MG Car Repair: weber down drafter run-on, zenith carb, brake servo

Question
To whom it may concern: I have a 1980 MGB with a weber down draft carburetor...it has bad run-on. I see that the original zenith carb had a anti run on valve but my weber has only one vacuum port going to the vacuum advance on the distributor. Does the anti-run-on valve run in-line with the gas and shuts it off to the carb? Or does the carb have a vacuum port above the carb float to close the jet hard to stop gas from spilling in to a just turned off engine?  Thanks in advance..Sean Walsh

Answer
The anti run-on valve on a Zenith carb cuts the fuel after the float chamber but before the jet, so it cuts the engine immediately.

The Weber downdraught carb has a small vacuum pipe on the side that communicates with the distributor vacuum advance module.  It has nothing to do with cutting the fuel.

You could fit an aftermarket anti-run-on valve into the big rubber vacuum pipe going from the manifold to the brake servo, using a plastic Y-piece.  This would dump air into the inlet manifold whenever you switched the ignition off, so the engine would stop immediately.  

I suggest you contact Moss and ask them for the necessary parts.  They may have a better suggestion.