Rev limiter (power loss) at 4,000 RPM ?

Hey folks!
I've got an 1983 XJ750 here and I have this intermittent issue with the bike.

The RPM seems to be limited at 4000. When I go above 4000rpm, the rpm just drops and it feels like a misfire, without any backfire. This happens in all gears, including neutral. Yesterday I also noticed black smoke coming out of exhaust (only when the 4000 rpm is applied) which means the fuel is not burning properly, as far as I know.

Example: Let's say I am in 5th gear on the highway doing about 85 km/h at 4000 rpm. I turn the throttle grip, RPM goes to about 4100 and then starts going down FAST, bike is still rolling at the same speed. It sounds like it's misfiring, very similar feel to a car's rev limiter. When I release the throttle a little, RPM goes back to steady 4000rpm.

This issue happens randomly. I can have this happening for 3 days. Then one day I start the bike and it runs perfectly fine to high RPM. Then 3 days later the 4000rom issue stars again.

Oil is new. Carbs are super clean. spark plugs are brand new.

Any ideas?

Thanks

 

valves in spec....??? carbs colortuned ??????

 

I didn't get to the valves yet. However, since it's an intermittent issue like that, I think I have a good reason to believe there is an issue with spark rather than carbs/valves...
Am I wrong?

 

It could be electrical, however a vacuum leak at the throttle shaft seals, or a chunk of crap floating around in the carbs, or any number of other reasons. The best (though often the longest) way is to focus on one system at a time and thoroughly investigate.

One thing that would be easy to check and can often cause similar symptoms is the sidedstand switch. They tend to get gummed up to the point where a bump in the road causes the switch to trip cutting power to the ignition, and then the spring slooooowly works the switch back into place so the igintion gets power again. You cna bypass the sidestand switch by placing a jumper between the two terminals on the harness end of the switch plug. If that solves the problem then you can remove the switch, remove the switch boot, and clean the switch with contact cleaner before reinstalling it.

 

Thanks
The sidestand switch idea came to me yesterday but for some reason I decided that it can't be that. I will check it today

 

Or perhaps if electrical a weak spark that is intermittent caused by a defective ignition circuit. Lots of possibilities here to from coil / wire secondary or caps to low voltage in the ignition circuit from an old fuse box or intermittent high resistance such as in the kill switch or associated wiring. Have you inspected or replaced the fuse box?