Engine cranks but won't start

You will need

• Nothing — this step is by eye

Do this

1. 1Check the fuel tank actually has gasoline in it, and look closely at the fuel: if it is darker than fresh fuel, smells sour or like varnish, or has been sitting more than about 30 days, treat it as suspect.
2. 2Confirm the fuel shut-off valve (if fitted) is in the open position.
3. 3Confirm the spark plug boot is pushed fully onto the plug — it should click and resist a light pull.
4. 4Satisfy whichever safety controls your machine actually has. On a walk-behind, hold the operator-presence bail against the handlebar. On a rider, sit on the seat with the blades/PTO switched off, the parking brake on, and the transmission in neutral. A generator, pressure washer or pump has neither bail nor seat switch — instead set the engine switch to ON and take the load off it: unplug anything drawing power, or leave the pressure-washer trigger released.
5. 5Set the choke correctly: full choke for a cold engine, no choke for an engine that has been running. If the machine has a primer bulb instead, press it three to five times.
6. 6Check the engine oil level. Several engines carry a low-oil sensor that will prevent starting, and running one low will destroy it regardless.

What did you see? What it means

• I found something wrong, corrected it, and the engine started

  The engine was never faulty — one of its operating conditions was not being met. Nothing needs replacing.

  Likely cause: The engine was not faulty — an operating condition was not met

• The fuel is old, dark, or smells like varnish

  Gasoline degrades within weeks. As it evaporates it leaves a gum that blocks the very small passages inside a carburetor. This is the single most common cause of a small engine that ran fine last season and will not start this one.

  Likely cause: Stale fuel

• Everything above checks out and it still will not start

  The operating conditions are satisfied, so the fault is a genuine failure in spark, fuel, or compression. Now we test.

  Next test: Test for spark

You will need

• Spark plug socket (13/16 in for most walk-behind engines, 5/8 in for many overhead-valve and handheld engines)
• Socket wrench or plug wrench
• An inline spark tester if you have one — it gives a far clearer answer than eyeballing the plug

Do this

1. 1Pull the plug boot off and remove the spark plug.
2. 2Look at the plug before you do anything else, and note what you see — a light tan or gray tip is healthy; black and sooty means it has been running rich; wet with gasoline means fuel is reaching the cylinder; oily means oil is getting past the rings or valve guides; white and blistered means it has been running too hot.
3. 3If you have an inline tester, fit it between the boot and the plug, refit the plug, and give the starter one pull.
4. 4Without a tester: push the plug back into the boot, then rest the plug's metal body on clean, bare metal on the cylinder head, well away from the plug hole. Do not hold it in your hand. Paint and oil will insulate it and give you a false 'no spark' reading.
5. 5Give the starter one short pull — or one quick bump of the electric starter — and watch the plug's electrode gap. That is all it takes: a working ignition shows its spark straight away, so there is no need to keep cranking with the plug out. Do this in shade — bright daylight easily hides a weak spark.

What did you see? What it means

• Bright blue spark, snapping consistently

  The entire ignition system is working — coil, magnets, kill circuit, and plug are all fine. That rules out about half of the possible causes. The fault is in fuel delivery or compression.

  Next test: Confirm whether fuel is reaching the cylinder

• Weak, thin, orange or yellow spark, or it comes and goes

  There is ignition output, but not enough of it. A weak spark will often fire a warm engine on the bench and then fail to ignite a proper fuel-air charge under compression, which is why an engine like this can seem to start intermittently.

  Next test: Track down a weak spark

• No spark at all

  Something is stopping ignition entirely. Before condemning the coil, we have to rule out the kill circuit — the safety wiring is a far more common cause than a failed coil, and it costs nothing to check.

  Next test: Separate the kill circuit from the coil

You will need

• Small wrench or nut driver
• Possibly a screwdriver to lift the blower housing

Do this

1. 1Trace the single thin wire that runs from the ignition coil back toward the handlebar controls or the key switch. This is the kill wire, and it usually ends in a spade terminal at the coil.
2. 2Disconnect that wire at the coil and make sure the bare end cannot touch any metal.
3. 3Repeat the spark test from the previous step.
4. 4Be ready: with the kill wire disconnected, the stop control no longer works. The only way to stop the engine is to close the fuel valve or pull the plug boot.

What did you see? What it means

• Spark came back once the kill wire was disconnected

  The coil is fine. Something in the stop circuit is grounding it out — a safety switch, the bail or brake cable adjustment, the key switch, or a damaged wire. That is a much cheaper repair than a coil.

  Likely cause: The stop circuit is grounding out the ignition

• Still no spark with the kill wire disconnected

  The stop circuit is cleared of blame. The fault is in the ignition system itself, so we look at the coil's air gap and the flywheel next.

