I second the zoom 66 cc slant head motors. Lots of power.
Run the first gallon about 20 to 1 on cheap out board motor. It will spit sputter etc. After that the next gallon was 60 to 1 opti 2 then I went to 80 to 1, now Im at 100 to 1 on opti 2 and the engine runs superb. After a gallon or 2 it is BROKEN IN. I run my bike at 75% to 90 % throttle most of the time on main roads and I back off a bit on the side rodes.
Some of the companies on here are bs with their motor prices. I talked with one and he was tring to sell a leser kit china motor for 350. Its still a china motor kit and nothing was done internally to it. What a joke. There is basically no service after the sale and you warranty is us here on the forum to help with your quesitons etc.
This topic need to be a sticky!