I have an 85 200x, that runs well but at the top of the rpm's it starts to sputter and i feel the power drop off.
The previous owner did an air box mod, put a White Bros. megalloy slip-on on it and may or may not have done a rejet. The C-clip is located in the #4 position, I have not opened the carb to check the jets. Also the spark plug is more black in color. The exhaust does seem to have more carbon build up than I would expect, with this in mind I thought possibly that previous owner was running crappy gas so I up the octane, added seafoam to help clean out the system. Bike does run better but still has the sputter at the top of the rpm range.

thanks for any direction on this.

I did check the timing as well, good there.

 

previous owner never re-jetted after modifying the air box and exhaust install, runs great now.