  Next test: Check the coil air gap and the flywheel key

You will need

• Feeler gauge, or a business card as a rough substitute
• Socket set for the blower housing bolts
• Flywheel holding tool or strap wrench if the flywheel nut has to come off

Do this

1. 1Remove the blower housing — the shroud on top of the engine that covers the recoil starter — to expose the flywheel and the coil sitting beside it.
2. 2Rotate the flywheel by hand until its magnets sit directly under the coil's legs.
3. 3Slacken the two coil mounting bolts just enough that the coil is drawn against the magnets, slide the feeler gauge between the coil legs and the flywheel, then tighten the bolts and withdraw the gauge.
4. 4Most manufacturers specify somewhere between 0.006 in and 0.014 in (0.15 mm to 0.35 mm). Confirm the exact figure in your engine manual rather than assuming — it varies by engine family.
5. 5Now inspect the flywheel key: look for the engine having been stopped abruptly by striking something. If you can rotate the flywheel slightly independently of the crankshaft, or the keyway looks distorted, the key has sheared.
6. 6Refit the plug lead and retest for spark.

What did you see? What it means

• The gap was wrong; after setting it the spark came back

  The coil was mechanically out of position rather than failed. No parts needed.

  Likely cause: Coil air gap was out of specification

• The flywheel key is sheared or the keyway is damaged

  The flywheel has slipped on the crankshaft, so the spark is no longer arriving at the right moment in the cycle. The key is a deliberately soft sacrificial part designed to fail this way and protect the crankshaft.

  Likely cause: Sheared flywheel key — ignition timing is out

• Gap is correct, key is intact, still no spark

  With the kill circuit ruled out, the air gap correct and the flywheel timed properly, the coil itself has failed internally. Coils commonly fail this way once warm, so an intermittent version of this fault is still the same part.

  Likely cause: Ignition coil has failed

You will need

• New spark plug of the correct type for your engine
• Feeler gauge
• Plug socket

Do this

1. 1Fit a new plug of the type specified for your engine. Do not substitute by physical appearance — reach and heat range matter, and the wrong plug can contact the piston.
2. 2Set the gap with a feeler gauge before fitting. Common small-engine gaps run from about 0.020 in to 0.030 in, but confirm yours in the manual.
3. 3If the old plug was black and sooty, oily, or its electrode was visibly rounded off rather than square-edged, it was overdue regardless.
4. 4Refit and retest for spark, then try to start.

What did you see? What it means

• A new plug restored a strong spark

  The plug was worn or fouled. Note that if the old plug was oily or heavily sooted, something caused that, so keep an eye on how the new one colors up over the next few hours of running.

  Likely cause: Spark plug was worn or fouled

• The spark is still weak with a new plug

  With a known-good plug fitted, weak output points at the coil. Check the air gap first, since a coil sitting too far from the flywheel produces exactly this symptom.

  Next test: Check the coil air gap and the flywheel key

You will need

• A can of starting fluid, or a small amount of fresh gasoline in a spray bottle

Do this

1. 1Recall the plug's condition when you removed it earlier. Wet with gasoline means fuel is reaching the cylinder. Bone dry means it is not.
2. 2Remove the air filter cover and the filter element so you can see into the carburetor throat.
3. 3Give one short burst of starting fluid directly into the carburetor throat.
4. 4Refit the plug and its boot, then crank the engine.

What did you see? What it means

• It fired for two or three seconds, then died

  This is close to conclusive. The engine ran on fuel you supplied directly, so spark and compression are both good and the fault is in the machine's own fuel delivery. Everything upstream of the cylinder is now the suspect.

  Next test: Find where fuel delivery is failing

• It fired and then kept running, though it was hard to start

  Fuel is getting through, but not freely enough to start the engine unaided. That points to a partial restriction rather than a complete blockage — typically gum in the carburetor's small starting passages.

  Likely cause: Partially restricted carburetor

• It did not fire at all

  Fuel was placed directly in the cylinder and there is confirmed spark, yet there was no combustion. That leaves compression, or ignition timing, as the remaining explanation.

  Next test: Measure compression

You will need

• Small container to catch fuel
• Pliers for hose clamps
• Screwdriver for the carburetor bowl nut

Do this

1. 1Vent test first, because it is free: slacken the fuel cap about half a turn and try starting again. A blocked cap vent creates a vacuum in the tank that stops fuel flowing after a few minutes of running.
2. 2Confirm the fuel shut-off valve is open, then disconnect the fuel line where it meets the carburetor, holding it over your container.
3. 3Watch the flow. A gravity-fed engine — most walk-behinds with the tank above the engine — should give a steady stream, not a dribble.
4. 4If the engine is fuel-pumped rather than gravity-fed, which is usual on riders and on engines with the tank below the carburetor, crank it briefly and check whether the pump delivers fuel.
5. 5If there is an inline fuel filter, note which direction the arrow points and whether the element is discolored.
6. 6Finally, if fuel is reaching the carburetor, remove the float bowl at the bottom of the carburetor and look inside for water, rust, or a green-brown varnish deposit.

