Sonnds like it might be a gas flow problem, I just got through rebuilding my 1980 Maxi, the petcocks on these things can clog pretty easy, and if the bike set for a while before you got it, you might need to clean it. and be sure to run an in line fuel filter on your bike. also check the screen in your breather box. mine was plugged. mine runs great now, and tops out about 30 with me on it, and I weigh about 230. clean your carb again, if you don't have an inline filter. good luck.