Andrew and Ilikeabikea have it down it's in the gears - but also there are some things you can do. First break the engine in. Always have a good plug - I think the NGK B5HS is about perfect. Good wire and boot. (you don't want to miss a cycle at all).I had my bike with an 80cc engine for a little while and I get up to about 27mph. Does anyone know of any adjustments to make to make it go faster LOL. It says they go 40mph and up, but Id be happy to go over 30mph.
Yeah... I found that my bike runs best "LEAN" with the needle all the way down, (c-clip at the top)
I also "port matched" the intake manifold & exaust pipe to the cyinder ports for better flow.
Try a 36-T rear sprocket.