10HP Tecumsah won't run smoothly unless choke half closed?

This is on a Coleman Maxa 8500 generator. If choke is wide open, it accelerates, then lets off, backfiring, then accelerating again...continually repeating this with the RPM going up and down quickly. BUT IF the manual choke is then set to half closed, the engine smooths out, and runs steady. I have run a half of a can of SeaFoam thru it with no effect and no change to this condition. Its hard to believe the carb is gummed up after running so much SeaFoam thru it. The air cleaner/filter is clean & flowing well.