Weed eater, trimmer or chainsaw won't start or won't stay running
Two-stroke won't start, won't idle or dies at full throttle? Check the fuel mix, fuel lines, plug, spark-arrestor screen and carburetor in the order that finds it fastest.
Two-stroke engines — the kind on most string trimmers (weed eaters), leaf blowers, edgers, hedge trimmers and chainsaws — fail in a small number of predictable ways: old or wrong fuel mix, cracked fuel lines, a gummed diaphragm carburetor, a fouled plug, or a blocked spark-arrestor screen.
The pattern of the failure tells you where to start, so the first question is what the machine actually does. One rule that saves engines: never run a two-stroke on straight gasoline, even for a moment — the oil in the mix is its only lubrication.
Safety
Remove the spark plug boot before touching a cutting head, chain, blades, or any other driven attachment. Work outdoors — two-strokes run rich and their exhaust is dense. And a rule that saves engines: never run a two-stroke on straight gasoline, even for a moment. The oil in the mix is the engine's only lubrication.
Start at the first test. Each result tells you which test or which cause to go to next, so you never run a test you don't need. Torque figures, gaps and clearances differ from engine to engine, so wherever one is given as a range, confirm the exact value in your engine's manual.
Start here
What exactly does it do?
Each failure pattern points at a different system. Thirty seconds here saves pulling a carburetor that was never the problem.
You will need
Nothing — you are describing the machine's behavior
Do this
1Try to start the machine the way the manual says: full choke, prime if it has a primer bulb, pull until the first cough, then half choke and pull again.
2Note what happens: nothing at all, a cough that dies, starts but dies at idle, or starts but bogs and dies the moment you open the throttle.
3Note how the starter rope feels compared with when the machine was healthy — normal snappy resistance, or unusually easy to pull.
What a healthy machine shows
A healthy machine coughs within a few pulls on full choke, then starts on half choke and idles.
What did you see? What it means
No sign of life at all — not even a cough
Either fuel is not reaching the cylinder or there is no spark. We check the cheap and common things first: the mix, the primer, and the lines.
Starts, idles rough or only on choke, dies when the choke comes off
Running only on choke means running lean — the carburetor is not delivering enough fuel. That is almost always gummed metering passages or stiffened diaphragms inside the carburetor, or air leaking in through a cracked fuel line.
Idles fine, but bogs or dies the moment I open the throttle
Full throttle needs full fuel flow AND a clear exhaust. A blocked spark-arrestor screen or a lean high-speed circuit both produce exactly this. The screen takes two minutes to check, so it goes first.
The starter rope pulls noticeably easier than it used to
Loss of pull resistance is loss of compression. On a two-stroke that usually means scored piston and cylinder — the classic result of running on straight gasoline or a far-too-lean mix. This is the one to rule out before spending money on anything else.
Check the mix, the primer, and what the primer shows you
Two-stroke fuel problems are the most common cause of every symptom on this machine, and the primer bulb doubles as a free diagnostic tool: it shows you whether fuel can actually move through the system.
Safety
Work with the engine cool and away from ignition sources. Wipe up spilled mix.
You will need
Fresh two-stroke mix in the ratio your manual specifies (commonly 50:1, some older machines 40:1)
Do this
1Smell and look at the fuel in the tank. Mix older than about 30 days, or fuel that smells sour or varnish-like, is a suspect regardless of anything else.
2Confirm it IS mix. If there is any chance the tank was filled with straight gasoline, stop — do not try to start it again until the tank is drained and refilled with proper mix.
3Press the primer bulb slowly five to eight times and watch it: it should draw fuel through and fill, with bubbles clearing after a few presses.
4Note what the bulb does: fills with fuel, stays empty no matter how many presses, or has visible cracks and leaks.
5Confirm the ignition switch is ON — on many trimmers and saws the stop switch position is easy to knock without noticing.
What a healthy machine shows
Fresh mix of the right ratio in the tank, a primer that fills with fuel and holds it, and the switch ON.
What did you see? What it means
The mix is old, sour-smelling, or of unknown ratio
Old mix both starves the engine (gummed carburetor passages) and under-lubricates it as the oil degrades. Fresh mix is the first repair, and often the only one needed.
The primer pulls fuel through the whole supply path — tank filter, then line. If it cannot fill, that path is broken: a cracked line letting air in, a clogged in-tank filter, or a split bulb. This is the classic handheld failure.
Ethanol-blend fuel hardens and cracks handheld fuel line in a few seasons. A cracked line leaks air rather than fuel, so the engine leans out or the primer stops drawing — and the crack is often hidden inside the tank where the line flexes.
Safety
Drain or work over a container; fuel will drip when lines come off.
