Drain old or contaminated fuel

Home mechanic (basic tools) · Time: 30 to 60 min (estimate) (estimate)

Petrol goes stale when it stands for weeks or months: it loses the light parts that help starting and leaves sticky varnish in jets and injectors. Water gets in through condensation, a bad cap or dirty fuel, and sinks to the bottom of the tank. This job removes bad fuel from the tank, the carburettor float bowl and the lines so you can start again with fresh fuel.

Safety

• Petrol is the biggest fire risk in any workshop. Work outdoors, engine cold, far from flames, sparks, cigarettes and running engines. Keep an ABC extinguisher near you. Never use water on a petrol fire.
• Fuel running down from a tank lands on the engine and exhaust. Let them cool fully first.
• Use a hand siphon pump. Never start a siphon by mouth; swallowing petrol can kill.
• Wear gloves and glasses. Wash petrol off skin with soap and water.
• Store drained fuel in a closed, labelled fuel can. Take it to a collection point; do not pour it on the ground or into drains.

You need

• Enough approved cans for a full tank (K-Light 125 tank: 11.8 ± 0.5 L).
• Fresh fuel of the grade your manual gives (K-Light 125: unleaded 92 or 95).

Steps

1. Turn the ignition off. On fuel-injected bikes also disconnect the battery negative lead so nothing can switch the pump on.
2. Carburettor bike with a tap: put the tap hose into a can and drain the tank through the tap on ON, then RES. A vacuum tap needs PRI.
3. Bike with no tap (most fuel-injected bikes): remove fuel through the filler with a hand siphon pump. Push the pump hose to the lowest point of the tank. Do not open the pressure line.
4. Pour a sample into the glass jar and let it stand for a minute. Water sits as a clear layer or bubbles at the bottom. Rust, dirt or a dark varnish colour confirm the problem.
5. Carburettor: put a hose on the float bowl drain screw (or hold the jar under it), loosen the screw a few turns and drain the bowl. Tighten it gently.
6. If you found water, rust or a lot of dirt: remove the tank, rinse it with a little fresh fuel, swirl, and drain again until it comes out clean. Look inside with a torch for rust (see Remove rust from a fuel tank).
7. Replace or clean the fuel filter and tap strainer: see Replace the fuel filter.
8. Refit everything, add fresh fuel and check for leaks.
9. Carburettor bikes: turn the tap ON and wait for the bowl to fill. Fuel-injected bikes: reconnect the battery, turn the key on and let the pump prime two or three times before cranking.

Check your work

• No drips at the tap, drain screw or hoses.
• The engine starts and idles. It may need a few more seconds of cranking while fresh fuel reaches the jets or injector.

If it did not fix the problem

• Still rough or will not idle: varnish may be in the jets. See Clean the carburettor, or for injection Check the fuel injector.
• After long storage, also follow Bring a bike back into use after storage.

Reference values (K-Light 125)