What Causes Gasoline to Drip Out of Your Carburetor

The carburetor is where the fuel mixes with the air to be burnt. Carburetors are on almost all types of combustion engines. When an engine sits for a long period of time without being ran, the fuel will start to break down. Make sure you replace the fuel in your tank before you start your engine after cleaning the carburetor otherwise you will just recreate your troubles.
A tiny piece of dirt is most likely stuck in the needle and seat of the float valve, preventing the valve from fully seating. Or else your float valve may need to re re-adjusted, or replaced (due to wear on the needle valve and seat), to lower your fuel level in the carburetor float bowl.
However, I'd like to give you a bit more background on this important question. I used to think that the float valve in the carburetor operated like the float valve in the tank of a toilet: i.e., when water in the tank got low (as in after flushing), the valve would open until the tank filled and then close again. However, it is important to realize that (unlike our toilet tanks) the needle in the carburetor float valve is always closed. What varies is the pressure that the carburetor float exerts on the needle: relatively high as the fuel level increases, and relatively low as the fuel level decreases.
This means that the fuel pressure (on the inlet side of the valve) is in constant equilibrium with the pressure exerted by the needle (on the carburetor side of the valve), so that a very constant level is maintained in the float chamber.
To help prevent this problem from happening, again. It's a good idea to put a good inline fuel filter between your gas tank and carburetors that keeps your fuel system very clean going to your carburetor's float valve and seat, which also prevents having other problems in your carburetor, too.
In addition to dirt causing the valves to stick open, I have seen a few cases where the brass seat of the float valve needs to be polished (smoothed out) by pressing and rotating a short pointed hardwood dowel into its small orifice. Dowels of 3/16” diameter work well. I have even had to clean up brand new seats in this fashion before they would stop leaking.

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