filter, a clogged spark arrestor screen in the muffler, or a partially clogged carburetor main jet. A clogged fuel filter or fuel cap vent can also restrict fuel flow under load.
Full guide: lil-engine.com/fix/weed-eater-bogs-down-under-load
The primer bulb stays flat, pulls air, or fills but won’t hold — fuel runs back into the tank.
Confirm the tank has enough fuel to cover the pickup filter inside the tank. If the fuel level is below the pickup, the primer will pull air.
Pass: Tank has adequate fuel — move to step 2.
Fail: Tank is low — refill with fresh 2-cycle mix and retest the primer.
Check all fuel lines for cracks, softness, or loose connections. A cracked line between the tank and the primer draws air instead of fuel. Pay attention to the lines at the tank outlet, primer bulb, and carburetor. Replace any cracked lines as a set.
Pass: Fuel lines are supple and sealed — move to step 3.
Fail: Lines are cracked or soft — replace all fuel lines as a set. They harden together and replacing one leaves cracked others.
The primer bulb has two one-way check valves — one lets fuel in, one lets fuel out. If the inlet valve fails, the bulb won’t fill. If the outlet valve fails, fuel runs back into the tank. Remove the bulb and test by blowing through each port (fuel should flow one way only). Replace the bulb if either valve fails.
Pass: Check valves are working — move to step 4.
Fail: A check valve has failed — replace the primer bulb. They are inexpensive and usually sold with new gaskets.
A fine screen at the carburetor fuel inlet can clog with debris and prevent the primer from pulling fuel through. Remove the fuel line at the carburetor inlet and inspect the screen. Clean it with carb cleaner or replace it if damaged.
Pass: Inlet screen is clear — move to step 5.
Fail: Screen is clogged — clean or replace it and retest the primer.
A clogged fuel filter in the tank restricts fuel flow enough to prevent the primer from working. Pull the filter from the tank and inspect it. If it is dark or gummed, replace it.
Pass: Filter is clean — the primer circuit is intact; recheck the bulb and lines.
Fail: Filter is restricted — replace it.
Most likely cause: Cracked or hardened fuel lines drawing air, a failed primer bulb check valve, or a clogged fuel filter. The carburetor inlet screen is a less common cause but should be checked if the lines and bulb are good.
Full guide: lil-engine.com/fix/weed-eater-prime-bulb-will-not-hold-gas-when-i-push-it
Fuel is visibly leaking from the trimmer — from the tank, lines, carburetor, or primer bulb.
Remove the fuel cap and inspect the O-ring or gasket. If it is cracked, flattened, or missing, fuel will seep from the cap when the trimmer is tilted. Replace the cap or just the gasket if available.
Pass: Cap gasket is intact — move to step 2.
Fail: Gasket is cracked or flattened — replace the fuel cap or gasket. This is a common leak source.
Check all fuel lines for cracks, especially at the tank outlet and carburetor inlet where they are stretched over barbed fittings. Squeeze each line — if it is hard, brittle, or has visible cracks, it is leaking. Replace all fuel lines as a set.
Pass: Fuel lines are supple and sealed — move to step 3.
Fail: Lines are cracked or brittle — replace all fuel lines as a set. They harden together and must be replaced together.
Fuel can leak from the carburetor body if the gasket between the carburetor and the engine is dried or cracked, or if the diaphragm cover gasket is failing. Remove the carburetor and inspect all gaskets. Replace any dried or cracked gaskets with a rebuild kit.
Pass: Carburetor gaskets are intact — move to step 4.
Fail: Gaskets are dried or cracked — install a carburetor rebuild kit with new gaskets and a new diaphragm.
A cracked primer bulb can seep fuel, especially around the base where it meets the housing. Inspect the bulb for cracks and confirm it is seated properly. Replace the bulb if it is cracked or loose.
Pass: Primer bulb is intact and seated — move to step 5.
Fail: Bulb is cracked or loose — replace the primer bulb and its gasket.
Inspect the fuel tank for cracks, especially at the seams, mounting points, and where the fuel lines enter the tank. A cracked tank will drip fuel continuously. Replace the tank if it is cracked — do not attempt to seal it with epoxy or tape, as fuel will dissolve most adhesives.
