New FMF
 

Just got my Powercore 4 and installed it love it. Nice and loud but is it normal for it to backfire when you slow down?

Yep. No baffling to stop it, just the way it is.

You can get some backfire relief by tweeking the fuel screw a bit to richen up the pilot circuit when you close the throttle. You may affect throttle response if you go too far. Good luck.

Did you re-jet?

Mine backfired ever so slightly on decel.

Aside from the air/fuel mixture screw did you follow the instructions for attaching it to the header. Your suppose to use high temp sealant and let it set for 24-hours before firing it up. An exhaust leak can cause that too.

Mine can backfire pretty loud on decel, especially on a cold start.
I does sound good though ;)

Just giving you a heads up on the Powercore 4 it WILL melt your airbox (Happened to me) so what I did is around where you join it into the header I put some exhaust tape to keep the heat inside of tailpipe. Just a friendly suggestion! :)

That really happened to you?
I have had the powercore 4 on the klx for over 12 months and had no such issue or even heard of that issue.

Really? Might of just got unlucky or something. The side of the airbox that is closest to the tail pipe near the joint is all melted up, not a huge hole but it's still a hole. I've had mine for, 10,000 KM (6000 miles?) with the Megabomb header. I'd post a picture but I can't seem to upload it.

That's odd, I've done around 4-5k miles and no signs of melting of the air box.
Did you bend the rear right foot peg mount outwards for a better exhaust fitting?

Then again I haven't added any head pipe I'm still using the stock header, maybe that's what has caused the melting...