Right, so the working principle here is that fuel being pumped into the fuel bowl on the carb is regulated by the float and needle valve assembly (they work together...as a team...).
As the level of fuel in the fuel bowl rises, the float rises with it (because it is floating on top of the fuel...which is why it is called a float...I think...).
When the float reaches a certain level, the tab at the base of the float (the one which you adjust to set the specs outlined in the FSM so that the fuel level in the bowl remains at 1/2 up in the sight viewing window) pushes down on the needle valve assembly, closing the needle valve and preventing the further flow of fuel into the bowl.
So, either:
1) float is not set correctly...so it is not shutting off the needle valve assembly
2) the needle valve assembly is not working correctly and does not close
3) the fuel pump is forcing fuel into the bowl, over riding the float/needle valve
Do this:
1) Replace needle valve or if the needle valve was replace during carb rebuild, double check and make sure all the parts are present and assembled correctly
2) Recalibrate the float setting as per FSM
3) Reinstall air horn and test
4) check your fuel pump. As Lambcrusher says, you should see 3 leads coming from the pump. One is a return line to the tank, so that when the needle valve closes, fuel being pumped is directed back to the gas tank. Remove the gas cap on the gas tank (where you add gas during fill up) and remove that return hose at the fuel pump and blow into hose - it should be clear and someone should hear the air coming out the filler cap- if it's not clear then you won't be able to blow. Put hose back on fuel pump and reinstall gas tank cap. EDIT: maybe try this last, since you probably don't have access to air...
