Could be a # of things that would cause that. Plugs, timing, fuel starvation, vacuum leak. Process of elimination. I just went thru it with my truck and I still have a lil issue......If I let it warm up fully via choke, truck runs like a raped ape, but if I rev it up and kick the choke off right as the temp gauge passes 115 it will really bog down. Changed the plugs, timed it, lowered tghe idle a bit. then found a vacuum leak at the carb base gasket.