I have a 72 or 73 Ford 2000. I replaced the Holley with a Zenith, but I can"t seem to get the fuel through the carb. If I add a little gas to the cylinders, it"ll start and stay running. Once it has sat a little while though it won"t restart. I can pull the plugs and they are dry. I"m guessing there"s some type of vacuum issue that is keeping the fuel from being pulled up through the updraft carb. The gasket between the carb and the intake manifold is new and the nuts are tight. There are no leaks on the carb or fuel lines going to it. The intake manifold gasket looks undisturbed. When the tractor is running, it doesn't sound like it has a vacuum leak. Anyone have any ideas? This has me baffled.