It's basically a detuned '75-'76 elsinore engine with a few modifications. Does it have a CDI or points?? Some had points. Later ones had CDI. If it has points, the timing is not very adjustable except for points gap. If you have points, clean them (running old soft dollar bill through them several times often works) and set the gap. Also, you could have worn out top end. The last time I bought one of these engines, someone had put the piston in backwards and snagged the ends of the rings on the exhaust port. They are very straightforward piston port engines. If you are getting good spark, and timing isn't way off, I tend to look for carburetion or air leak issues. As Joe said, check the seals. You do have to split the cases, though, to replace the seals. Start with choke on, no throttle. Keep us posted. I think the Odyssey's were way cool machines.