Somebody please point me into the right direction. I have tried a lot to get my '99 400ex to stop back firing on deceleration. Carb is super clean, all jets are perfect. Back fires only after you gas it good and then let off.

Upgrades: Hmf Slip on, FMF powerbomb, no airbox lid, and k&n filter. Yes I know k&n isn't the best. Everything on the quad is clean.

Jet sizes: 172 main, 45 pilot. Don't know about the needle position. Air fuel screw is 3 1/4 turns out.

Could this be an air leak? I mean it does not back fire when you are in like 4th-5th gear and let off slowly. Not super slow, just not an immediate let off.

Also, exhaust packing. Would this cause/worsen back firing? I can see some black packing coming through the core. I'm guessing it needs a change.

Air cut valve: I don't know if this has gone bad, and don't know how to check if it is working properly.

Header crush gaskets: Don't know if the previous owner changed out the crush gaskets when he installed the after market header (fmf powerbomb.)

I'm close to sea level: 550 ft.

I'm at a loss, please lead me to the right direction.

 

Try a 175 main. When your FA screw is more than 3 turns out you need a bigger main.