Stihl MS 391 Won’t Start (Starting Fixes)
If your STIHL MS 391 will not start, stop pulling and check the controls, fuel, and spark plug first. Use fresh 50:1 fuel, follow the marked cold- or warm-start sequence, and avoid repeated choke pulls that can flood the engine. If those checks fail, test spark and compression before buying parts or adjusting the carburetor.
A dealer diagnosis can cost more than a few basic checks, and guessing at a carburetor or ignition coil can waste money. I work from simple causes toward harder ones: control settings, fuel, air, spark, then compression. That order helps you find a fault without taking apart a sound engine.
Start safely and use the correct starting sequence
A sound starting routine prevents injury and avoids creating a second problem while you diagnose the first. The MS 391 is a 64.1 cm³ chainsaw; its engine size does not change the basic checks, but the manual and marked controls determine the correct starting positions.
Before each attempt, work outdoors on firm, level ground, away from fuel spills and ignition sources. Wear eye and face protection, hearing protection, gloves, chainsaw leg protection, and protective boots. Check the chain brake, throttle lockout, stop switch, handles, chain catcher, bar, chain, and fasteners. Do not start or use the saw if a safety part is damaged.
Set the saw securely on the ground with the bar and chain clear of people and objects. Engage the chain brake. Never drop-start it.
Cold-start procedure
A cold start uses the control setting marked for a cold engine. Follow the sequence shown on the saw and in the applicable manual; control markings and instructions take priority over a generic description.
Set the control to cold-start or choke. Pull until the engine first fires, then move the control to the prescribed warm-start or half-choke position. Pull again until it starts. Briefly blip the throttle as directed to return the engine to idle. Keep the brake engaged until you are ready to cut.
Warm start and a flooded engine
A warm engine usually needs the manual’s warm-start setting, not full choke. Repeated full-choke pulls can wet the spark plug and make starting harder.
If you suspect flooding, stop pulling with full choke. Follow the manual’s flooded-engine procedure. If you remove the plug, keep spilled fuel away from sparks and hot parts; inspect, dry, or replace the plug as the manual allows. Do not test for spark near fuel.
Next step: If the saw still will not fire after a correct attempt, move to fuel and air checks before replacing parts.
Check fuel, air, and the spark plug
Fuel, air, and a clean, correctly fitted spark plug are basic needs for combustion. A fault in any one can cause no-start symptoms, but the checks below help separate a simple service issue from a deeper engine problem.
Start with the fuel. The specified mix is 50:1 gasoline to STIHL two-stroke oil: 20 mL of oil per 1 L of gasoline. Use fresh fuel and the fuel specification in the manual for your saw. Never use straight gasoline, a guessed ratio, or old, contaminated mix. If the fuel’s age or condition is unknown, drain it as directed in the manual and add correctly mixed fuel.
Remove and inspect the air filter according to the manual. A clogged or damaged filter can restrict airflow. Clean it using the method specified for that filter; do not reinstall a wet filter if the manual requires it to be dry. Check the plug for damage or heavy deposits. Use the specified plug type and gap from the manual, rather than guessing.
| Check | What to look for | Next action |
|---|---|---|
| Fuel | Stale, dirty, or wrong mix | Replace with fresh 50:1 mix |
| Air filter | Dirt, damage, restriction | Clean or replace as specified |
| Spark plug | Damage, deposits, wet fuel | Clean or replace; verify type and gap |
| Choke use | Full choke used on warm engine | Follow warm-start procedure |
A wet plug after repeated choke pulls can point to flooding, but it does not prove the carburetor is bad. A dry plug can mean fuel is not reaching the cylinder, though the plug and other conditions still matter. Treat either sign as a clue, not a final diagnosis.
Next step: Correct fuel, filter, or plug issues, then make one proper starting attempt. If it remains dead, test ignition.
Test spark, then check compression
Spark is the electrical ignition that lights the fuel-air mixture. Compression is the pressure the piston builds inside the cylinder. Testing these after the basic checks helps narrow the cause without relying on a guessed replacement part.
Use a proper spark tester and follow its instructions. Keep the test area clear of spilled fuel and vapors. Do not hold the plug or lead by hand while cranking, and do not create an open spark near fuel. If the tester shows no spark, check the stop-switch position and visible wiring or connection damage. A failed test can point to an ignition fault, but internal testing may require service tools and a manual.
Compression testing needs a suitable gauge and the correct procedure for this model. There is no single threshold provided here that can safely diagnose every MS 391. Compare the result with the applicable service information, or have a qualified technician assess it. Low compression or signs of internal damage call for service, not repeated pulls.
