So I have a 2003 800 RMK. I have a heel klicker primary clutch kit, MBRP can, SLP cheater head(50/50 mix 90 octane, 110 octane), big wheel kit, and an ATACC. The sled idles fine and is running awesome on the trail, but when I start tearing around in any deep powder it starts to sputter and lose most of its power. Does anyone have an idea of what may be causing this???

Just a quick answer but have you had the carbs out lately???? Check for water!!!/snow ingestion.

Yeah I would check the carbs then the airbox/filters, if you run prefilters, clean them out really good, make sure you are not getting any snow in your intake tract. Do you have the vents in your hood backed with prefilter material? If not, you should try that, helps a lot.

Is this a sled you just bought, or one you have ridden before? If new check the gears they may not match the track. Check that your exhaust valves are clean and move free. My 800 seized a valve in the housing, it ran like junk when under a load but on a trail it was unnoticable. Also you might check your clutches to make sure they are fully engaging. Mark a shiv on each clutch with a permanent marker outside to inside. If the line disapears clutching and gearing are correct ,if not you are not getting full shift. Going through this same problem with my 800 right now. What a pain!!

Last but not least those 8's were known for crank seals.....compression check and visually check your PTO side piston and cylinder. Is your jetting correct for the pipe, can and head???

I would make sure that your exhaust where it comes out under the sled still has a sheild to deflect the snow so it can breathe in deep snow. If it doesn't then the pwoder is choking it out possibly?