On my 05 YFZ 450 I just got a hmf put on and run an open air box with pro flow k&n and it is jetted at 170. After going full throttle and let off of the gas and slows down there is a backfire. Happens more often then I would like. I had the pipe and jetting done at the dealer so what can I do to take the backfire away? Is it too lean or what?

 

go up one size on your Piolit jet, and turn your air screew out 3 turns, YFZ's are jetted lean from factory.

 

Yea get a size up Pilot (which is 48) and just adjust the the fuel screw till' you have fresh crisp throttle response

 

I'd try going out on the fuel screw before changing the pilot. If that does not work, then change the pilot.

 

I have an 04 Bruin 350, and it is stock. The same thing happens to me when going from full throttle to letting off the gas, it will backfire. Is there anything I can do to stop this?