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I recently purchased a non running 05 or 06 zx50.  After getting it running, I replaced the exhaust with a new stock unit because the previous owner had gutted the back end of the muffler which made the bike unpleasantly loud!  The air filter assembly was also missing so I purchased all new stuff for that too.  When I installed the new filter assembly the bike would not run.  It would start but would not rev above idle.  I got it to run by lowering the float level and removing the air adjustment screw.  Now it runs fine as long as I keep the speed at 30mph.  If I try to run 35Mph it will do so for about 1/4 mile and then start to sputter and eventually die.  If I put my finger over the hole where the air screw goes it will start right up and I'm on my way.  If I don't cover the hole for the air screw it won't start. Does anyone have any ideas?  I was wondering if a smaller main jet would help.