Tell me a little more about what you have and what you have done with it .
1) What ignition does it have ?
2)Is it mechanical or auto advance ?
3)Have you verified the igniton timing and how did you do it?
4)What spark plug are you using and what is it gapped at ?
5)What prop are you using and what RPM are you getting at full throttle ?
6) What is the carb number (WT 76) ?
7) What ignition battery are you using?