String Trimmer — Won't Start

Before you begin

Trimmer cutting heads spin at very high RPM. Keep hands well clear of the cutting head during any check, and disconnect the spark plug wire before touching the head, guard, or fuel system. Wear eye protection.

Spark plug test — safety first

Keep the plug pointed away from the plug hole and any spilled fuel while testing spark — a spark near fuel vapor can ignite it.

Ignition switch in Off position

It sounds obvious, but this is one of the most common reasons a trimmer 'won't start' — the switch gets bumped to Stop during storage or transport and is easy to overlook.

1. Flip the switch to Run/On before attempting to start.
2. If it still won't stay in the Run position, the switch itself may be faulty and need replacement.

Incorrect or missing 2-stroke oil mix

2-stroke engines rely on oil mixed into the fuel for lubrication — there's no separate oil reservoir like a 4-stroke mower engine. Running straight gasoline, even briefly, can cause serious internal damage, not just a no-start.

1. Drain any straight gasoline or incorrectly mixed fuel completely.
2. Mix fresh gasoline with 2-stroke oil at the ratio specified in your owner's manual (commonly 50:1, but confirm for your model).
3. Use a marked mixing bottle to get the ratio right rather than eyeballing it.
4. If straight gas was run for more than a few seconds, keep an ear out for unusual noise on the next start — that could indicate lubrication damage worth having checked.

Parts that may help: 2-stroke engine oil, marked fuel mixing bottle

Stale 2-stroke fuel mix

2-stroke mix separates and degrades faster than straight gas — old mix is one of the most common trimmer no-start causes, especially after seasonal storage.

1. Drain old fuel into an approved container.
2. Mix a fresh batch at the correct ratio and refill.
3. For future storage, mix only what you'll use within 30 days, or add a fuel stabilizer rated for 2-stroke mix.

Parts that may help: 2-stroke engine oil, e.g. Sta-Bil, Sea Foam

Cold-start procedure not followed

Trimmers need fuel primed into the carburetor before a cold start, and the choke set to restrict air until the engine catches.

1. Press the primer bulb until you can see fuel moving through it, typically 6-10 presses.
2. Set the choke to full/cold-start, pull the cord until it fires (may take a few pulls, sometimes catches briefly then dies).
3. Once it fires, move the choke to half or run position, then to full run once it's idling steadily.

Faulty on/off switch or wiring, not the ignition coil

Since spark returned once that wire was isolated, the coil is fine — the switch itself, or the wire between the switch and coil, is grounding out the ignition even in the 'Run' position.

1. Inspect the switch's internal contacts for corrosion or wear — these small slide switches are a common failure point after a season or two of grit exposure.
2. Check the wire's full length for cuts, fraying, or a loose connector.
3. Replace the switch if the wiring itself checks out — it's a low-cost, straightforward swap on most trimmers.
4. Reconnect the wire properly once repaired rather than leaving it disconnected.

Parts that may help: replacement on/off ignition switch

Faulty ignition coil

Since spark still didn't return with the kill circuit isolated, the coil itself is the likely failure point.

1. Clean the flywheel magnets and coil laminations with fine sandpaper if accessible — surface corrosion can weaken spark.
2. Check the coil's air gap against your engine's spec with a feeler gauge if adjustable on your model.
3. If that doesn't restore spark, replace the ignition coil.

Parts that may help: engine-model-specific ignition coil, feeler gauge set for coil air gap

No spark — plug or ignition coil

Trimmer spark plugs are inexpensive and fail more often than the ignition coil, so start there.

1. Replace the spark plug with the correct type/gap for your model — this resolves most no-spark cases.
2. If a new plug still gives no spark, the ignition coil likely needs testing with a multimeter or replacement.

Parts that may help: engine-specific spark plug, engine-model-specific ignition coil

Fouled spark plug — likely flooded

A wet, fuel-smelling plug usually means the engine is flooded from excess priming or choke use.

1. Remove the plug, wipe it clean or swap in a fresh one.
2. With the plug out, pull the starter cord several times to clear excess fuel from the cylinder.
3. Reinstall the plug, set the choke to run (not full choke) this time, and try starting again.

Parts that may help: engine-specific spark plug

Cracked fuel line or primer bulb

Small-diameter 2-stroke fuel lines and primer bulbs are especially prone to cracking from heat and ethanol fuel — a crack lets the system draw air instead of fuel, so priming never fully works.

1. Replace the fuel line and primer bulb as a set — they're inexpensive and usually sold together in a repower/carb kit.
2. Route the new line exactly as the old one was, noting the routing before removal.
3. Test the primer bulb holds fuel and doesn't leak before reassembling the housing.

Parts that may help: fuel line and primer bulb repower kit

Clogged air filter

A blocked filter starves the small 2-stroke engine of air, which is enough to prevent starting even with good fuel and spark.

1. Foam filters: wash in warm soapy water, air dry fully, lightly oil before reinstalling.
2. Replace if torn or heavily saturated.
3. Clean every few uses in dusty/dry trimming conditions.

Parts that may help: engine-model-specific air filter, foam air filter oil

Possible crankcase seal leak — needs pressure/vacuum testing

Without a pressure/vacuum tester, a crankcase seal leak (crankshaft seals, carburetor mounting gasket, or a crack in the crankcase itself) can't be reliably confirmed at home, but it's a common cause of a 2-stroke that has fuel, spark, and a clean carb yet still won't run properly.

1. This is a good candidate for a local small engine shop — this specific test (crankcase pressure/vacuum) is quick for them to run with the right equipment and will confirm or rule this out definitively.

If this doesn't resolve it, this is a good candidate for a local small engine shop rather than continued DIY diagnosis.

Crankcase seal or gasket leak

A leaking crankcase can't build the pressure needed to properly pump the fuel/air mixture into the cylinder, which prevents reliable running even with good fuel and spark.

1. Common leak points are the crankshaft seals (both ends) and the carburetor-to-cylinder mounting gasket — inspect these first as they're the most accessible.
2. Replacing crankshaft seals typically requires splitting the crankcase, which is a substantial repair — this is a reasonable point to have a shop handle it unless you're experienced with 2-stroke rebuilds.

Parts that may help: crankshaft seal kit

If this doesn't resolve it, this is a good candidate for a local small engine shop rather than continued DIY diagnosis.

Low cylinder compression — piston or cylinder wear

With crankcase sealing, fuel, and spark all ruled out, low cylinder compression points to worn piston rings or cylinder wear — internal wear rather than a bolt-on fix.

1. Given the teardown involved, this is worth a shop's opinion on repair cost versus the value of the trimmer before investing further time yourself.

Parts that may help: small engine compression tester

If this doesn't resolve it, this is a good candidate for a local small engine shop rather than continued DIY diagnosis.

Needs professional diagnosis

You've ruled out switch, fuel mix, priming, spark, fuel lines, air filter, crankcase sealing, and (if tested) compression. Remaining causes are specific enough to your engine model that a shop's diagnostic equipment will get there faster than continued guessing.

1. This is a good candidate for a local small engine shop — bring your notes on what's already been ruled out.

If this doesn't resolve it, this is a good candidate for a local small engine shop rather than continued DIY diagnosis.