Just got a 2000 400EX with a big gun and K&N filter. Does any one know the jet size to get me in the ball park? So I do not have to buy a hand full of jets. Air box lid is on. Stock pistion

 

Although every engine/pipe configuration is different, most people seem to be landing anywhere between a 158 to 165 main jet. You also must take altitude into consideration.

You shouldn't worry about buying a handful of jets, they are quite inexpensive. Anywhere from 2 - 4 dollars a piece, depending on brand. Plus, you can always return the unneeded ones - that's what I usually do.

ogre

 

When I first installed my WB R4 and the K&N on my 400EX with stock piston I was running a 152 ('700 above sea level) and had a good plug reading.