Stihl MS 250 Cranks But Won't Start — Fuel & Spark Diagnosis
⚡ Quick Answer
To diagnose a Stihl MS 250 that cranks but won't fire, first confirm ignition by performing a spark test using an NGK BPMR7A plug. If spark is present, inspect the spark arrestor screen in the muffler for carbon blockage, a very common Stihl failure that prevents the engine from breathing. If the saw still won't fire, verify fuel delivery by checking for hardened carburetor diaphragms or a disconnected impulse line located behind the carburetor.
What do you want to do?
Most Likely Cause
Clogged Carburetor Internal Screen
The micro-filter screen inside the Zama carburetor often traps debris from degraded fuel lines. If restricted, fuel cannot reach the metering chamber, preventing the engine from drawing a combustible mixture.
Start here — this is the most common reason for this symptom.
Why This Problem Happens
The Stihl MS 250 utilizes a relatively simple piston-ported two-stroke cycle, meaning it relies on precise crankcase compression and fuel atomization. When the engine cranks but fails to fire, the internal combustion triangle—compression, spark, and fuel—is incomplete. In these units, ethanol-blended fuels often degrade the internal rubber components of the Zama carburetor, specifically the fuel metering diaphragm and the tank vent, leading to a lean condition where the engine cannot draw fuel into the combustion chamber. Conversely, if the fuel delivery system is intact, the issue often lies in the ignition module air gap or a fouled spark plug, where the high-voltage pulse from the coil fails to jump the gap under the pressure of the cylinder's compression cycle. Furthermore, the MS 250 is susceptible to impulse line leaks, where the pulsing air pressure from the crankcase fails to actuate the carburetor pump, rendering the fuel delivery system completely stagnant despite adequate compression. Understanding the relationship between these physical pressure pulses and fuel draw is critical to diagnosing this classic no-start scenario.
Common Symptoms
- Dry spark plug after repeated cranking attempts
- Engine fires briefly when prime fuel is added to the intake
- Lack of resistance when pulling the starter rope
- Strong gasoline odor after cranking indicating flooding
- Visible spark jump is thin, orange, or non-existent
- Carburetor air intake throat appears wet with fuel
Before You Buy Parts
Quick, free inspections — check these first. Most repairs don't actually need replacement parts.
Repair Overview
45–90 minutes
$5–$40
First Checks — Start Here
- 1Verify fuel is fresh 50:1 mix and less than 30 days old
- 2Check spark plug for wetness (flooding) or bone-dry tip (starvation)
- 3Test for a crisp blue spark grounded against the cylinder head
- 4Inspect spark arrestor screen for heavy carbon buildup
- 5Ensure the Master Control Lever correctly engages the choke flap
- 6Check the impulse line connection for cracks or detachment
Safety Warnings
- !Always ensure the kill switch is in the 'Run' position and keep the spark plug away from the spark plug hole to prevent accidental ignition of fuel vapors.
- !Ensure the throttle is held wide open during testing to allow proper air intake for an accurate reading.
- !Perform this test in a well-ventilated area away from open flames or hot exhaust components.
- !Use caution when removing the fuel lines to avoid spills; keep a rag handy to absorb any residual fuel.
- !Ensure the kill wire is not pinched during reassembly of the cooling shroud.
- !Work on a clean, white surface to ensure no tiny internal springs or check valves are lost during disassembly.
- !Using an incorrect spark plug heat range which leads to fouling
- !Adjusting the carburetor L and H screws before fixing the underlying fuel delivery issue
- !Ignoring the fuel tank vent as a cause for intermittent starting issues
Always disconnect the spark plug, work in a well-ventilated area, and consult a professional if unsure.
Most Likely Causes
Ranked by likelihood — tap each to see diagnosis details.
The micro-filter screen inside the Zama carburetor often traps debris from degraded fuel lines. If restricted, fuel cannot reach the metering chamber, preventing the engine from drawing a combustible mixture.
The vent allows air to replace consumed fuel. If it clogs, a vacuum forms in the tank, preventing fuel from flowing to the carburetor even if the lines are clear.
🔧 Step-by-Step Fix Guide
Spark Verification
Remove the Bosch or NGK spark plug and reconnect it to the high-tension lead, touching the threaded base against a clean engine bolt. Pull the starter rope and observe for a strong, bright blue spark, ensuring you do not touch the wire during testing.
Always ensure the kill switch is in the 'Run' position and keep the spark plug away from the spark plug hole to prevent accidental ignition of fuel vapors.
Cylinder Pressure Test
Use a dedicated small-engine compression tester to verify that the cylinder produces between 120-150 PSI. Anything significantly lower indicates a ring failure or scored cylinder wall requiring a top-end rebuild.
Ensure the throttle is held wide open during testing to allow proper air intake for an accurate reading.
Fuel System Flow Check
Disconnect the fuel line at the carburetor inlet and check for steady flow from the tank. If no fuel flows, verify the fuel pickup body (filter) in the tank is not saturated with varnish or debris.
Perform this test in a well-ventilated area away from open flames or hot exhaust components.
