Rotation undulation, loss of power

[perfekt08]

Hello everyone. I have a GL 1500 from 1989. After a few years of riding my bike started losing power for no reason. Symptoms: no power, the tachometer enters the red zone. After the next firing, full power returns and unexpectedly the failure repeats itself. I checked the Hall sensor, ignition coils, engine control unit, fuel system - everything works. I'm running out of ideas where to look for the cause of the problem.
Maybe someone had a similar problem, all help appricieted. English isn't my tongue lounguage so sorry for mistakes.

[WingsandThings]

Could it be the clutch slipping?

You say after the next firing it is ok for a while but then fails again. Do you mean if you shut the bike off and start it up again immediately it is ok, or is it afer it cools down it is ok for a while.

What type of oil are you using, some engine oils have friction modifiers in them and will cause clutch slippage.

If your tachometer enters the red zone the problem would seem to be in the drive train not the engine.

[perfekt08]

Thanks for the answer.

It is definitely not a slipping clutch, because when the failure does not occur, there are no problems with the ride.

The problem occurs both when the engine is cold and when it is warm. The fault manifests itself many times during firing - when the ignition is turned on, the engine speed indicator "jumps quickly a few millimeters at a time" and I already know that the motorcycle will not have power and slightly unscrewing the throttle causes the tachometer pointer to max out. Doesn't matter warm or cold engine.

If the indicator does not "jump" when the ignition is turned on, you can start it and drive even several hundred kilometers until the fault appears suddenly, for no reason. Then you can clearly feel the lack of work of the engine cylinder on the right side.

The engine is flooded with oil dedicated by Honda.
Could the failure be in the electrical system?

[WingAdmin]

Is the tachometer just going up, or is the engine actually racing?

[perfekt08]

When gently unscrewing the handle, the tachometer goes to the red field, while the engine does not rapidly increase the speed. I noticed that in this "emergency" mode the exhaust gas is very irritating, as if complete combustion is not taking place.

As soon as the key is turned to the power position, the tachometer pointer jumps higher then normally. After engine is fired up it operates uneven and lacks of power. There is a ripple in the 700 to 1500 rpm range. After gently unscrewing the throttle handle, the pointer swings sharply towards the red field while the engine does not rapidly increase rotation. You can drive it, but there is no power.

On the other hand when all works, after turning the key to the power position the tachometer does not jump up so much, then after firing up the tachometer shows about 800 rpm and you can drive - full power; smooth revving. You can drive a lot of kilometers and nothing happens, but at some point, without reason, the engine enters "emergency" mode, the tachometer ripples and the engine loses power.

[WingsandThings]

It may be that you have a problem in the wiring that goes to the tachometer. Possibly a short or a pinched wire somewhere.
I think i read that on the 89 the tach is connected to the negative side of one of the ignition coils.
I would try and find a wiring diagram for the 89 that shows the wiring for the tach
and try tracing the wires looking for possible damage.
You may be able to check for continuity to ground on the wires at the plug going into the tach, but it would have to be when the bike was in "emergency mode"

I am not very good at the electrical stuff though, so you should wait for a second opinion..

[perfekt08]

Thank you for your answer. I'm following the trail of a short circuit with the tachometer. I'm looking for a diagram. I'll let you know after the review.

[perfekt08]

The engine started to work properly, so it was difficult to find a short circuit.

I started by disconnecting the plugs and inspecting them. The harness that goes from the tachometer side to the set of plugs on the right side above the fan was very dirty, and some of the plugs were not fully fastened. I cleaned and reassembled them. I cleaned the buttons from the cruise control and radio also.

After warming up engine I adjusted engine idle. A lot of water blew out of the muffler on the right side. Water also poured out of the hose under the carburetor.
Today I was on a test drive. The engine runs at full power, the tachometer pointer does not move when the ignition is turned on as before. Drove almost 300 miles and plan to test the motorcycle extensively. I’ve ordered wiring diagrams for this model year hope it will be easier to trace where the cables run.

It should be clear by a week whether the contacts were the cause of this failure.

[perfekt08]

End of pleasure. Today the drive failed.

After firing, nothing indicated any problems. I drove about 10 miles and while driving suddenly the tachometer pointer was in the red zone and the engine started to jerk and lost power. I downshifted to second gear and slowly drove home.
While driving, I tried to change gears and in third gear the engine jerked, increasing and decreasing power.

I have no idea where to look for the cause of the error.

[WingsandThings]

When the problem occurs and you have a loss of power, try and test for spark at the spark plugs. See if it is firing on all cylinders.

[WingsandThings]

This thread should help you test the wiring to the tach and there is also wiring diagram

viewtopic.php?t=37219

[perfekt08]

Thanks @WingsandThings I checked wiring and followed the trail of the ECM module.

I bought two used ones, one from 1989, the other younger.

After installing the older module, it was even worse - the engine was not firing on all cylinders and the tachometer was waving.

I connected an ECM from a younger model, which has a slight difference in wiring. It is missing one plug - a single one with a brown cable (see attachment).

After starting, I observed the behavior of the engine, which runs smoothly, the tachometer maintains about 900 revolutions. After warming up, the engine turns on the fan and all indicator lights work.

After driving about 500 miles, the engine runs smoothly, acceleration is dynamic, the tachometer now maintains equal and appropriate revolutions for engine operation.

I sent the original ECM module from my motorcycle for repair. The initial diagnosis is a damaged 2SD1071 transistor. Whether it can be repaired - time will tell.

I am sending a photo of the dismantled controller and asking for your opinion on whether the motorcycle can be used without this single plug.

[WingsandThings]

Glad to hear you are making progress.

Unfortunatly I do not know what the extra plug in the picture is for and I can't find any info on it. Hopefully someone here will know.