Yamaha yfz backfire

[YFZ450]

i have a 06 yfz450 with a 05 bottom end it has a heavy duty crank and a 06 top end i just put it all back together it runs great i had a 170 jet with a 48 main jet, i put a 168 jet back in because i had a guy that used to be a factory mechanic on a race team look at it and he told me to do so. Now with a 168 jet it when i rev it, it backfires not all the time just every once in awhile i got the clip on the needle on the 4th setting and im running straight turbo blue. evrything is in time

[handyhiker]

Is the backfire out of the carb or muffler?

[mywifeknowseverythin]

i have a 06 yfz450 with a 05 bottom end it has a heavy duty crank and a 06 top end i just put it all back together it runs great i had a 170 jet with a 48 main jet, i put a 168 jet back in because i had a guy that used to be a factory mechanic on a race team look at it and he told me to do so. Now with a 168 jet it when i rev it, it backfires not all the time just every once in awhile i got the clip on the needle on the 4th setting and im running straight turbo blue. evrything is in time

Hows your Valves for Clearance....????? Also,,,,,Sometimes you could be getting some Cold air into the Exhaust at the head....The Cold air hitting the Hot air inside could cause some popping....Have you done a Plug Chop to see what your Jetting is Doing????/ Rich or lean???? There are just too many things that may be wrong....I would suggest you seek out the Cheapest Fix First and move up the ladder....

[YFZ450]

it doesnt backfire any other time just when its sitting there idling and i rev it up it will back fire , it wont do it with the 170 jet but the bike has more power with the 168, its not like a backfire, it will shoot a flame out the exhaust.

[mywifeknowseverythin]

You may be running a little lean.....Or you may have an Exhaust Valve open or Closing Slowly.....

[YFZ450]

ok, its doesnt do it anymore. Now i notice it tops out really fast with the 168 jet and i tapping off the rev limiter with the 170 it took awhile beforethe rev limiter would hit, the bike is super fast and hits the limiter quick