I put a White Bros. E-series pipe, K&N filter, and cut 4- 2" holes in the air box. I now backfires and runs bad. I have not messed with the needle or jets. Does anyone have any suggestions?

 

I would think you would need to richen up the jetting.

 

Call White Bros. and see what they recommend for jetting. You should have bought a jet kit for the intake and pipe mods those needles have a performance design. On 400's if they have a 148 main jet and you add a pipe and intake you go up to a 155 or so.

 

helloooo!
When the air flow is increased by the amount you mention, which is to say a lot, you certainly need to increase the fuel mix to compensate. You can't get the right info here or any where else but from your own engine

 

Which quad are you talking about, Raptor or 400ex?
But yeah, like the others said increase the air flow then you must increase the fuel flow. Raptor has 2 carbs so good luck if its that.

 

I would replace the 140/145 main jets with 150/155 right now! and test ride & read the plug. I'd assume you're at sea level or thereabouts so you need at least the above sizes but only a test/plug read can tell you that. It may indicate even fatter jetting (main) is needed. Don't fool with the jet needle until the full throttle jetting is corrected or you'll be out in left field without a clue. Then, when 3/4 to full throttle performance is okay, you can check the part of the band controlled by the jet needle(1/4 to 3/4 throttle). I'm betting it will be okay as is.

 

I'm at 5000 ft elevation. Go much richer then suggested cause I run 145 and 150 which is spot on for my altitude and I'm running the stock filter. Where did you get your K&N???