try running the truck with the gas cap off,
if it runs ok then, you have ventilation/evaporation
system blockage. The charcoal canister or related lines could be blocked up. It just may be the carb is ok.
I have lived this problem, I can tell you the sight glass
was always perfect as well. <img src="/forums/images/graemlins/oink.gif" alt="" />

fast enough to beat its own shadow