Flat power band after 4K RPM

[P41N]

Hey there, I just picked up my new to me 91 K100RS and I was putting it around town and notice a loss of "power" or falls flat after 4000 RPM in every gear. it still accelerates but not at the same rate.

Im not really sure where to start. I did search the forum but didn't find much about it except it could either be a throttle sync or maybe a fuel filter clog? any help or direction would be much appreciated!

[The Mighty Gryphon]

Have you replaced the fuel filter?  NAPA Gold 3032.  Not real expensive and a good idea.

Have you checked the inside of the air box for a rodent dumpster?  How dirty is the air filter?  They can last a long time, but it's worth checking. 

How is the Hall Effect Sensors plate slot aligned with the cutout in the timing chain cover? 

How old are the spark plugs?  They aren't that expensive to replace.