The engine needs three things to start: enough cranking speed, fuel in the right proportion and a spark at the right time. Most no-start complaints are a failure in one of those, and the cheapest tests come first.
Does it crank?
If the starter turns the engine normally, the battery and starter are probably healthy enough. If it clicks, turns slowly or does nothing, start with the battery, terminals, kill switch and side-stand switch.
A scooter with a brake-light interlock also needs the brake lever held. Check the obvious switches before dismantling anything.
Is there fuel?
A fuel-injected bike needs pressure; a carburetted bike needs fuel in the bowl. Listen for the pump prime, check the tank valve and confirm the fuel is not stale. A bike that has sat for months may have a clogged filter or injector.
Do not spray fuel into the intake to force a start. It tells you almost nothing and creates a fire risk.
Is there spark?
Remove the plug, fit it to the cap and ground it against the engine. A visible, strong spark is a useful yes. No spark sends you to the plug, cap, coil, wiring and ignition input. A plug that is wet with fuel suggests the ignition or compression side, not the fuel supply.
Is there compression?
A compression test separates a fuel or ignition fault from a mechanical one. Low compression can be valves, rings, a head gasket or a timing fault. If the engine spins unusually easily, stop cranking and test.
Checklist
- Battery voltage at rest and while cranking.
- Kill switch, side-stand switch and clutch or brake interlock.
- Fuel level, fuel quality and pump prime.
- Spark at the plug.
- Air intake for a blockage or a disconnected hose.
- Compression if the first four checks pass.
- Fault codes before replacing sensors.
Questions
Why does it start cold but not hot?
Heat changes resistance and clearances. A failing coil, a tight valve or a sensor that drifts with temperature can all cause it. Test when the fault is present.
Can a weak battery cause a no-start with a good starter?
Yes. Modern electronics need stable voltage. A battery that turns the engine slowly may still be too weak for the ECU and injectors.
Should I replace the spark plug first?
A plug is cheap, but it is not a diagnosis. Read the old plug, test for spark and check fuel and compression before buying parts.