Measure and set the carburettor float height

The float and the needle valve hold the fuel level in the bowl. A level that is too high floods the engine and can leak fuel out of the overflow tube, or into the engine and oil. A slightly high level makes the mixture a little rich, and a low level makes it a little lean. Each carburettor has its own float height, found in the manual. This page adds the method to Check the carburettor float and needle valve.

Safety

• Fuel is flammable. Drain the bowl into a container and keep flames away.
• Do not blow hard into the fuel inlet. Use very light air only.

You need

• The float height from your bike's manual and whether it is measured with or without the bowl gasket.
• A ruler or depth gauge.

Steps

1. Take off the float bowl. Look at the float. A metal float tang can be bent. Most plastic floats cannot be adjusted, though some have a metal insert that can.
2. If the manual says to measure with the gasket, fit it. If not, leave it off.
3. Turn the carburettor so the float hangs and the tang just touches the needle valve, with the needle not pushed in. A light way to find the point is to put a tube on the fuel inlet and blow very gently while raising the float slowly with one finger. The moment the air stops flowing is the float height point.
4. Measure from the carburettor face to the top of the float. Compare with the manual value.
5. Bend the tang up or down to match. For double floats, many manuals ask only that the float arm is parallel with the gasket surface. Check your manual.
6. If no value is available, set the level just below the top of the float bowl, with no fuel coming from the overflow tube. Adjust from there.
7. Check the float and pin for wear, dents and rust. Water that froze in a metal float can crush it, and a crushed float makes the carburettor overflow. A float that is cracked, soaked or crushed must be replaced.
8. After the job, fit the bowl and check with fuel on.

Check your work

• The float height matches the manual and no fuel comes from the overflow tube.
• The engine runs without fuel smell.

If it did not fix the problem

• Overflow continues: replace the needle and seat. See Fuel pours out of the carburettor overflow pipe.
• Fuel found in the oil: change the oil. See Oil smells of fuel.