Several threads on this forum concerning the power valve, which as mentioned by 4onthefloor, could well be the problem. Wrong gasket or body warpage can cause power valve problems.
Carter built LOTS of different YF carbs. There is another thread on this forum which lists the YF's which are applicable to Chevrolet engines.
Stromberg and Zenith are other options, and from a quality standpoint, even better than the YF; but the correct ones tend to be pricey.
Jon.
