just a little background on my bike, its got 38000 total miles, and about 20k on a hop up kit. which included cam, 42mm mikuni carb, bored out, port and polish etc. whats happening is when i jump on the throttle, about 2/3rds of the way through my rpm range its dies out and lets out one big backfire. it only does it through first and second and gear, and when i get a good roll on the throttle it still happens, just not as drastically. any ideas? the 20k have been completely reliable and trouble free.

 

Could be an ignition problem. I'm no expert though. You might be able to get better answers in the EVO classic section of the forum.

 

Sounds like you're leaning out at higher throttle ranges. Could be you have too small a main jet, or trash in the carb, or wrong needle, or clogged/malfunctioning accelerator pump. Those are the things I'd check first, along with making sure there are no air leaks at the manifold.

Cheers!