Backfire is blowing the carb off the intake.
 

I have a 1999 grizzly 600. Twice the motor backfired and pushed the rubber sleeve that connects the carb to the intake off of the intake. I loosened the clamp and put it back on both times. A friend told me to check the spark. The spark seemed weak. My friend said if the spark is too weak, the cylinder loads up with gas. Then when it does fire it will backfire. I replaced the spark plug. No backfire for two days. Then it backfired and pushed the rubber sleeve and carb off the intake again. Any help would be greatly appreciated. Thanks in advance!

What is the condition of the battery? Have you checked to ensure the intake boot and airbox boot have no holes/cracks....and the intake boot is well secured to head?

If the boots are ok, you might clean the carb. Could be a tiny bit of crud partially plugging up the pilot jet causing a lean condition. Lean condition can cause backfires on deceleration. Bad for the motor too.

Battery seems fine. It starts the motor everyday. I haven't checked the boot yet. I worked late. I will check it tomorrow.
Are you saying that the backfire can be caused by a loose boot, hole in the boot, or a cracked boot?

I just purchased this ATV about 6 weeks ago from a small engine mechanic. He said he cleaned the carb about a month prior. The boots could be the cause of the backfire? Haven't had time to check them yet.

Yes, damaged or loose boots can/will cause this backfiring but...it could be a carb issue as well. There are many folks the "say" they "cleaned" the carb, and actually did but, may not have done it correctly...Carbs are a bit tricky to clean correctly.....:eek::D