Hi there I just rejetted my 04 KFX400 and have seen a loss of power. And I'm not sure why. I started the whole process by removing my air box lid and replacing it with a K&N air box lid cover and pro flow K&N air filter kit. I also at that time changed my main jet from a 135 to a 160 main jet. I noticed a power increase, but the quad would back fire on deceleration which I believe indicated that the unit is running to rich. From there I intalled a new Yoshimura RS-3 comp. slip-on pipe and also noticed a little increase in power, but the unit was still back firing in deceleration. I then proceeded to change the main jet to a 155. I took my quad out for a ride and I lost alot of mid range power. The bike did start and Idel better, but the power is just not there. Any suggestions on what jetting I should be at. Should I go down to a 152.5 like the yoshimura specs suggest? I live in Bismarck, ND at and ride at about 1300 ft. above sea level.