Ok her is the story I bought a 99 Grizzly 600 from a friend that he could not get started. I have worked on it and have gotten it to start. I cleaned the carburetor and replaced the plug. When I ride it I can oly give it about 1/2 throttle befor it starts sputtering and backfiring. When I release the throttle no problems only when I get past 1/2 throttle. I have tried everything I know and I cannot figure it out. By the way I got a great deal on this atv I only paid $600 for it.

 

Sounds like its running lean. Try this...if you pull the choke on, while its sputtering...does the symptoms get better, or worse?
If it gets better, then for sure its too lean. Look at the main jet...it could be too small, or plugged. What is the number stamped on the side of the main jet? Its posible it was setup for high altitude.
Sounds like you got a great deal.

 

Ok my jet # is 145. Also there are 5 positions for my clip it was set at the third one from the top if that helps. The whole choke thing it is raining here and I could not get out to check it.

 

What's your elevation?

 

I live in arkansas so my elevation is about normal. I have fixed my problem and I will let you know the details. I took apart the carb and set my clip one notch higher. I kinda got lucky because I was not sure whether to go up or down. I am gonna check my valve clearance also because I hear a clicking comming from the top end. For 600 bucks you cannot beat the deal that I got. I took it out in the rain and got it up to about 45 in my subdivision so I think it is running pretty good. Thank you everyone for the help.