9. Why does dirt get trapped in the float valves of carburetors, rather than move on through as the floats raise and lower the needle against the seat?

The needle of the float valve is actually always being pressed up against the seat by the buoyancy of the floats. As the fuel level lowers, the pressure against the seat is somewhat less, and the pump is able to force some more fuel into the float chamber. As the fuel level rises, the buoyancy once again is sufficient to close off the fuel flow, in opposition to the pressure from the pump.

The fact that the needle is always being pressed against the seat explains why a piece of dirt will remain held between the needle and the seat once it works its way in.

It's sometimes possible to get rid of such a piece of trapped dirt by draining all the fuel from the carburetor and then running the pump for a few seconds. The incoming fuel will sometimes flush the dirt past the needle valve and out of harm's way. If this quick fix doesn't work, you'll have to take the carburetor apart and remove the particle.

While the carburetor is still apart, it would be a good time to polish the seat by taking a short length of 3/16" wooden dowel rod (pointed on the end) and pressing it against the seat while rotating it back and forth. This dressing action sometimes assures a better seating of the needle. - Updated: November 4, 2003