has white bros e-series pipe...k&n filter and holes drilled in box...i was told by guy i brought it from that he did not jet carb because didn't need to but tends to backfire every so often when i let up on gas...is this not a lean condition from to small a primary jet or air/fuel mixture misadjusted...jet kit is like $50 seems like cheap insurance against siezed piston.....any advice.....thanx

 

It deffinately needs to be rejetted. Not just the primary and air/fuel screw but you need to go bigger on the main too.
I'm surprised it doesn't cut out on the top end and run really hot without being rejetted.l