the bike runs fine as long as you are running it wide open throttle. when you try to run 1/4 to 1/2 throttle it dies then starts up and runs fine when
my son goes back to wide open throttle.

sounds like a clogged pilot circuit. take out the pilot jet, and try to look through it, if you cant see daylight, then get a new one and give it a shot. if you can see through it, then it could just be really really lean on the pilot. did it just start doing this or has it done it all along?

Try running a 45# pilot jet should clear it up and if you want more bottom end you need to change gearing I ran a 13 front and 48 rear on my sons 85 and it made a big difference compared to the stock 14-47 sprockets.

thanks for the replies.  the pilot jet was plugged