standing fuel in intake

hi, I have rebuilt a 1972 351 Cleveland, I am having a problem with standing fuel in the intake, I have put several carburetors on, and have experienced the same results, I also pulled a plug to see if it was fouling and the plug was clean and no fuel on the plug, it seems to me that the fuel is not burning and just sitting in the intake, I would sure appreciate some suggestions on what the problem might be, is it possible that I am a tooth off on my timing chain? kind of just throwing things out there. any help would sure be appreciated. thanks much.