Hi,the way it works is.....petrol gose from tank into filler pipe into carb it fills the bowl at the bottom of carb float rises and shuts off the petrol by means of a inlet valve when float pushes it shut,
so if there is petrol coming out of the over flow pipe from the carb bowl it is iver the valve is worn(can be replaced)or the float is not at the right level(to high) or float has a hole so sinks(get a small container put petrol in it and put float in if it floats its ok)the other thing is the gasket that gose between the bowl and and top part of bowl make shore its not broken,
hope this helped all the best.