A Remedy For Stopping A Leaking Carburetor Float Needle Valve And Seat, on my 76 Kawasaki KZ 900 and 79 Kawasaki KZ 1000 ST.
Do you know what causes gasoline to drip out of the over flow drain tube of your carburetor?
The short answer is that 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-adjusted to obtain a correct fuel level in your carburetor's float bowl. Or if your if your carburetor's needle valve and seat are excessively worn, they may start leaking and need to be replaced.
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, while the fuel level in the float bowl is being maintained. 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 working in constant equilibrium, as it's equalizing with the pressure exerted by the float valve needle (on the carburetor side of the valve), so that a very constant level is maintained in the float chamber.
Understanding that the needle of the float valve is always being pressed against the seat is important in understanding why any tiny bit of dirt can cause so much trouble. Once a piece of dirt gets caught between the needle and the seat, it will be held there and interfere with the needle valve's ability to control the fuel level, until such time as fuel is drained from the carburetor and the piece of dirt is flushed away. This is the reason for the flushing procedure of the fuel system, by removing the drain plug from the bottom of the float bowl and let enough fuel run out. Which will usually rinse off the the debris that is on the needle valve and seat, providing the debris is small enough to go through the needle valve seat orifice, as the fuel is draining out of the float bowl drain screw hole.
To help prevent this problem from happening, again. It's advisable to put a good reliable inline fuel filter between your gas tank and carburetors to keep your fuel system as clean as possible that's going to your carburetor's float valve and seat. Which will, also, prevent other problems from occurring in your carburetor, too, from dirty fuel.
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, sharply 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.
