- Leaking float valve: The float valve in the carburetor should shut off the fuel flow when the float bowl is full. If it's leaking, it can allow excess fuel to flow into the crankcase.
- Incorrect float height: If the float height is set too high, the fuel level in the float bowl will be too high and the float may push the valve open, allowing fuel to flow into the crankcase.
- Sticking float valve: The float valve may be sticking in the open position, allowing fuel to flow into the crankcase even when the float bowl is full.
- Worn valve guide: The valve guide in the cylinder head may be worn, allowing oil to flow past the valve and into the crankcase. This can cause the crankcase oil to become contaminated with fuel.
