I wouldn't change carbs. Check for vacuum leak at the manifold. Do you have good compression, you might have a valve not closing completely. What brand of intake gaskets are you using? I used SCA brand aftermarket and they came aprt after about 700 miles and caused a really bad vacuum leak. Is your carb new or used? make sure the base gasket is good and the base plate of the carb straight. What kind of base gasket are you using? If you have a thin gasket try a thicker one from like what GM uses.
That should keep you busy for awhile. Good luck