K100 1983 power problem

[PiotrK100]

Here is the situation.
Never ending story with rebuilding for cafe racer, many small issues and now I stuck with one problem. Bike runs smoothly, but over 100-110 km/h does not accelrate and engine does not gets hight rpm. Feeling like someone keep the bike from rear side.
But, what is weird that from time to time bike resores full power and can get 140km/h easily. I checked spark plugs, commpression test and changed fuel pressure regulator. Nothing. Have no idea where to looking for because nothing reasonably commes to my mind.
Any suggestions?

[E30_Crazy]

Could be a few things, time to start testing, inspecting and cleaning.

- Loose and/or dirty FI computer connector.
- Sticky AFM door.
- AFM connector.
- 4-pin connector under front-left of the fuel tank.
- Fuel pump may be on it's way out, or getting partially/temporarily clogged.
- Fuel filter and lines in the tank.
- Dirty injectors. 
- Any number of vacuum leaks from throttle body rubbers, crank breather, etc.
- Check/clean common grounds under tank, if any grounds for the various sensors are getting interupted, it could be messing with you AFR as commanded by the FI computer.
- Wouldn't hurt to check all fuses, relays, and their pins/spades. Try swapping in a new fuel pump fuse, I've heard stories about some fuses appearing good, but not carrying the required load.
- Check your alternator, voltage regulator, and brushes, you could be dropping voltage, or over-voltage.

[Laitch]

Along with E30's laundry list of the usual suspects, it might be possible that the clutch is intermittently slipping. If there is no evidence of oil at the transmission's weep hole, review the clutch's adjustment. Did this moto get a new clutch disc? The current disc might be worn out. The pressure plate or spring might be failing. It is unclear to me whether you've noticed a rise in the revs during the times when you were trying to accelerate but failing to do so.

[PiotrK100]

Thanks.
Clutch is not the issue because I can reach allmost 100km/h on second gear, and engine is not having higher rpm.

[Laitch]

I can reach allmost 100km/h on second gear, and engine is not having higher rpm.

Just to clarify, you assert that the problem happens after 100km/hr. I'm indicating that a clutch can hold until that speed and slip after that speed in any chosen gear and also slip unpredictably for a couple of reasons. If you've eliminated that possibility, E30's list gives you plenty of other choices.