Welcome to JUST ANSWER small engines, let me try to help you !! You are dealing with older engine, Have you rebuilt the carburetor ? Have the valves been set, valve lash being .008 to .010 in. on intake, and .017 to .020in. on exhaust. The only running on heavy choke sounds like the carburetor, here is a link to the disassembly if you need it.http://outdoorpowerinfo.com/repairs/ You could pull the carburetor float bowl and just try cleaning the needle valve, float and the main jet and see if it takes care of the problem. Let me know. Thank you.