it wont run!!!!!! got spark got gas, got compression. when she died, i tried to restart and nothing but a backfire anybody got any hints? thanks

 

How are you determining you have compression? If you're just using the old thumb over the spark plug hole then it may still feel as if you have some compression but there may not be enough to fire the 400. Mine acted the same way when the stock piston finally died.
It might be something else like a bad plug but if you can't find anything else try to get your hands on a compression tester to rule that out.
If you're not cranking at least a 100 it's probably your piston.

 

I also had a few backfires on my 2000 scrambler 400. It was because the piston had broken off at the port holes in the back. You may be getting a little compression, but with part of the piston missing, you won't get enough to fire her up. If so I recommend a Wiseco Piston, they are forged aluminum, the stocker is a cheap one. I have rebuilt my engine 4 times over a 2 year period. I now have it bored to .160 over which is the biggest piston that I could find. I also have a fmf fatty pipe and turbine core silencer, 39mm flatslide carb, hotseat hypershift clutch kit, v-force 2 reeds, and hotseat seat cover. I like my polaris but unfortunately I am very famalier with rebuilding the engine and the bottom end. If I can help you in any way, just let me know.

 

GUYS THE WIESCO PISTON DONT BREAK BUT THEY WEAR OUT THREE TIMES AS FAST. STOCK PISTON CAN AND DO RUN 2000 MILES AND THEN SOME. BUT THATS ONLY IF YOU KEEP AIR CLEANER CLEAN . IT MUST BE REPLACED EVERY 100 MILES AND SOMETIMES SOONER IF CONDITIONS ARE VERY DUSTY. I HAVE OWNED 8 POLARIS 400 TWO STROKES AND I RIDE 200 MILES PER MONTH ON AVERAGE