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Dirt bike
won't start

Most no-starts are found trailside in under an hour with the tools already in your pack. What decides whether it takes an hour or a weekend is the order you check things in.

The rule: Narrow one variable at a time, cheapest and most likely first. Never open the engine over something you have not ruled out on the outside of it.

Walk me through it →

The guided version asks one question at a time and skips what your bike's own chart already answers.

Always confirm intervals, torque specs and part numbers against your own service manual. This site is a shopping aid, not a repair manual.

Four categories, two that matter

Air, fuel, spark, compression. Air is ruled out in seconds — step one below. Compression is worth skipping as a diagnostic on a single-cylinder engine: almost every modern four-stroke has auto-decompression, which makes the numbers meaningless. Compression testing belongs to two-stroke tuning, not to a no-start.

That leaves fuel and spark as the real path, and that is what the steps below narrow down.

  1. Step 1

    Air filter

    Seconds to check and it rules out a whole category. An engine needs air before anything else on this page matters.

    1. Pull the seat or side panel and look at the filter.
    2. Confirm it is actually fitted, not soaked, and not packed solid.
  2. Step 2

    Two-stroke or four-stroke?

    The two split here for one step, then rejoin at fuel.

  3. Step 3

    Start with the filter

    Fault found

    Fit a clean filter and try again before going further. If it still will not start, come back and carry on from the next step — you have at least ruled air out.

    Air filter →
  4. Step 4

    The fifteen-minute two-stroke inspection

    On a two-stroke this is quick and tells you a lot, so it comes before fuel.

    1. Take the exhaust pipe off.
    2. Take the left side cover and ignition cover off.
    3. Look over everything now exposed, unhurried.
  5. Step 5

    Carburetted or fuel injected?

    Fuel delivery is the first real check on any bike. How you test it depends on which system you have.

  6. Step 6

    Fix what you found first

    Fault found

    Deal with the damage before carrying on. Diagnosing past a fault you have already found only adds variables.

  7. Step 7

    Carburettor and fuel supply

    1. Confirm the carburettor is clean inside.
    2. Confirm fuel actually flows from the petcock to the carburettor.
    3. Confirm the fuel is fresh — old fuel is a common and quiet cause.
  8. Step 8

    Listen for the fuel pump

    No tools needed. Hit the start button and listen — the pump should prime for a second or two.

  9. Step 9

    Check for spark

    1. Pull the spark plug.
    2. Check whether it sparks.
  10. Step 10

    Fuel is not reaching the engine properly

    Fault found

    Clean the carburettor, restore flow, or drain the stale fuel — whichever you found. Then try again.

    Fuel line →
  11. Step 11

    Fuel pressure test

    The single most informative test on a fuel-injected bike. It answers the fuel question outright rather than by inference.

    Grease the connector O-ring before you pull the fuel line apart. They run dry, go sticky, and tear — and they are an awkward size to source.

    1. Find the fuel line connector — on most KTMs it is on the left side.
    2. Grease the O-ring, then disconnect.
    3. Fit the fuel pressure tester.
    4. Crank the engine and read the gauge.
  12. Step 12

    Power at the fuel pump plug

    Work outside-in. Check the wiring before you open the tank — a wiring fault is far cheaper to find than a tank you have already pulled apart.

    1. Take the seat off.
    2. Find the fuel pump plug. Three wires: power, ground, and the level sensor.
    3. Probe each wire in turn with a test light or meter while hitting the start button. You do not need to know which is which, and you will not damage anything.
  13. Step 13

    Read the oil

    Cheap, quick, and it can end the diagnosis outright.

    1. Drain the oil.
    2. Pull the oil filter.
    3. Look hard at the oil, the drain plug, and the filter for metal.
  14. Step 14

    Ignition side

    Fault found

    With fuel confirmed and no spark, the cause is one of a short list — or a combination: the ECU, the stator, the ignition coil, the regulator/rectifier, or wiring that has chafed or broken.

    Spark plug →
  15. Step 15

    The fault is in the wiring, outside the tank

    Fault found

    Trace and repair the wiring to the pump. Leave the tank alone — the pump is not the problem.

  16. Step 16

    Open the tank

    Power is reaching the pump and pressure still is not arriving, so the fault is inside.

    1. Drain the fuel. Note what you smell — stale fuel points at a tired pump, clogged filters, or both.
    2. Listen for anything loose. A rattle means something has come adrift in there.
    3. Draw the pump and sending unit out. Do not force it; they can be stubborn.
  17. Step 17

    Two-stroke or four-stroke?

    Valve clearances are a four-stroke step.

  18. Step 18

    Stop — bottom end failure

    Stop here

    Gold flakes anywhere means the bottom end has failed. There is nothing further to diagnose and nothing to gain by cranking it again; the engine has to come apart.

  19. Step 19

    Inspect the pump assembly

    Go through all of it. More than one of these fails at once often enough to be worth the extra minutes.

    1. Regulator — rust here blocks pressure even while the pump spins happily.
    2. Inline filter — should be seated in its housing. Loose is your rattle.
    3. Prefilter — if it is damaged or displaced, the pump has been drinking debris.
    4. Level sensor — look for broken wires.
    5. Plastic filter housings — a hairline crack bleeds off pressure with everything else working.
    6. Fuel lines — factory lines often will not take the pressure the pump makes.
  20. Step 20

    Valve clearances

    By this point you have usually either found the fault or you are looking at something obviously wrong.

    1. Take the fuel tank off.
    2. Take the valve cover off.
    3. Measure the clearances.
    4. Check the valve timing while you are in there.
    Valve shims →
  21. Step 21

    Teardown diagnosis — last resort

    End of the trail

    Everything outside the engine has come back clean. From here you are buying gaskets, so go in the order below and stop the moment you find your answer.

    1. Ignition cover off. Look for chafed wires, damp connectors, wires pulled out of connectors. Check the stator and flywheel, turn the engine over and listen for grinding.
    2. Outer and inner clutch covers off. On a four-stroke check the crank bearings — any play at all is a problem and can absolutely stop it starting.
    3. Borescope through the plug hole, if you have one that fits.
    4. Exhaust off and look at the valves. Bent or broken shows up here, as does heavy carbon — which means it is burning oil, itself a cause of a no-start.
    5. Cylinder head off. The last step, and the point of no return.
  22. Step 22

    Fuel pump or its filters

    Fault found

    A failed pump is far and away the most common cause of a no-start on a fuel-injected dirt bike. Replace the regulator and fuel lines while you are in there — they cost little against the work of getting back in. Prefer metal filters to plastic: they resist cracking and can be back-flushed.

    Fuel pump →Fuel filter →Fuel line →
  23. Step 23

    Leak-down test

    Optional, and only worth doing if you already own the tester.

  24. Step 24

    Top-end wear

    Fault found

    Tight clearances mean the top end has worn significantly. That is your answer.

    Piston kit →
  25. Step 25

    Likely a bent valve

    Fault found

    Loose clearances are unusual. More often than not it means a valve is bent and no longer closing fully.

  26. Step 26

    Reading the leak-down

    This test misleads people. Carbon on a valve face can hold it slightly open and produce a dramatic number that means very little — it may clear with cleaner, or with a good hot ride. And a clean result still does not rule out valve train or timing problems. Treat it as evidence, not a verdict.

  27. Step 27

    Something is not sealing

    End of the trail

    Before condemning the engine, try cleaner and a hot ride — carbon is a common and harmless explanation. If it persists, carry on into the teardown steps.