High RPM power loss

[RuppRanger]

Hey Guys-

Ive recently been doing a bit of spring maintenance on my 1981 CB750 and have had a High rpm power loss issue that I have been struggling to get my head wrapped around. Checking the valves I found that the clearances although not perfect were suitable for enough so I moved on to the ignition timing, as it turned out after doing a static alignment I found the CDI was a little out of sync. after correcting this I started the engine and saw an immediate difference in acceleration, top end performance and reduced backfire. It was night and day how ever after about 5-6 miles I found the bike was going to back to its old way of losing power around 6-6500 RPMs, on occasion catching power but generally running real sluggish. Upon getting home I pulled off the cdi cover and checked it and found all was in line as I had adjusted it.
i did change the oil at the same time as this adjustment but I don't see how that would change things much. I am about past my trouble shooting capabilities so any input would be very appreciated. Let me know if you are curious of any other details about the bike. Thanks!

[Re-run]

Seems we missed this post at first.

Anyways, Were clearances tighter than spec? That would cause problems once everything warmed up all the way. Tight valves are bad. So they must be within spec.

What is your air intake like? Is it the stock box and filter or are there pods on it?

I would pull the plugs and see what color they are. it would give an indication of either too lean or too rich.

Also, how old are the plugs? Are they a few years old? Might be time to replace them just in case. NGK works best in these bikes.

These are some of the easiest things to start with. And might give us the direction to go.