What did you see? What it means

• It started or ran normally with the fuel cap loosened

  The cap's vent is blocked. The tank cannot draw air in as fuel is used, so a vacuum forms and starves the engine. This classically presents as an engine that runs for ten or fifteen minutes and then dies.

  Likely cause: Blocked fuel cap vent

• Little or no fuel comes out of the line

  The restriction is upstream of the carburetor — the tank outlet screen, the shut-off valve, the inline filter, or the line itself, which can collapse internally as it ages while still looking fine from outside.

  Likely cause: Blocked fuel supply upstream of the carburetor

• This engine has a fuel pump and it is not delivering

  A vacuum-operated pulse pump is driven by crankcase pressure through a small hose. Either that pulse line has split or the pump's internal diaphragm has hardened and stopped sealing.

  Likely cause: Fuel pump has failed

• Fuel reaches the carburetor fine, but the bowl is dirty or varnished

  Fuel is arriving and stopping there. The carburetor's internal passages — some no wider than a pencil lead — are blocked with the residue left behind by evaporated gasoline.

  Likely cause: Carburetor is blocked with fuel varnish

You will need

• Compression gauge with the correct thread adaptor for your plug
• Plug socket

Do this

1. 1Before reaching for the gauge, note how the starter felt. An engine with lost compression spins noticeably fast and free, and a recoil rope will feel light with no firm bounce at the top of the stroke.
2. 2Remove the plug, thread the compression gauge into the plug hole finger-tight, and make sure it seats.
3. 3Hold the throttle wide open so the engine can draw a full charge of air.
4. 4Crank for five or six full compression strokes and read the highest figure the gauge holds.
5. 5Interpret it with care: many small engines have a compression release or a decompression feature built into the camshaft to make starting easier, which deliberately lowers cranking compression. A healthy engine of that type can read far lower than an automotive engine would. Check your manual for the figure that applies to yours.
6. 6As a general guide, most small four-stroke engines need roughly 60 psi to run at all, and a healthy one usually reads well above that.

What did you see? What it means

• Very low compression, or the engine spins freely with no resistance

  The cylinder is not sealing. On a small four-stroke the usual cause is a valve — either the clearance has closed up so a valve is held slightly open, or a valve is sticking in its guide. Both are far more common than worn piston rings.

  Next test: Inspect valve clearance and valve movement

• Compression is within specification

  Spark, fuel and compression are all confirmed present, and the engine still will not fire. That combination points at ignition timing rather than any of the three individually — the spark is happening, but at the wrong moment.

  Likely cause: Sheared flywheel key — ignition timing is out

You will need

• Feeler gauge set
• Wrench and screwdriver, or a socket set, for the rocker adjustment
• New valve cover gasket, since the old one rarely reseals

Do this

1. 1Remove the valve cover from the top of the cylinder head.
2. 2Turn the engine over by hand until the piston is at top dead center on its compression stroke — both valves fully closed — then continue about a quarter turn past it, which takes any compression-release mechanism out of play.
3. 3Slide a feeler gauge between each rocker arm and its valve stem in turn. You are looking for a light drag, not a loose fit and not a jam.
4. 4Compare against your engine's specification. Typical small-engine figures fall around 0.002 in to 0.006 in on the intake and 0.004 in to 0.008 in on the exhaust, with the exhaust usually the larger of the two — but these vary by engine and must be confirmed in the manual.
5. 5While the cover is off, push each valve open by hand and let it return. It should move smoothly and snap shut. A valve that moves sluggishly or hangs is sticking in its guide, usually from carbon or varnish.

What did you see? What it means

• Clearance is too tight or has closed up to nothing

  The valve is being held slightly open, so the cylinder cannot seal. This is an adjustment, not a failure — no expensive parts are involved.

  Likely cause: Valve clearance has closed up

• A valve is sticking, hanging open, or moving roughly

  Carbon or fuel varnish has built up on the valve stem and it is binding in its guide. Often recoverable by cleaning, though a badly scored stem or guide needs more work.

  Likely cause: A valve is sticking in its guide

• Clearance is correct and both valves move freely

  The valve train is not the cause, which leaves the sealing surfaces themselves — the head gasket, the valve faces, or the piston rings. Distinguishing between them requires removing the cylinder head.

  Likely cause: Compression loss inside the engine