STIHL Outdoor Power Equipment String Trimmer Won't Start: How to Fix It Yourself
STIHL trimmers are two-stroke engines that need proper fuel mix, spark, and air to run. The most common no-start cause is old or improperly mixed fuel. These engines are very sensitive to fuel quality. If it has been sitting for more than a month, start with fresh fuel.
⚠️ Safety First
- Work in a well-ventilated area when handling fuel.
- Keep hands and clothing away from the cutting head — it can spin unexpectedly.
- Use the correct 50:1 gas-to-oil mix. Straight gasoline will destroy a two-stroke engine.
- Let the engine cool before refueling.
Parts You May Need
Step-by-Step Repair
- 1
Follow the correct starting procedure
Set the choke to the full-choke (cold start) position. Press the fuel primer bulb 5-7 times until you see fuel in the bulb. Pull the starter rope briskly until the engine fires briefly (you will hear a pop or partial start). Then move the choke to the half-choke or run position and pull again. Many no-start issues are simply incorrect starting technique.
- 2
Drain and replace the fuel
Empty the old fuel into an approved container. Mix fresh premium unleaded gas with STIHL two-stroke oil at a 50:1 ratio (2.6 oz of oil per gallon of gas). Never use fuel with more than 10% ethanol (E10). Pour in the fresh mix, press the primer bulb again, and try starting. Fresh, properly mixed fuel solves more trimmer problems than any other fix.
- 3
Check the spark plug
Remove the spark plug with the included wrench. If the tip is wet with fuel, the engine is flooded — dry it off with a clean rag and let the cylinder air out for a few minutes. If the plug is dry, fuel is not reaching the cylinder. If the electrode is damaged or heavily fouled, replace the plug. Reinstall and try again.
- 4
Clean or replace the air filter
Pop off the air filter cover (usually a snap-on cover on the side of the engine). Remove the filter and tap it on a hard surface to knock loose debris out. If it is very dirty or damaged, replace it. A clogged air filter makes the fuel mixture too rich and the engine will not start or will bog down immediately.
- 5
Check the spark arrestor screen
The spark arrestor is a tiny screen in the muffler that catches hot particles. Over time, carbon builds up and blocks exhaust flow, preventing the engine from starting or running. Remove the muffler cover, pull out the small mesh screen, and clean it with a wire brush or replace it. This is an often overlooked fix that takes two minutes.
If That Didn't Work
- Flooded engine from too many choke pulls (wait 15 minutes and try with no choke)
- Cracked fuel lines (common on older trimmers — they harden with age)
- Faulty ignition coil
- Clogged carburetor jets from ethanol fuel residue
When to Call a Professional
If fresh fuel, a new spark plug, and a clean air filter do not get the trimmer running, the carburetor may need a professional rebuild or replacement. STIHL dealers have trained technicians who can diagnose the issue quickly.
Related Repairs
Disclaimer: This guide is for informational and educational purposes only and does not constitute professional repair advice. All repairs are performed at your own risk. RepairItFree.com is not liable for any personal injury, property damage, device damage, voided warranties, or other losses resulting from the use of this information.
If you are unsure about any step — especially anything involving electricity, gas, water, or refrigerant — stop and consult a qualified professional. Your safety is more important than saving money.
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