+1 on the Rick Ritter suggestion. I have bought 2 exhaust kits from him for out 250 trailblazers; he will set you straight on that! Google "ritter cycle". He is in OH or IL i believe.
He will probably tell you to put aftermarket piston in the 400; they are known for breaking piston skirts.
You may also consider converting to premix and using polaris gold oil; we've done that on all 3 of our Tb's and love it.
FYI the 'airbox mod' is simply removing the limiter in the airbox lid snorkel. You can do that before any mods

.
Good luck.
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2012 Sportsman 500, Red.
Highlifter HD Springs all around, Moose pregressors on front (not activated). EPI 'patv1' primary clutch spring, UNI air filter w/ PJ-1 oil. K&N Model KN-303 oil filters. Bikemaster foam grips.
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My ATV-GURU bumpers ARE FOR SALE, front and rear, BRAND NEW. Me a reasonable offer and they are YOURS.
(As you probably already know, they are currently unavailable from TheAtvSuperstore.com /ATV-Guru for an undisclosed amount of time).
http://www.polarisatvforums.com/foru...500-800-a.html