make sure the auto decompressiopn is realeasing, the problem should be in the head (valvetrain) , starter motor, or stator. that the only things, other than the odvious bad gas as JT already said, i wouldnt use the injector clean stuff i would disassemble the carb blow out the jets with compressed air, clean everything up with B-12 carb cleaner.
For better help on the subject check out the forums on
www.atvriders.com thats the largest and most helpful forum on the net for atvs. My username on there is the same as on here.