Test fuel supply with a small shot of fuel

This test separates a fuel fault from every other fault. A small amount of fresh fuel put straight into the engine should make it fire, rev briefly and die. If it does, the engine has spark and compression and the fault is in the fuel supply (tank, tap, line or carburettor). If it does not fire, the fault is elsewhere. This test is for carburettor bikes. On fuel-injected bikes, use Check the fuel pump (fuel injection) instead.

Safety

• Fuel is very flammable. Work outdoors and keep a fire extinguisher in reach.
• Use only a LITTLE fuel. Raw fuel that reaches the hot exhaust can ignite or blow off a silencer.
• Keep a finger near the engine stop switch.
• Put out any smoking and flame. Wipe up spilled fuel before you start.
• Do not use this test on a bike that already smells strongly of fuel: it may be flooded.

You need

• Fresh fuel in a clean small container, and a dropper or small oil can.
• Plug spanner.
• A fire extinguisher.

Steps

1. First prove spark and compression: Check for a spark safely and Test engine compression. Do not skip this.
2. Remove the air filter cover or element if it blocks the intake. Pour a few drops of fresh fuel into the carburettor throat, or through the plug hole (refit the plug and cap before cranking).
3. Fit the plug, plug cap and any covers. Turn the key on.
4. Crank the engine or kick it over. A good engine fires, revs for a moment, and dies when the fuel is used.
5. Repeat once with a few more drops. Do not keep feeding fuel.
6. If the engine fires: the fuel path is at fault. Check in order: tank vent and fuel, tap, filter and line: Check the fuel tap, lines and flow; then the carburettor: Clean the carburettor, Check the carburettor float and needle valve.
7. If it runs only with the choke on and dies when the choke is off: the idle jet or its passages are probably blocked: Clean the carburettor.
8. If the engine does not fire at all: stop feeding fuel. Look again at spark, compression, ignition timing and air leaks: Find a fault step by step.

Check your work

• You know whether the problem is fuel or not fuel, from one clear test.
• The bike is dry of spilled fuel and the plug cap is fitted before you leave it.

If it did not fix the problem

• Fuel shot makes no difference: Engine turns over but will not start.
• Engine is wet and smells of fuel: Clear a flooded engine.