vertigo

I am hoping someone can help me out. Last year I purchased a Yamaha WR500 real cheap. It was a little taken apart, had to put gas tank and exhaust back in etc... It will turn over and run and you can get about 1/3 throttle out of it than it sputters and won't go any faster.
I thought it might be a carb problem so I bought another carb on ebay and it didn't make a difference. Adjusting the carb screws don't seem to have any effect either.
I bypassed the overheat sensor and that didn't do anything either. Any suggestions? I'm not running a waterbox, I have this little glass pack type exhaust that was off a Kawasaki 550, would that matter?

Thanks
Adam