I need some help! We recently got a 98 grizzly 600 quad, and when we started it the first time i knew the timing was off. i fixed that and it started up good the first time, but when i shut it off gas started pouring out of the carb from the overflow. since then it hasnt started. it turns over and occasionally backfires. i checked the spark plug and it has good spark, then i rebuilt the carb. still doesnt start. what could it be? please help, this quad is starting to really frustrate me!!!