A damaged starter assembly can also prevent normal cranking. If the cord does not retract, pulls unevenly, or fails to turn the engine, stop and inspect it rather than forcing it.
Next step: No spark, abnormal cranking, or low compression is a reason to pause and diagnose with the service manual or a qualified repair shop.
Diagnose what happens when the saw fires
The way an engine starts and then runs is useful evidence. It does not name the failed part by itself, but it helps choose the next check and avoid risky carburetor adjustments.
If the MS 391 fires briefly and dies, recheck fuel freshness and the air filter. A restricted fuel filter or line, or a carburetor fault, may also limit fuel delivery. Those parts need inspection using the model’s service procedure; do not assume a carburetor replacement is the fix.
If the engine races, will not idle, or the chain moves at idle, stop using it. Do not try to correct the problem by turning mixture screws by ear. The listed engine speeds are approximately 2,800 rpm at idle and 12,500 rpm maximum. Adjustment requires a tachometer and the limits in the manual; have a qualified service provider do it if you lack the tools or procedure.
| Symptom | Checks to make | Do not do this |
|---|---|---|
| No firing | Controls, fuel, plug, then spark | Buy an ignition part without a test |
| Fires, then dies | Filter, fuel delivery, carburetor diagnosis | Keep pulling with choke |
| Races or will not idle | Stop; check for fuel or air fault | Tune mixture screws by ear |
| Chain moves at idle | Stop and service the saw | Continue operating |
A lean-running engine can overheat and seize. Stale fuel, restricted fuel supply, or an air leak may contribute. Stop if the saw surges, races, loses power, or runs unusually hot. Continued operation can destroy the engine and make control harder.
Next step: Use symptoms to guide inspection, but do not run a saw with unsafe idle behavior or signs of overheating.
Two diagnostic examples and when to get service
These examples show how to use symptoms as clues, not proof. They are common diagnostic patterns, not claims that every saw with the same symptom has the same fault. Work through the checks in order and stop when a safety or internal fault appears.
Example 1: It will not fire after many choke pulls. Stop repeated attempts. Check whether the engine is warm, use the manual’s flooded-engine steps, and inspect the plug. Then confirm fresh, correctly mixed fuel and the air filter. If it still will not start, test spark safely.
Example 2: It starts, then races or dies. Stop running it. Check the fuel and air filter, then look for a restricted fuel path or a carburetor issue using the service procedure. A racing saw may have a serious fuel or air problem; do not compensate with mixture-screw changes.
Call for qualified service if the saw has no spark after basic control checks, damaged wiring, a starter fault, low compression, suspected air leak, or internal damage. Paying for a targeted test can be cheaper than replacing several parts without evidence.
Next step: Before cutting after a successful start, verify chain tension, chain-brake function, and chain lubrication. Use dedicated bar-and-chain oil and confirm oil delivery as directed by the manual. Do not cut with a dull, loose, mismatched, or oil-starved chain.
Frequently asked questions
These short answers cover common starting questions for the MS 391. Use the applicable STIHL manual for control positions, maintenance steps, and service limits; equipment details can vary by market and configuration.
Why will my MS 391 not start after many pulls?
Check the control position, fuel freshness and mix, air filter, and plug. Repeated choke pulls may flood the engine; follow the manual’s flooded-start procedure.
What fuel mix does the MS 391 use?
The specified ratio is 50:1 gasoline to STIHL two-stroke oil, or 20 mL of oil per 1 L of gasoline. Follow the fuel specification in your saw’s manual.
Should I use choke for a warm start?
Use the warm-start position specified in the manual. Full choke on a warm engine can make starting harder.
What does a wet spark plug mean?
It may indicate flooding after repeated choke pulls, but it does not prove a carburetor fault. Follow the manual’s flooded-engine steps and inspect the plug safely.
Can I adjust the carburetor by ear?
No. Do not make uncontrolled mixture-screw changes. Adjustment may require a tachometer and the service limits for the saw.
What if it starts and then dies?
Check fresh fuel and a clean air filter first. A blocked fuel filter or line, or a carburetor fault, may need service diagnosis.
Why does the saw race or move its chain at idle?
Stop using it. Racing, poor idle, or chain movement at idle needs diagnosis; do not continue cutting or try to tune around it.
What bar and chain fit an MS 391?
Use a combination approved for your saw and fitted sprocket. Match bar pitch, chain pitch, gauge, and drive-link count to the bar stamp and manual; do not choose by engine size alone.
When should I stop troubleshooting at home?
Stop if there is no spark after basic checks, damaged starter parts, low compression, suspected internal damage, overheating, or unsafe idle behavior. Have the saw assessed by a qualified technician.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
