I have installed a Dr. D slip-on pipe on my 2006 Yamaha YFZ450. I have no other mods on the bike. I changed the main jet to 172 and I am using a 45 pilot jet. The bike is very sluggish in the first 25% of the throttle. It runs fine if you ease into it or at higher speeds. Hitting the throttle hard from a stand still will likely cause it to stall. I ran it for a while and checked the spark plug and the color is good.

What should I change next to get it to run correctly? I have not adjusted to fuel screw. Should I adjust the fuel screw or the pilot jet?

 

try to adjust the fuel screw first to see if that help if not try putting a 165 or a 160 with the fuel screw out 2 turns put a 48 pilot jet and the needle on the 3rd clip. how far are you off sea level

 

Go one clip position richer on the needle, and that will solve your problem.

 

Changing the clip position did the trick. The problem was when I got my carb apart the '06 needle only has one groove. I had to get an '04/'05 needle with 7 clip positions. Thanks for the feedback.

BTW..No dealers around me carried the needles.