Motorcycle Cranks But Won't Start: Fuel, Spark, Compression, Air
Updated: Sep 6
An engine that cranks at normal speed but never fires has everything it needs to start except one of four things: fuel, spark, compression or air. Test in that order, because fuel and spark are where the answer usually lies and both can be checked in minutes without removing anything expensive.
Difficulty | Time | DIY cost | Tools | Safety |
Level 2, easy DIY | 30 to 90 minutes | $0 to diagnose, spark tester about $10 to $25 | Spark plug spanner, spark tester or spare plug, multimeter | Fuel and fire hazard, high voltage ignition |
If you have not yet confirmed which branch you are on, start at the motorcycle will not start diagnostic guide. This page assumes the starter spins the engine over at a healthy speed.
Before you touch anything
You are about to work around fuel and around an ignition system that generates tens of thousands of volts. Both deserve respect.
Work outdoors or in ventilated space. No smoking, no naked flames, no grinders running nearby.
Keep a fire extinguisher rated for fuel fires within reach.
Fuel injected machines hold pressure in the fuel line even after the engine is off. Do not crack a fuel union open casually.
Never hold a plug lead or a plug body with bare hands while the engine is cranking. Use insulated pliers or a proper spark tester.
A removed plug sprays fuel vapour out of the plug hole while cranking. Keep that vapour away from the spark you are creating.
Let a hot engine cool before removing plugs, both to avoid burns and to avoid damaging threads in a hot aluminium head.
Check one, is it getting fuel
Start here because it is the cheapest and most common answer, especially on a machine that has been standing.
Look in the tank. Not the gauge. Gauges and fuel senders fail.
Check the petcock if the bike has one. Turn it to reserve as well as on. A blocked main pickup with a clear reserve pickup is a classic standing-bike fault.
Listen for the fuel pump on a fuel injected machine. Turn the ignition on without pressing the starter. You should hear a short whirr or buzz lasting a second or two as the pump primes. Silence there points at the pump, its relay, its fuse or its wiring rather than at the engine.
Smell the exhaust and check the plug. Remove a spark plug after several seconds of cranking. If it is wet with fuel, fuel is arriving and your problem is spark or compression. If it is bone dry, fuel is not arriving.
Consider the fuel itself. Petrol that has sat for many months, especially ethanol blended petrol, can lose the light fractions that make cold starting possible, and can leave varnish in carburettor jets. Old fuel is a genuine cause, not an excuse.
On a carburetted machine that has been standing, a blocked pilot jet is one of the most frequent single causes of a bike that cranks, will not idle and will not start. The passage is very small and varnish closes it easily.
Check two, is it getting spark
An inline spark tester is the safest way to do this, and it is cheap. The alternative is to remove a plug, reconnect it to its cap, rest the plug body firmly against bare engine metal so it is grounded, and crank while looking for a spark at the electrode. If you do it that way, disable the fuel first by pulling the fuel pump fuse or relay on an injected machine or turning the petcock off on a carburetted one, and position the plug well away from the open plug hole, because that hole is venting fuel vapour while you crank. The inline tester avoids all of this, because the plug never leaves the head.
Ground it properly. A plug held loosely against a painted or oily surface will not spark even when the ignition is healthy, and you will chase a fault that does not exist.
Look for a strong blue-white spark that snaps across the gap. A thin, orange, intermittent spark suggests a weak ignition rather than a healthy one.
Check every cylinder on a multi-cylinder engine. One dead cylinder rarely stops a start on its own, but it tells you where to look.
No spark on any cylinder points upstream: kill switch, ignition switch, a safety interlock, a fuse, the ignition coil supply, the crank position sensor, or the engine management.
Coil testing by resistance is a genuine test, but the pass and fail resistance values are specific to your model and are published only in the factory service manual. Measure the resistance if you like, then compare it against that document rather than against a figure from a forum, because coil specifications vary widely between machines.
