It starts great and idles fine. I have changed sparkplug cleaned out gas tank. It has No top end and starts to backfire.
Wolud it be a valve problem?
or maybe a carburater problem?

Where could I find information or manual for fixing it?

 

My manual says under "Poor Medium-Hight speed peeformance"

Incorrect fuel level in carb bowl.
Incorrect jet needle clip position (if adjustable)
Clogged or loose main jet.
Clogged air filter
Engine oil viscosity too high or oil level to high.

 

Thanks, So I will clean the carb. see if that helps.

 

It now runs better then when I bought it 5 years ago! I did a total cleaning of the carb and fuel system! Thanks again.