I have an 86 C-10 that I bought as a project. The had been sitting for some time, and I finally got it running. I have cleaned the carbs thoroughly (twice!), adjusted the valves, synced the carbs and got the engine running. The problem I have now is when idleing or running at part throttle with no load or coasting down to a stop, the engine surges to the point that the bike will actually buck back and forth. When running with a load (I.E. hard acceleration) the engine seems to run fine. Everything looks ok with the ignition system, and the plugs look normal.
Check the airbox to carbs boots for leaks. Check the vacuum caps on the carbs to make sure they aren't cracked. Make sure your air filter is clean and the airbox has no cracks/leaks. Other than that, possibly pull the carbs again and double check the slow circuit. It has no pressure behind it, so any kind of little bit of crud can mess it up. Especially running ethanol Gas in it.
Does using the enrichment/choke lever even out the surging at all??
Airbox to carb boots are brand new, the vacuum caps are ok, as well as new vacuum lines to the petcock and air valve. Once the engine has warmed up, the choke just kills it.
I'm gonna double check the valve adjustment and do a compression check next I think.