Check three, has it got compression
Compression is third because it is the least likely of the three and the most work to test. Two clues come free.
First, the sound. An engine with no compression cranks unusually fast and evenly, with none of the normal rise and fall as each cylinder comes up on its stroke. Riders often describe it as spinning too easily. Second, the resistance you feel if you turn the engine over by hand on a machine that allows it.
Before cranking an engine with the plugs out, disable both the ignition and the fuel. Pull the fuel pump fuse or relay on an injected machine, or turn the petcock off on a carburetted one, and ground the plug leads or unplug the coils so they are not firing open circuit. Raw fuel and vapour are pushed out of the open plug holes while you crank, and an unloaded coil firing nearby is the ignition source you do not want.
A proper test uses a compression gauge threaded into the plug hole, throttle held wide open, engine cranked for several revolutions. The number it gives you is meaningless without your engine's specification, which again lives in the factory service manual. What is universal is the comparison between cylinders on a multi-cylinder engine: readings should be reasonably close to one another, and one cylinder far below the others is a real finding regardless of the absolute figure.
Check four, can it breathe
Rare, but genuinely happens, and it costs nothing to look. A completely blocked air filter, a rag or a glove left in an airbox after work, a mouse nest in an intake over winter, or a blocked exhaust will all prevent a start. Remove the airbox lid and look. Machines stored outdoors are the usual victims.
Check the fuel tank breather too. A tank that cannot admit air as fuel leaves it will draw a vacuum and starve the engine. If the bike runs briefly then dies and starts again after you crack the filler cap open, you have found it.
Specifications you need to look up for your bike
Value | Why you need it | Where to find it |
Spark plug type and gap | Correct plug, and judging a used one | Owner's manual |
Spark plug torque | Refitting without damaging the head | Factory service manual |
Ignition coil primary and secondary resistance | Judging a coil test | Factory service manual |
Compression specification and service limit | Judging a compression reading | Factory service manual |
Fuel pump pressure | Judging a fuel pressure test | Factory service manual |
We do not print these numbers. They vary by model and year, they exist only in licensed documents, and a wrong plug torque strips a cylinder head.
When to stop and take it to a shop
Stop if the engine cranks with a knock, a scrape or a rhythmic metallic noise, because continuing to crank can make the damage worse. Stop if you find fuel leaking. Stop if the machine has an immobiliser and the security light is behaving oddly, since those systems usually need a dealer tool. And stop before removing a cylinder head on the strength of a single compression reading, because a leak-down test will tell you where the pressure is going first.
Related checks
Frequently asked questions
My bike cranks but will not start after sitting all winter. Where do I look first?
Fuel, then spark. Standing machines lose starts to stale petrol, varnished carburettor jets and a battery that is strong enough to crank but weak enough to give a poor spark. Fresh fuel and a fully charged battery resolve a great many spring no-starts before any parts are bought.
The plug is wet with fuel. Is that good or bad?
It means fuel is reaching the cylinder, which narrows the problem to spark or compression. It can also mean the engine has been flooded by repeated cranking. Dry the plug, and on many machines you can clear a flooded cylinder by cranking briefly with the throttle held wide open, which reduces the fuel being drawn in.
Why does it start with a squirt of easy start but not on its own?
Because that proves the engine has spark and compression and is simply not receiving fuel. It narrows your problem usefully. It is also hard on an engine, so use it as a diagnostic clue rather than a habit.
Can a dead battery cause a no-fire even though the starter spins fine?
It can. Voltage sags while the starter draws current, and on a fuel injected machine the engine management, pump and coils all need adequate voltage during that same moment. A battery that cranks the engine but cannot hold voltage under load produces exactly this symptom, which is why the full guide asks you to measure the battery before going further.
About this guide
Published by MotoGearRater. Universal procedure only, with no model-specific specifications. Not reviewed by a qualified motorcycle technician, and we say so rather than implying a review that has not happened. Read our editorial standards.