Pass: Tank is intact — recheck all fittings and connections above.
Fail: Tank is cracked — replace the fuel tank. Do not use a cracked tank; it is a fire hazard.
Most likely cause: Cracked or hardened fuel lines, a failed fuel cap gasket, or a dried carburetor gasket. A cracked fuel tank is less common but requires immediate replacement.
The engine starts and revs normally, but the trimmer head doesn’t rotate.
Confirm the trimmer head is properly seated and locked onto the drive shaft. Remove the head, clean the mounting surface, and reinstall it. If the head uses a bolt, confirm the bolt is tight. A loose or incorrectly mounted head can slip on the shaft.
Pass: Head is properly mounted — move to step 2.
Fail: Head was loose or incorrectly mounted — clean, reinstall, and tighten. Retest.
On straight-shaft trimmers, a flexible drive shaft inside the tube transmits power from the engine to the head. Remove the shaft from the engine end and inspect the flex shaft for breakage or stripped ends. A broken or stripped flex shaft is the most common cause of a non-spinning head.
Pass: Flex shaft is intact — move to step 3.
Fail: Flex shaft is broken or stripped — replace the flex shaft. On curved-shaft trimmers, the shaft and tube are often one assembly.
String trimmers use a centrifugal clutch that engages at a certain RPM. If the clutch shoes or springs are worn or broken, the engine revs but the clutch doesn’t engage the drive shaft. Remove the clutch drum and inspect the shoes and springs. Replace the clutch if the shoes are worn below specification or the springs are broken.
Pass: Clutch shoes and springs are intact — move to step 4.
Fail: Clutch is worn or broken — replace the clutch assembly. Do not run the trimmer with a slipping clutch; it will damage the drum and shaft.
On straight-shaft trimmers, an angle gearbox at the head end transfers power from the flex shaft to the head. The gears can strip or the grease can dry out. Remove the gearbox cover and inspect the gears. If the gears are stripped, replace the gearbox or the gear set. If the grease is dry, repack with lithium grease.
Pass: Gears are intact and greased — move to step 5.
Fail: Gears are stripped or grease is dry — replace the gear set or gearbox, and repack with fresh lithium grease.
The coupling between the engine output shaft and the flex shaft can strip if the set screw loosens or the shaft rounds off. Remove the coupling and inspect both the coupling and the shaft ends for stripping. Tighten the set screw with Loctite or replace the coupling if it is stripped.
Pass: Coupling is intact and tight — recheck the clutch and gearbox.
Fail: Coupling is stripped or set screw is loose — replace the coupling and tighten the set screw with thread-locking compound.
Most likely cause: A broken or stripped flexible drive shaft, a worn centrifugal clutch, or a stripped angle gearbox. The head attachment being loose is the simplest cause and should be checked first.
Full guide: lil-engine.com/fix/string-trimmer-engine-running-but-string-spool-trimmer-head-isnt-spinning
The trimmer line won’t advance when you bump the head, or it runs out and won’t reload.
Open the spool and check that the line is wound in the correct direction (arrows on the spool show the way) and is not tangled or crossed. Line wound the wrong way will not feed. Rewind the line following the arrows on the spool.
Pass: Line is wound correctly — move to step 2.
Fail: Line is tangled or wound wrong — rewind it following the direction arrows on the spool. Do not overfill the spool; leave a small gap.
The bump knob at the bottom of the head pushes a spring-loaded plate that advances the line. If the spring is broken, the knob is worn, or the plate is jammed with grass, the head won’t feed. Disassemble the head, clean all parts, and inspect the spring and knob for wear. Replace the bump knob and spring if worn.
Pass: Spring and knob are intact — move to step 3.
Fail: Spring is broken or knob is worn — replace the bump knob and spring assembly. Clean all grass from inside the head.
Trimmer line can melt and weld together inside the spool from heat generated during use, especially if the line is too heavy or the head is run at low RPM. Pull the line from the spool — if it is stuck together or fused, cut the welded section off and rewind the remaining line.