You will need
Needle-nose pliers or a piece of bent wire to fish the filter out of the tank
Good light
Do this
1Empty the tank into a container.
2Look at every visible section of fuel line, and flex it gently. Healthy line is soft and rubbery; failing line is stiff, discolored, or shows fine cracks when bent.
3Fish the fuel pickup (the weighted filter) out of the tank with the wire and inspect the line inside the tank — this hidden section cracks first.
4Look at the filter element itself: discolored or crusted means clogged.
5Squeeze the primer bulb and inspect it for splits or stiffness.
6While the tank is empty, look inside for debris or water sitting at the bottom.
What a healthy machine shows
Soft, intact lines from tank to carburetor, a clean filter, and a supple primer bulb.
What did you see? What it means
A line is cracked, stiff or deteriorated — or the filter is clogged
Found it, or at least the first fault. Lines, filter and primer are replaced together as cheap consumables; replacing one and leaving three-season-old others is a false economy.
The supply path is fine, which moves suspicion inside the carburetor — its metering diaphragms and passages. One more check first: spark, because it is faster and free.
The plug tells you what has been happening in the cylinder, and the spark test splits the remaining possibilities in half.
Safety
Do not hold the plug in your hand — the ignition can give you an electric shock. Rest it on the cylinder head, away from the open plug hole. Two-stroke mix vapor ignites readily.
You will need
Plug wrench (usually supplied with the machine) or a 19 mm / 3/4 in socket
Spark tester if you have one
Do this
1Remove the plug and read it: wet and smelling of fuel means fuel is arriving (and possibly flooding); bone dry means it is not; black fluffy soot means rich running or too much oil in the mix; gray-brown is healthy.
2Clip the plug back into its boot and rest its metal body on bare metal on the cylinder head, away from the plug hole — do not hold it in your hand. Switch the ignition ON, give the starter one short pull, and watch the gap. That is all it takes to see a spark. Shade helps — a weak spark is invisible in sunlight.
3If the plug is heavily fouled, worn, or of unknown age, swap in a new one of the type your manual lists before drawing any conclusion — handheld plugs are a couple of dollars and a frequent culprit.
What a healthy machine shows
A visible blue spark at the gap from a single pull.
What did you see? What it means
No spark, even with a known-good plug
Check the simple things on the way to the coil: the stop-switch wiring on handhelds is thin and pinches easily under housing screws. If the switch and wiring are sound, the ignition module has failed — they are sealed units and are replaced, not repaired.
Fuel and spark are both present, so the engine is flooded — usually from over-priming or over-choking. Clear it and it will very often start; if it floods again immediately, the carburetor's needle is not sealing and the carburetor needs work.
Spark is fine and fuel is reaching the carburetor but not getting through it. With the lines already checked, that is the diaphragm carburetor itself: gummed passages or stiffened diaphragms.
Two-strokes burn oil, and the fine mesh screen in the muffler outlet collects that carbon until the exhaust cannot get out. A blocked screen strangles the engine exactly at full throttle while leaving the idle almost normal — and it is a two-minute check.
Safety
Let the muffler cool completely before touching it.
You will need
Screwdriver or small socket for the muffler outlet cover
Small wire brush
Propane torch (optional, for burning the screen clean)
Do this
1Find the small cover or deflector on the muffler outlet and remove it — the arrestor is the fine mesh screen behind it.
2Hold the screen up to the light. You should see clearly through the mesh.
3If it is carboned over, either brush it clean, heat it with a torch until the carbon burns to ash and brush again, or replace it — they cost very little.
4While it is off, glance into the exhaust port: heavy carbon inside the muffler suggests a lot of rich running or oil-heavy mix.
5Refit and test at full throttle.
What a healthy machine shows
Light clearly visible through the whole mesh, and full-throttle power restored.
What did you see? What it means
The screen was carboned over; cleaning it restored power
That was the whole fault. Worth asking why it coked up: oil-heavy mix, low-quality oil, or long periods of idling all accelerate it.
The screen was already clear — still bogs at throttle
Then the bog is fuel: the high-speed circuit of the diaphragm carburetor is not delivering. Same destination as a lean idle — the carburetor needs service.
Before condemning the engine, make sure the easy pull is not something trivial — and if compression really is gone, stop spending money on carburetors and plugs, because they will not fix it.
You will need
Small compression tester with a 10 mm/14 mm adaptor if you have one — otherwise your hands and judgment
Do this
1Remove the plug and look for heavy scoring residue: aluminum smeared on the plug threads or glittering in the plug well is a bad sign.
2Refit the plug. Pull the starter slowly and feel for the firm bounce of compression on each revolution.
3With a tester: most healthy handheld two-strokes read roughly 90–120 psi; below about 90 psi they start poorly, and below about 70 psi they generally will not run. Treat your model's manual as the authority if it states a figure.
