bad midrange stumble and backfires

[hankohlson]

so ive got a 2000 800 non ves in an iqr cahssis thats got a real bad stumble in the mid range. its ported with reeds, custom fit gen2 slp pipe with a stinger silencer. no lights but i put the arctic cat 200 watt voltage reg in it. I rebuilt the motor top and bottom. ran great for the first tank or so but then got this problem. it has good idle and once you get it wide open it rips. it barely runs and back fires in the mid though. not rideable. went up on the neeld and pilot and cleaned the carbs. ran better for a minute but then it was back.

could it be :

exhaust backpressure?

timing?

electrical ? tach works fine?

temp sensor?

tps?

ideas?

thanks guys!