Stihl MS 170 Cranks But Won't Start — Fuel & Spark Fix
⚡ Quick Answer
On a Stihl MS 170, start by checking the spark arrestor screen inside the muffler; a carbon-clogged screen is the most common reason these saws refuse to fire. If the screen is clear, check for spark using an NGK BPMR7A plug gapped to 0.020" (0.5mm). If you have spark but no fire, inspect the fuel line for hairline cracks near the tank entry and ensure the Zama carburetor's internal diaphragms haven't hardened due to ethanol fuel.
What do you want to do?
Most Likely Cause
Clogged Carburetor Internal Screen
The Zama carburetor contains a microscopic inlet screen that traps debris. If ethanol fuel has sat for over 30 days, varnish buildup will block this screen, preventing fuel from reaching the metering chamber.
Start here — this is the most common reason for this symptom.
Why This Problem Happens
The Stihl MS 170 utilizes a simple, yet finicky, stratified-charge 2-stroke engine that relies on precise fuel atomization and strong electrical timing to initiate combustion. When the saw cranks but fails to fire, it is usually because the volatile components of stale fuel have evaporated, leaving behind a gummy residue in the Zama C1Q carburetor’s tiny check valve and metering diaphragm. This disrupts the pulse-driven fuel delivery system, resulting in a lean condition where the air-fuel mixture lacks the necessary fuel vapor to ignite under compression. Simultaneously, the MS 170’s ignition module is highly sensitive to carbon buildup on the spark plug electrode or a failing ignition coil lead wire. If the ignition timing or spark intensity is compromised, the charge will not detonate. The small combustion chamber of the MS 170 requires a crisp, blue spark; if the plug is fouled or the air gap is incorrect, the engine will spin indefinitely without reaching a power stroke, as the compression ratio is insufficient to force ignition without that precise spark event.
Common Symptoms
- Dry spark plug after repeated cranking attempts
- Spark plug saturated with black, oily carbon
- Engine pops briefly on choke then dies immediately
- No visible spark when testing against cylinder head
- Fuel tank line shows signs of cracking or hardening
Before You Buy Parts
Quick, free inspections — check these first. Most repairs don't actually need replacement parts.
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Repair Overview
45–90 minutes
$15–$50
First Checks — Start Here
- 1Verify Master Control Lever is in the 'Cold Start' (full down) position
- 2Inspect spark arrestor screen in the muffler for carbon blockage
- 3Test for spark using a dedicated tester — look for a crisp blue arc
- 4Check spark plug gap — must be exactly 0.020 inch (0.5 mm)
- 5Inspect fuel line and impulse line for cracks or softening
- 6Ensure 50:1 fuel mix is fresh (less than 30 days old)
Safety Warnings
- !Keep hands clear of the spark tester wire while cranking to avoid a high-voltage electrical shock.
- !Do not operate the saw for long periods with a loose cap to prevent fuel leakage.
- !Perform this in a well-ventilated area away from open flames, as raw fuel will spill.
- !Wear safety glasses, as carb cleaner is highly irritating and pressurized spray can blow back into your eyes.
- !Do not use a metal feeler gauge as it will magnetically lock to the flywheel and produce an inaccurate gap.
- !Ensure the throttle is held in the wide-open position during the compression test for an accurate reading.
- !Using old gas that has undergone phase separation
- !Adjusting the carburetor high and low needles before ensuring fuel delivery
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 Zama carburetor contains a microscopic inlet screen that traps debris. If ethanol fuel has sat for over 30 days, varnish buildup will block this screen, preventing fuel from reaching the metering chamber.
The rubber fuel line inside the MS 170 tank often cracks at the bend near the fuel filter. This allows air to be sucked into the system instead of fuel, resulting in a lean condition that prevents starting.
🔧 Step-by-Step Fix Guide
Spark Test and Plug Replacement
Remove the shroud and pull the plug wire. Use a calibrated ignition spark tester to see if you have a strong blue spark while cranking the engine; if not, replace the NGK BPM7A plug and inspect the ignition lead.
Keep hands clear of the spark tester wire while cranking to avoid a high-voltage electrical shock.
