The motor is a 12 hp briggs and straton. while motor is running we have fuel coming out the weep lines of a Mikuni carb, about the size of a 175 to maybe 250 motorcyle. When the engine is shut off it will sypheron the tank dry. The float and needle valve wont over come the syphoning effect of the fuel comimg out of the tank via the weep holes of the carb. . Can somebody tell me what is the cause of this problem or how to fix the problem? Thank You very much and MOW ON!!!!!