Problems with a Polaris Scrambler 90CC

[vt700guy]

I'm working on my brothers Srambler 90 for him and can't seem to get a problem figured out. When you give it gas it acts like it wants to bog down, but if you hold the gas at full throttle it will pick up and run wide open after a few seconds. This gets better after I ride it for a while but never completely goes away. Any ideas on what it might be?

FYI here's what I have done so far:

Drained all the old gas
cleaned the carb
cleaned the filter
new plug