Johny is right. I had a problem with the mechanical fuel pump overpowering my float valves giving the symptoms that you describe and solved it with 3mm of additional gasket between the pump body and the block. Later, once I had rebuilt the carbs, changed the rubber pipes for Gates Barricade, installed an additional fuel filter just before the carb's "Tee" piece and gotten rid of every last miniscule rubber speck I was able to remove the additional gasket and all is now well.