Honda 250 engine running question

[vbb71zoso]

This will start and idle fine. Then when taking off it seems to have very little power. As it gets going you can feel the power getting stronger. Its kinda like misfiring in the begining. It has electronic ignition, and the carb is freshly cleaned. This is the only thing I can think of. Since the exaust was rusted very bad, I made a pipe out of a old 650 nighthawk pipe . The original header is being used, I just used the silencer. Could this be the cause?

Any help would be great....

Thanks Vince