well whare should i start. how bout this. i have a 1991 indy 400 was out riding it last year and locked the crank up so i replaced the entire bottom end crank and block. the parts i used were from a 500. so this year it was running ok but noticably loosing bottom end power due to loss of compression so i went ahead and upgraded the top end to the 500, jugs head and new pistons. prior to the "upgrade" the engine had deicent top end performance, 85mph at about 9200 rpm i know a lil high but it loved it. so now with the fresh top end it runs awesom in the bottom end lots of torque and responce but as for the top end, right around 7000 rpm it starts missing and backfiring out the intake. i re jetted the carbs to 280 mains still have the 40 pilots in it. checked the plugs several times over and over seems to be burning clean not too lean not to rich. i have taken apart the primary clutch cleaned everything everything moves good, weight pins are slightly worn but not jamming, secondary is working good, so i am out of ideas, any insight to my problem would be apreciated.