Impulse Line Inspection
Remove the carburetor assembly to inspect the rubber impulse line that connects the crankcase to the carburetor body. If you see signs of dry rot, cracking, or if the tube is disconnected, replace it immediately to restore pump pressure.
Use caution when removing the fuel lines to avoid spills; keep a rag handy to absorb any residual fuel.
Ignition Coil Air Gap Adjustment
Loosen the ignition module mounting screws and insert a business card between the coil laminations and the flywheel magnets. Tighten the screws while holding the coil against the card, then remove the card to achieve the perfect air gap.
Ensure the kill wire is not pinched during reassembly of the cooling shroud.
Carburetor Cleaning/Diaphragm Inspection
Disassemble the Zama carburetor and check the diaphragms for stiffness. A hardened metering diaphragm will fail to pulse correctly, preventing proper fuel metering.
Work on a clean, white surface to ensure no tiny internal springs or check valves are lost during disassembly.
What To Test Next
If fuel delivery checks out, the next thing to inspect is spark — remove the spark plug, ground its threads to the engine block, and crank the engine to look for a visible blue spark.
Component Reference
Carburetor & Fuel Intake
The carburetor mixes air and fuel for combustion. A clogged jet or stuck float is the #1 cause of small engine starting and surging problems.
Spark Plug & Ignition System
The spark plug ignites the air-fuel mixture. A fouled plug, wrong gap, or failing ignition coil will prevent the engine from firing.
Oil & Lubrication System
Regular oil changes are critical. Low or dirty oil causes engine wear, overheating, and eventual seizure. Check the dipstick before every season.
Air Filter & Intake
A clogged air filter starves the engine of air, causing rich running, power loss, and hard starting. Inspect and clean or replace it each season.
Photos for educational reference. Actual parts may vary by make and model.
Common Mistakes to Avoid
Replacing the carburetor without first testing the impulse line for vacuum leaks
Using an incorrect spark plug heat range which leads to fouling
Adjusting the carburetor L and H screws before fixing the underlying fuel delivery issue
Ignoring the fuel tank vent as a cause for intermittent starting issues
Get the Exact Parts for Your Machine
Tell us your equipment's year, make, and model — we'll search for parts that actually fit, not generic ones.
Finding parts for
Stihl MS 250 cranks but
Required Gear
Matched to your Stihl MS 250 cranks but — click to find the exact fit on Amazon.
Stihl MS 250 Carburetor Rebuild Kit
This kit includes the internal screen, gaskets, and diaphragms necessary to resolve fuel delivery issues caused by a clogged carburetor or hardened internal components.
Ignition Coil
If spark verification fails and air gap adjustment does not correct the issue, the coil itself is likely internally shorted and must be replaced to restore ignition.
Fuel Tank Vent
A degraded or clogged tank vent prevents air from entering the tank as fuel is consumed, which creates a vacuum lock that starves the engine of fuel during operation.
Impulse Line
The impulse line is a common failure point that loses its sealing ability over time; replacing it restores the proper vacuum pulses required to drive the carburetor fuel pump.
Crankshaft Seal Kit
If the cylinder pressure test reveals low compression or an air leak, these seals are required to maintain the necessary crankcase pressure for the engine to pull fuel and ignite properly.
Affiliate Disclosure: Lil Engine may earn commissions from qualifying purchases.
Pro Tips from the Community
Real-world fixes from Reddit, YouTube & forums.
"If your MS 250 won't start after a few pulls, immediately check the spark arrestor. It's the 'silent killer' of Stihl saws and takes 30 seconds to clean."
YouTube comments"The impulse line on this model is notorious for popping off the engine side. If you have spark and a clean carb but no fuel, that's your culprit."
Reddit r/chainsaw"Always use the 'Master Control' properly. 1 pull on full choke until it 'pops', then move to half-choke immediately. If you stay on full choke after the pop, it will flood every time."
ArboristSite.com ForumPrevent This From Coming Back
Always use non-ethanol fuel to prevent carburetor diaphragm decay
Change the fuel pickup filter every season to prevent sediment buildup
Store the chainsaw with the tank drained or treated with high-quality stabilizer
Keep cooling fins clean of sawdust to ensure the ignition module doesn't overheat
Prevent This From Happening Again
While you're fixing it, stock up on the maintenance essentials that keep your small engines running right.
Spark Plug
The cheapest, fastest fix for most small engine starting problems.
Get on AmazonCarburetor Cleaner
Cleaning the carb is the #1 fix for surging and stalling engines.
Get on AmazonFuel Stabilizer
Prevents the stale fuel problems that cause most seasonal no-starts.
Get on AmazonAffiliate Disclosure: Lil Engine may earn commissions from qualifying purchases.
Frequently Asked Questions
The MS 250 uses a modern electronic ignition module, so it does not have points or a condenser that require adjustment or replacement.
Video Help
Helpful videos showing these repairs in action.
Share Your Repair
Real-world fixes help everyone. Share what worked and your repair becomes part of Lil Engine's community database.
Related Problems
More Small Engines GuidesRelated Maintenance Articles
Did this help?
Your feedback helps improve guides for everyone.