No Acceleration at 40MPH

[anesde]

I did a search and couldn't find anything so I figured I'd start a new topic. I've been getting to know my 2003 gz250 in the past month and from what I've read about top speeds and acceleration I think something is wrong. On flat roads the bike does not get above 45 even with wide open throttle in fourth gear.

It wasn't running when I bought it but all I did was put a new battery, new spark plug, and dismantled/cleaned/reassembled the carburetor. I did go an extra half turn on the pilot screw and shimmed the needle (not a true shim, just switched the position of the washer per one of the topics on this forum). I also changed the oil to synthetic (bike had about 6000 miles when I bought it). The engine sounds good but power doesn't translate to the wheels.

I weigh ~170 lbs so I don't think its a weight problem. I did take the windshield off but I don't think wind resistance should keep me from hitting 60-65.

At a loss for what it could be - any suggestions?