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It appears as though i have the same issue. I went through the regular checks of the usual suspects; Cam chain was in optima forma, tensioner was functioning correctly and the valve clearances were O.K. The direction of the noise indeed pointed towards the primary chain. I do not understand however, how a rattling primary chain can cause power loss above 3000 rpm.

Help!!! I have the same thing going on with my bike. Did you ever find the solution?

Robvolle wrote: Help!!! I have the same thing going on with my bike. Did you ever find the solution?

bad news my friend, i had this exact same issue with my 1979 kz440 and we were diagnosing it and running the bike, it too lost power just as described. had a stock wiring harness and battery and everything, unfortunately my connecting rod let loose and blew my motor apart. i would definitley not run it anymore and check the cylinder walls with a scope thing.

i just read this "i know 6 years late to help the poster" but had all the same issues then today un earthed that the crank balance chain had snapped not sure if its the route of all my running issues but 100% where all the noise was coming from !

I think I posted..apologies if i double posted..I may still be in newbie purgatory...

Anyway, mine makes the same noise sometimes.. It happened to be the carburetor choke plates flapping in the bore.
Lift the seat, unscrew the air cleaner, and start her up. Use your phone's camera to see if your plates are flapping in the breeze.