4If you can see the piston through the exhaust port with the muffler off: vertical scoring on the piston skirt confirms it.
What a healthy machine shows
Firm, springy resistance on every pull, or a tester reading in the healthy range.
What did you see? What it means
Compression is clearly low, or the piston is visibly scored
The piston and cylinder are worn or scored — straight gas, lean mix, or simple high hours. On handhelds this is usually the end of the economic road, and it is better to hear that now than after buying a carburetor.
A cause is named here only after the tests above point at it. Open “Repair steps” for the fix.
Stale or wrong fuel mix
Two-stroke mix ages faster than straight gasoline: the gasoline fraction oxidizes and gums, and the oil separates. The machine then starts hard, runs lean, and gums its carburetor — while also being lubricated worse than it should be. Draining and refilling with fresh mix at the ratio on the machine's label fixes a remarkable share of handheld complaints, and costs almost nothing.
Difficulty
Est. time
20 min
DIY?
Yes
Parts you may need
Spark plug — optional. If the old mix fouled the plug, a fresh one removes the last variable.
Repair steps (3)
1
Drain the old mix (5 min)
Empty the tank into a container and dispose of it properly. Press the primer a few times to pull the old fuel out of the lines too.
2
Mix fresh fuel properly (10 min)
Use fresh gasoline and a quality two-stroke oil at the ratio your machine specifies — commonly 50:1 (2.6 oz per US gallon). Mix in a can, never in the tank. If the machine sits between uses, mix small batches or use a stabiliser.
3
Restart with the correct procedure (5 min)
Full choke, prime until the bulb fills, pull to the first cough, then half choke. If it now starts and idles, run a tank of fresh mix through before judging anything else.
Cracked fuel lines / clogged in-tank filter
Handheld fuel line is thin-walled so it can flex inside the tank, and ethanol-blend fuel hardens it within a few seasons. Once it cracks, the primer draws air instead of fuel, the engine leans out or refuses to start, and the fault worsens with temperature. The in-tank filter clogs on the same schedule. Lines, filter and primer bulb are cheap consumables and are replaced as a set — sized by diameter for your model.
Difficulty
Est. time
45 min
DIY?
Yes
Parts you may need
Fuel line set — required. The cracked or hardened lines. Sized by inner diameter — match your model's specification.
In-tank fuel filter — required. Clogs with the same debris and age; always renewed with the lines.
Primer bulb — optional. Same rubber, same age, same failure — replace it while everything is apart if it shows any stiffness.
Repair steps (3)
1
Photograph the routing first (5 min)
Note which line goes to which carburetor stub — supply and return matter. A photo now saves real confusion later.
2
Replace the lines and filter (25 min)
Pull the old lines out of the tank, thread the new ones through the grommet (a little fuel on the line helps), fit the new filter to the pickup line inside the tank, and connect supply and return exactly as photographed.
3
Prime and test (15 min)
Fill with fresh mix, prime until the bulb fills and holds, and start. The bulb filling firmly is itself the proof the new path seals.
Flooded engine
Fuel and spark are both fine — there is simply too much fuel in the cylinder for it to fire, usually from over-priming or pulling too long on full choke. Clearing it takes five minutes and no parts. If the machine floods again immediately after a proper start procedure, the carburetor's inlet needle is not sealing and the carburetor needs service — but prove that by flooding twice, not by guessing once.
Difficulty
Est. time
15 min
DIY?
Yes
Repair steps (3)
1
Clear the cylinder (5 min)
Remove the plug, switch the ignition OFF, choke OFF, hold the throttle wide open, and pull the starter five or six times to pump the excess fuel out. Expect a fine mist from the plug hole.
2
Dry or replace the plug (3 min)
Wipe the plug dry or fit a new one, and refit the boot firmly.
3
Restart without choke (5 min)
Ignition ON, no choke, no priming, throttle slightly open, and pull. A cleared engine usually fires within a few pulls, runs rough for a moment while it clears, then settles.
Ignition module (coil) failure
With a known-good plug, a confirmed-ON switch, and sound stop-switch wiring, no spark means the ignition module has failed. On handhelds the module is a sealed unit beside the flywheel and is replaced rather than repaired. Before buying one, disconnect the stop-switch wire at the module and retest — handheld switch wiring is thin, and a pinched wire under a housing screw produces an identical no-spark and costs nothing to fix.
Difficulty
Est. time
1 h
DIY?
Yes
Parts you may need
Ignition module — required. The failed sealed unit, if the stop-switch test clears the wiring.
Spark plug — optional. Cheap enough to renew while the machine is apart.
