I recently bought a 1988 kawasaki GT 550 with the intension of using it for a daily commuter and smartening it up a bit. The problem is that when it starts to warm up the sound becomes muffled and acceleration is dampened. The power seems a lot less and some times as i'm giving it more revs to compensate for the loss of power it'll jump back up to normal power suddenly and jerk forward with a burst. But then just as quickly lose power again. The bike will still run, just pulling by. But as i slow or come to a set of lights i have keeping the revs up high to keep it from stalling.

This problem only occurs when its warmed up, i can get about 10 kilometers before it starts to have a problem. It starts fine and sounds great but as it warms it all seems to go out the window. what could it be?

With the sale i also got a write off spare parts bike that has most original parts intact.