When the vacuum tanks is full the float should open the vent valve and relieve the upper chamber of vacuum so the fuel can drain into the lower chamber through the flapper valve. A leaking vent valve won't permit suction to the fuel line. If the vent valve is good but the flapper is leaking, the vacuum will be equally directed to both the carburetor fuel line and the fuel tank line. Try putting a pint of fuel in the vacuum tank. starting the engine and plug the vent tube with your finger. If the engine continues to run but loads up with too much fuel until you allow the tank to vent, the trouble is probably the vent valve or mechanism that trips it. On speculation I'm thinking that the flapper valve could be tested by plugging the (1/8"NPT), port that goes to the fuel tank and attaching a vacuum cleaner, (duct tape and ingenuity?), to the engine side vacuum port. Once done, an empty vacuum tank should not suck a, (small), piece of paper against the carburetor outlet port. It may be too late for this year but remember that nothing will please Mum and the Munchkins more than finding a big box of DB parts under the tree. Happy Festivus. Nat
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