Since taking my 2003 Sportster 1200C out from winter storage, it pops, backfires and has very poor low speed performance.
I thought it an air leak so I replaced the carb to air intake seal and one of the vacuum hoses but it has not helped. I sprayed WD 40 on all the other air and vacuum ares but did not note any difference in idle as would be normal.
I disassembled the carb and cleaned it inside and out thinking it might have some gunk built up sitting over the winter. It did not help.
Have adjusted and readjusted the cables and idle.
I did put in a jet kit about 25,000 miles ago, could it be time for new jets, sort of an oddity seeing it ran GREAT when it was put up last fall.
Have not tried adjusting the idle screw (jet), could this be the culprit?
Getting VERY frustrated.

 

many times winter storage will form condinsate clean the carb. and tank

 

try runnin some sea foam through it. works on everything ive used it it!!

 

You may wanna take out the fuel tank filter and have a look at it.