I had a similar problem. I did a rebuild on the carb and when I went to put it back on I noticed there was a vacuum passage that comes out the base of the carb and goes to the manifold. The base gasket that came with the kit had no matching hole for this passage. The old gasket was also missing this hole. I cut out the new gasket and the truck has run well ever since.

if it dosen't need fixed, break it