I got a 04 yfz 450 with a duncan racing pipe and a pro flow air filter.It idles fine and runs good at 1/4 throttle and under but at 1/2 to 3/4 its really bogggy i dunno how it runs wide open since is a new top end less than 1hr on it. the jet sizes are 45 ,180 and a tiny one that is 90 pin is on the 4th setting the air/fuel screw is set at 2 turns out. i am about 2000ft above sea level and the temp is 65-85 out here. My plug is verry black not oily looks more like carbon.

Edited by Warzardo, 01 August 2011 - 04:55 PM.

I am having the same prob with my 04 yfz 450 so if u find out anything please let me know

180 main is too big try a 170 or 172

-looking for some jetting help-

bought a quad with some work done to it. It has a 478 big bore kit(jt piston), hotcams, k&n filter(lid on), hmf bill ballance slip on.  i noticed it backfired like crazy when i let off the throttle, so i upped the pilot from a 48 to a 50. that solved the backfire. she runs great at low rpm and pulls like crazy  but at top end it bogs badly. not sure what my main jet is now but my only guess is that i need to go bigger because at the top end it seems to be starving for fuel...


not sure where my needle is set or how many turns i am on my idle screw.

so before i tear into it, does anyone have any suggestions on the main jet, needle setting, and idle screw?

70-80 degrees, 600' above sea level.