Verify Fuel Tank Venting
Ensure the fuel tank vent is not blocked by testing the saw with the fuel cap slightly loosened. If it starts with the cap loose, the vent is clogged and must be replaced.
Do not operate the saw for long periods with a loose cap to prevent fuel leakage.
Inspect Fuel Pick-up Line
Use a small hook tool to pull the fuel filter and line out of the tank filler neck. Inspect the line for stiffness or cracking, particularly at the connection to the carburetor inlet.
Perform this in a well-ventilated area away from open flames, as raw fuel will spill.
Clean or Rebuild Zama Carburetor
Remove the carburetor by disconnecting the throttle linkage and fuel lines. Disassemble the metering side, remove the diaphragm, and spray carburetor cleaner through the internal ports.
Wear safety glasses, as carb cleaner is highly irritating and pressurized spray can blow back into your eyes.
Check Ignition Coil Air Gap
Loosen the two screws holding the ignition module. Insert a 0.010-inch plastic feeler gauge between the flywheel magnet and the module, then tighten the screws.
Do not use a metal feeler gauge as it will magnetically lock to the flywheel and produce an inaccurate gap.
Test Compression
Screw a compression gauge into the spark plug hole and pull the starter rope briskly 5-6 times. The MS 170 should show at least 110-120 PSI; lower numbers suggest worn rings or a scored piston.
Ensure the throttle is held in the wide-open position during the compression test for an accurate reading.
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
Using old gas that has undergone phase separation
Adjusting the carburetor high and low needles before ensuring fuel delivery
Forgetting to switch the master control lever to the full choke position
Over-tightening the carburetor mounting screws and cracking the plastic intake flange
If This Didn't Fix It
Still having trouble? These are the next things to check:
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Finding parts for
Stihl MS 170 chainsaw
Required Gear
Matched to your Stihl MS 170 chainsaw — click to find the exact fit on Amazon.
NGK BPM7A Spark Plug
Replacing the spark plug is essential to rule out a fouled or resistor-shorted plug that prevents the engine from firing despite having fuel.
Zama C1Q Carburetor Rebuild Kit
This kit provides the necessary internal diaphragms, gaskets, and screens required to restore the fuel metering function of the carburetor after it becomes clogged.
Stihl OEM Fuel Pickup Hose
The original fuel line is prone to cracking or hardening, which prevents the carburetor from drawing fuel and causes the saw to crank without starting.
Stihl MS 170 Impulse Hose
A failed impulse hose prevents the pressure pulses from the crankcase from operating the carburetor pump, making it a critical replacement part for starting issues.
Fuel Tank Vent/Duckbill Valve
Replacing the tank vent ensures the tank does not create a vacuum that restricts fuel flow, which is a common cause for fuel starvation in this specific model.
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Pro Tips from the Community
Real-world fixes from Reddit, YouTube & forums.
"The MS 170 doesn't have an adjustable high-speed jet. If it's running lean, you usually have an air leak at the crank seals or a cracked impulse line."
ArboristSite.com Forum"If you flood it, move the lever to the 'I' (On) position, hold the throttle wide open with your foot in the rear handle, and pull 10-15 times to clear the excess fuel."
YouTube comments"Check the 'sugar cube' fuel filter inside the tank. If it's discolored or slimy, it will restrict flow enough to prevent starting even if the carb is clean."
Reddit r/chainsawPrevent This From Coming Back
Always use 89+ octane non-ethanol fuel with high-quality 2-cycle oil mixed at a strict 50:1 ratio
Drain the fuel tank entirely before storing the chainsaw for more than two weeks
Replace the fuel filter and air filter every season regardless of visible wear
Use a dedicated fuel stabilizer like Stihl MotoMix during off-season storage
Prevent This From Happening Again
While you're fixing it, stock up on the maintenance essentials that keep your chainsaws running right.
Bar and Chain Oil
Proper bar oil prevents chain overheating and premature wear.
Get on Amazon2-Cycle Oil
Correct fuel-oil mix is critical — use fresh 2-cycle oil every season.
Get on AmazonChainsaw Chain
Keep a sharp spare chain — a dull chain is dangerous and slow.
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Frequently Asked Questions
Yes, it typically requires an NGK BPM7A. Avoid using resistor plugs unless specified, as they can interfere with the simple ignition system.
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