So many things can effect this, and you didn't put much info in your question, however, check these. What vacuum are you pulling at idle. What is your powervalve? Make sure the powervalve is above the idle vacuum. What is the timing at idle? If it is retarded to much the lack of advance can cause a very rich condition (this is the main argument for keeping vacuum advance in your car). Who set the carb up? You should be idling on the idle circuit and not into the transition circuit of the carb. Hope this helps. The 750 carb is a pretty good carb setup proberly so keep working on that tune. You can get the rich condition down to a livable level, may it ain't never going to be like mom's buick.