Rebuilt my 250r motor, split the case and new bearings, seals etc. Its all back together, except with a 265 ESR powervalve cylinder and cr250 ignition, but I can't get it started. At the bottom of the kickstart it backfires and doesn't start. It would seem like its getting spark due to the backfire. Could it be the jetting in the carb from switching cylinders? It has a keihin PJ carb, not sure which size and the main is 180. Any ideas? I'll try and find some calipers to figure out what size carb it has