Pass: Line is not fused — move to step 4.
Fail: Line is melted together — cut off the fused section and rewind. Use a lighter line weight if the problem recurs.
The line exits through small eyelets in the head. Grass and debris can pack into these holes and block the line. Clean all debris from the eyelets with a pick or compressed air. Confirm the eyelets are not worn or enlarged — oversized eyelets let the line slip back in.
Pass: Eyelets are clear and intact — move to step 5.
Fail: Eyelets are blocked or worn — clean them. If they are worn or enlarged, replace the head.
Confirm the line diameter matches the specification for your head. Line that is too heavy won’t feed through the eyelets; line that is too light will break and weld. Check the head or manual for the correct diameter (commonly 0.065–0.095 inch).
Pass: Line size is correct — move to step 6.
Fail: Line is the wrong size — replace it with the diameter specified for your head.
If the head is cracked, warped, or the internal tracks are worn, the line will not feed reliably even with new line and a clean spring. Replace the entire bump head with a new one matched to your shaft. Aftermarket universal heads are available if an OEM head is not found.
Pass: Head is in good condition — recheck the line and spring.
Fail: Head is worn or cracked — replace the entire bump head assembly.
Most likely cause: Line wound in the wrong direction, a jammed or worn bump head spring, or line melted together from heat. Blocked eyelets and incorrect line size are the next most common causes.
Full guide: lil-engine.com/fix/weed-eater-trimmer-line-wont-feed
The trimmer vibrates far more than normal during operation — your hands tingle or the whole unit shakes.
Stop the engine, disconnect the spark plug boot, and clean all grass and debris from under and around the trimmer head. Packed grass throws the head out of balance and is the most common cause of sudden vibration.
Pass: Head is clean and vibration is gone — the debris was the cause.
Fail: Head is clean but vibration persists — move to step 2.
Inspect the trimmer head for cracks, warping, or missing pieces. A damaged head is unbalanced and causes vibration. If the head is cracked or a piece is broken off, replace it. Confirm the head is mounted squarely on the shaft.
Pass: Head is intact and balanced — move to step 3.
Fail: Head is cracked or unbalanced — replace the trimmer head.
A bent or worn drive shaft (flex shaft on straight-shaft models) causes vibration that increases with RPM. Remove the shaft and roll it on a flat surface — a straight shaft will roll true; a bent shaft will wobble. Replace a bent flex shaft.
Pass: Drive shaft is straight — move to step 4.
Fail: Drive shaft is bent — replace the flex shaft. Do not run the trimmer with a bent shaft; it will damage the clutch and bearings.
A worn or broken clutch shoe can cause imbalance and vibration. Remove the clutch drum and inspect the shoes and springs. If a shoe is broken or a spring is missing, the clutch is unbalanced. Replace the clutch assembly.
Pass: Clutch is intact and balanced — move to step 5.
Fail: Clutch shoe or spring is broken — replace the clutch assembly.
Check all mounting bolts and screws on the engine, handle, and shaft. A loose engine mount or handle allows the whole unit to shake. Tighten all hardware to specification. Inspect the engine mounting isolators (rubber bushings) for deterioration and replace if cracked.
Pass: All hardware is tight and isolators are intact — move to step 6.
Fail: Hardware is loose or isolators are cracked — tighten all bolts and replace any worn isolators.
If all of the above are good, the crankshaft itself may be bent — usually from hitting a fence, rock, or other solid object. A bent crankshaft causes severe vibration that increases with RPM and cannot be repaired economically. On an inexpensive trimmer, engine replacement is the only practical fix.
Pass: Crankshaft appears straight — recheck the head and shaft for subtle imbalance.
Fail: Crankshaft is bent — the engine must be replaced. A bent crankshaft is not repairable and running the engine will destroy the main bearings.
Most likely cause: Debris packed under the trimmer head, a cracked or unbalanced trimmer head, or a bent drive shaft. A bent crankshaft from hitting a solid object is the most serious cause and usually requires engine replacement.
Generated by Lil Engine. Always follow manufacturer instructions and proper safety precautions. If unsure, contact a qualified professional.