Yes I should have said that this is a Q-jet carb. It came to me on an engine taken from a 74 Nova. It had a fuel pump that vented back to the fuel tank and I used the pump with the vent plugged for years. Recently I replaced the pump with a 1967 style pump with no vent.
I have looked inside the carb and while I don't see the gas entering the engine I believe that it is because the throttle plates appear to be wet. There are no fumes to be seen and no dripping to be seen but when I removed the carb, the right front port had gas in the intake manifold. It's not enough gas that the engine starts with any black smoke so maybe the gas is evaporating before getting into the cylinders. I just have to hold the throttle to the floor and spin it for a dozen spins or so to get it to start. Another clue may be that I'm only getting about 14 mpg.
I worked as a mechanic in the 1970's and tuned and repaired tons of Q-jet engines and never had this kind of problem. I've seen the gas drilpping into the engine when a float valve sticks and I don't see that here. I'm beginning to think that it is the new gas causing a problem somehow. I just don't see it boiling into the engine. Can it maybe ust be evaporating?
I wish that I had a known good spread bore carb to try on there and see if it corrects the problem.