My Kawaksaki 300 bayou 4x4 is not running right, please help!

It starts just fine. But when in about 2nd and 3rd and 4th gear, it doesnt have full throttle for some reason, the more I get into the throttle, it stays the same speed and even kind of bogs down. When its in nuetral and I rev it, it does the same thing, it will bog down for split second then rev all the way up. Anyone know what the proble might be? Any help is greatly appreciated.