My son saved hhis money and bought his own 4 wheeler. Idles real good and revs up to low range and medium range, but when hitting upper mid range and high range rpms it starts loosing power and sputtering. Plug comes out black and suity. Any suggestions. Has good compression. Thanks for looking  

cutting out is a sign that the fuel mixture is too lean, while bogging down is a sign that the mixture is too rich.

i would mark where the fuel/air mixture screw is setting now with a sharpie, and in 1/4 turn increments adjust it to the left.
let it run for a few minutes between 1/4 turns and check to see if it is running better.

keep up with how many 1/4 turns you make so that you can take it back to where it was originally and try the other way if no improvement to the left.