Vibration felt 5000-6000 rpm.

Ok guys. Been messing with this bike with the Huasheng 142f for a couple weeks now. Seem to have it tuned, engine mounted good, chain aligned etc etc. I can actually ride now without something going wrong. But since day one, I have had a vibration that channels somewhat thru the foot pedals, seat, and alot in the handlebars at an rpm range of 5000-6000. It is smooth before that range, and smooths out after that range. Is this normal? Or do I need to chase another problem. I did notice slight flex with the mounting plate. i can take the motor and angle it very slightly side to side. I am really hoping not to have to take this motor off again. I don't know about you guys, but everytime I turn a wrench on this thing, I seem to scratch it more. I wouldn't think this was normal, are your guys rides smooth all the way thru the rpm range?

Wow.. No input. Really? Anyway, I found the problem. I had not had the exhaust pipe clamped down. Apparently at the specified rpm range it caused the vibration to channel up thru the handlebars. The foot pegs were loose, thus causing the vibrations in the foot pedals. So, always remember folks, clamp that exhaust!