If you have a Rochester carburetor and if you're using gas laced with ethanol and if you're not driving it very often, it is very likely your power piston is stuck in the "down" position. If this is the case, you'll be running much richer than you should. The ethanol when mixed with unleaded gas dries leaving a sticky gummy varnish-like residue which will stick that power piston as tight as if you epoxied it in there. And because of the spring, it will always be in the down position, opening the power valve fully. Good luck.
