My Nova 454 was runing very rich had carb (750 vac. secondaries) apart several times there realy isn't much to a Holley. changed gaskets changed jets so many times I don't remember whats in there anymore. Got another 750 manual secondaries. placed it on and started the engine. I have not checked the carb as it was good on the engine it was removed from..now I have a backfire in the engine , it's not all the time but it happens. there is no cat on this car. it is dual ex. and the B/F is on the pass side. This just started today so havent had time to realy get into it any sugestions