Here is what I got:
2008 Outlaw 525S - FMF Power Bomb - FMF Factory Full Titanium 4.1 - Stock air filter and box , 4 quarter size holes in lid , covered with 12 x 12 outerwears sheet.
Starter Main Jet = 85
Main Jet (138) w/ Needle (B36)
Air screw 2 1/2 out
Has been backfiring on deceleration pretty bad , not just a little, but a lot. Other than that, the quad is a beast, and runs TOP.
Did some research over on the KTM forum and here is what they did:
In summary , you order KTM OEM part # KTM EXHAUST FLANGE 03 (SXS02540506) $28 from motosport , my fmf header pipe had some slight deformities and wasn't 100% round, so I went to Harbor Freight and purchased http://www.harborfreight.com/small-tail-pipe-expander-69550-9156.html ( small tail pipe extender) Add a little grease to it.
Using this, it opened the tip of pipe slightly so I could get the flange in (tight fit), using a rubber mallet I was slowly able to get the flange it, not all of the way, but tight, added some red gasket stuff to that connection for insurance, and let it sit for a day.
Next when I installed the head pipe , I made sure not to move it around from the muffler end of the pipe, as to not pull the flange off center. Bolted the flange , added new heavy heavy springs from home depot ( Century Spring C-173 , paid .99 for two in a pack on clearance.
Once done , I carefully installed muffler, making sure not to move the head pipe connection. added a slight bit more RVT . Let sit overnight. Everything is solid, no movement at all.
Next day, fired it up, once warm , got on it !! Pop gone !! No backfiring at all , finally .....
Went for a trail ride , still success :-)
Let me know if you have any questions, hope this help someone..