i have 97 xp not running, new top end. ran like crap. cleaned out strainers in carbs. then it ran great for 2 minutes then started sputtering again and would not start, despite repeated attempts. cleaned the carbs again, were perfectly clean. new gas. got it started this morning but would not stay running now especially on top end. ran before but really crappy, now it wont run at all. checked the plugs, getting good spark. checked flywheel for timming, seems good. backfires sometimes while trying to start. wondering if my rotory vavle is out of timing. timed it the way the manual said too. any one have any ideas. motor came out of my 97 spx was runnin great. only rebuilt engine because of dropping washer into engine. has good compression, new rings and honed. any help will be appreciated.