Repair steps (3)
1
Rule out the stop circuit (15 min)
Disconnect the stop-switch lead at the module and retest for spark. If spark returns, repair the switch or its pinched wire instead of replacing the module.
2
Replace the module and set the air gap (30 min)
Remove the starter housing, unbolt the old module, fit the new one, and set the gap between module legs and flywheel — a business card is the classic feeler for handhelds, but use the manual's figure if stated.
3
Test and reassemble (15 min)
Confirm strong spark, refit the covers, and start the machine.
Diaphragm carburetor needs service
Handheld carburetors (Zama and Walbro patterns) meter fuel through flexible diaphragms and passages finer than anything on a mower carburetor. Old fuel gums the passages, and the diaphragms themselves stiffen with age until they can no longer pump and meter — an engine that runs only on choke, needs constant re-priming, or bogs at full throttle with a clear arrestor screen is describing exactly that. The honest cost comparison: a rebuild kit is cheap but fiddly; a complete replacement carburetor for most handhelds costs only slightly more and removes all the fine-work risk.
Difficulty
Est. time
1 h 15 min
DIY?
Yes
Parts you may need
Carburetor rebuild kit (diaphragms and gaskets) — optional. The stiffened diaphragms and gaskets — the usual actual fault. Matched to the carburetor model stamped on its body (Zama/Walbro), not just the machine.
Complete carburetor — optional. Often the better value on handhelds: modest extra cost, no fine assembly, and a known-good baseline.
In-tank fuel filter — optional. Renew it with any carburetor work so the fresh carburetor stays clean.
Repair steps (3)
1
Identify the carburetor before ordering (10 min)
The carburetor model (e.g. a Zama or Walbro code) is stamped on the carburetor body itself. Kits and replacements are selected by that code plus your machine model — note both.
2
Remove, photograph, and service or swap (40 min)
Photograph the throttle linkage and both fuel lines before removal. For a rebuild: strip, spray-clean every passage, and fit the new diaphragms noting their stacking order. For a replacement: transfer the linkage and lines exactly as photographed.
3
Set and verify (25 min)
Refit, prime, start, and let it warm. It should idle without the cutting head moving and take full throttle cleanly. If your carburetor has adjustable H/L needles, set them per the manual — small fractions of a turn matter.
Blocked spark-arrestor screen
The fine mesh screen in the muffler outlet had carboned over until the exhaust could not leave the engine, which chokes power at exactly full throttle while barely affecting idle. Cleaning or replacing the screen restores the engine instantly. It cokes faster with oil-rich mix, cheap oil, or lots of idling — worth knowing so it does not come straight back.
Difficulty
Est. time
20 min
DIY?
Yes
Parts you may need
Spark-arrestor screen — optional. If the mesh is torn or will not come clean, the screen itself costs very little.
Repair steps (2)
1
Clean or replace the screen (10 min)
Brush the carbon off, or burn it to ash with a torch and brush again. Replace the screen if the mesh is damaged — never run without it; it exists to stop hot carbon leaving the exhaust.
2
Check your mix and habits (10 min)
Confirm the oil ratio matches the label and avoid long idling. If the screen coked in a single season, the mix is probably oil-heavy.
Low compression — piston and cylinder worn or scored
The engine has lost the compression it needs to run, and on a two-stroke that means the piston, rings or cylinder are worn or scored. The classic causes are straight gasoline (even one tank), a badly lean mix, or plain high hours. This is the diagnosis that stops you spending: no carburetor, plug or fuel line will fix it. Piston-and-cylinder work is possible on quality saws and trimmers, but on consumer-grade machines a repair quote usually lands close to the price of a better new machine.
Difficulty
Est. time
30 min
DIY?
Shop job
Repair steps (2)
1
Confirm with a proper compression test (15 min)
Before any final decision, have the compression measured with a gauge. Handheld two-strokes generally need roughly 90 psi or more to run properly; your model's manual is the authority if it states a figure.
2
Decide repair against replacement (15 min)
On a professional-grade saw or trimmer, ask a shop to quote a piston/cylinder replacement. On a consumer machine, compare that quote honestly against a new machine — and if the cause was straight gas, drain and properly mix the fuel for whatever comes next.
Want it done for your exact machine?
The free guided diagnosis walks you through these same tests one at a time, then names the part for your model with an honest confidence level and where to buy it — or tells you plainly when it can't confirm a number.
This is the same test sequence HandsOnFix's guided diagnosis uses. Spark, fuel and compression behave the same on every engine of this type, which is why the tests hold without knowing your exact model. Part numbers are different: they change by model, engine code and serial range, so this guide names part types only. For exact OEM numbers, find your model among our source-verified models or run the free diagnosis. A diagnostic aid, not professional or safety advice — see our Terms. Last updated October 9, 2026.
