i had kind of the same issue this weekend. it was my first real ride since i purchased my 99' sport 400 a month or so ago. i replaced the spark plug 2 weeks ago. after riding for a couple hours i got stuck and turned the quad off till my buddies got done goofing around and then when they came to winch me out my quad wouldn't start-i purchased a new plug today and it fired right up. from what i understand from a few different people is you really can't just putt around in the woods on a 2-stroke you have to open up the throttle every so often.

 

I checked the compression, and at 125, the owner's manual says it is OK. Replaced the fouled Denso with a brand new NGK, drained the old oil and replaced it with Amsoil Dominator 2-Stroke, and it fired right up. I too have learned that you ride a 2 stroke differently than a four stroke. I bought a spare plug, cause I am sure I'll need it. At $2.99 it is going to be expensive to ride this thing. Tomorrow night, hopefully, I can get some seat time and flog her really good.