Motorcycle Repair: engine repair, rear cylinder, e bike

Question
QUESTION: hey mike i have a 1980fxs lowrider 80ci. had engine rebuilt have changed out carb to an ss super e bike runs fine except when it gets up to temp then starts to breakdown back fire. if i bring the speed down it starts to work ok again. i have done a compression test and it appears the rear cylinder is only reading 90 psi where the front one is reading 140. do i have a broken ring or what do you think?

ANSWER: You are having a big difference in compression. How many miles are on the rebuild? When the S&S was installed, was it jetted correctly?
Mike

---------- FOLLOW-UP ----------

QUESTION: Not reall sure about the miles possibly 400 to 500 maybe. Not a whole lot when the problems started. As far as the carb goes I never had it jetted I just bolted it on seem to work fine, fired right up no hesitation or any problems since. Except when the backfireing starts it throws out through the carb.

Answer
With that few of miles your compression might b a little low since the rings have not broke in yet. Give it till about 200 miles.
Your carb problem is that it needs jetted. If you go to the S&S web site and the tech pages, you will find the info you need to jet your carb
Good luck and happy riding
Mike