I would highly recommend running some
SeaFoam through the crankcase, considering your comments about the old oil, and how the engine's performance improved with an oil change... You likely have a lot of gunk and deposits within the engine and the transmission. I've used it on all three of my bikes, and three of my four-wheeled vehicles, without any issues. It has not affected my clutch on any of my bikes, so don't worry about that.
The directions can be a little confusing, but it is 1 oz. of
SeaFoam per quart of oil. Check the color of the oil before adding it, then add it in, run the bike on the center stand, for five minutes, then check the oil's color: if it darkens (it will likely turn it black), change the oil, and filter, immediately.
SeaFoam will dissolve carbon, and varnish deposits within the engine, and the transmission. Some will be caught by the filter, the rest will be suspended within the warm (operating temperature) oil. After treating it, your engine and transmission won't need another treatment for 10,000+ miles, if even then. If you use a diesel oil (much higher levels of detergent; no friction modifiers to affect your clutch, either), with regular changes, you should never need another
SeaFoam treatment in the engine.
SeaFoam will also dissolve varnish in the gas tank, and the entire fuel system: 1 oz. per gallon of gasoline. It will also stabilize gasoline for up to one year. For fuel issues, add it to a full tank of gas, run the engine for a few minutes to circulate it through the system, then shut it off for at least a day. After that, run that tank as low as you dare, before filling up. Good luck! Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII