Yes, following my rich running problem back in July (it turned out to be a blocked return pipe fitting on the tank causing excessive fuel pressure) I now have a fuel starvation problem!

1983 K100, no modifications and all seemingly fine. After fueling up and 100km, including 110kph hwy cruising, it partially lost power. I stopped the motor and could see no problems, restarted and all good..... for another 20 km.
Again the bike slowed, felt like less than 1/4 throttle but cable fine. In neutral I blipped the throttle a few times and normal power returned. This continued for another 50km but throttle blipping no longer effective. Pottered home on what felt like 50cc.
Each time no rough running, no missing, still would Idle smoothly. I removed spark plugs, all show lean mixture, all 4 the same. Fuel pressure a steady 32psi.

If this had a carb I would diagnose blocked main jet! Fitting a new fuel filter has made no difference. All hoses are good and free of kinks. The tank vent works fine so not a tank vacuum. 

Obviously the fuel mapping would not allow such a drop in performance regardless of temperature sensor. Can I only suspect the injection control unit as faulty? Suggestions please

Air flow meter?

daveyson wrote:Air flow meter?

+1

Air leak down stream of the air flow meter?

Is something causing fuel starvation.

Fuel pressure regulator?

Fuel pump?

Fuel filter?

The simplest measure would be to disconnect the Jetronic plug, clean its pins and sockets with DeoxIT, reconnect it and verify that it cannot be loosened.

The same might be done for the ignition control unit.

Have you tested the units with a multimeter using the troubleshooting guide data?

All good suggestions thank you. I will try working through the diagnosis methodically. 

The only clues I have now is that the throttle blips seem to have had an effect (maybe "exercising" air mass meter?) and a rest seems to restore to normal (so possible heat related).

After changing fuel filter it appeared fixed on my 50 km test ride. Next day I was 80 km from home and it slowed to 110kph max and again I rode slowly home.

The return line from the FPR can collapse where it enters the tank especially if non OEM on hot days or in traffic. Also check all the other fuel lines for connections and kinks.
Regards Martin.

could the clip around the air filter box piping to plenum have been loose and sucked in one edge of that hose ?

or partially blocked injectors - fuel rail , after that return line failure 

if it could be injectors , possible to remove them and clean the filters on the fuel rail input side ? with out too much drama .

just a couple of suggestions

Again, thank you. 

Surely a return line blockage causes a rise in fuel rail pressure? (as I experienced with a blocked fuel return spigot at fuel tank). Fuel pressure appears to be normal at approx 32 psi

Of significance is the smooth idle even when in fault mode. All four pots equally lean.

I will have a look at the wiper in the air mass sensor.... Maybe it only records a fraction of the actual air flow and fuels to suit. I should be able to test resistance over the whole range of the flap movement. Or as suggested, high resistance on the air mass reading reaching the control unit.

Thinking out of the box,the ignition control unit under the tank might be the problem old age+heat can cause strange gremlins,worth a look/test to be sure

Well Laitch, I did as you suggested. The earth from Air Mass Sensor and Fuel control unit was a little high resistance. A little over 0.5 ohm, dropping to 0.2 after a thorough clean (tested at multi plug). 

Charlie has an uncanny insight with the suggestion that the hose clamp between air box and plenum might be loose! However, the hose was a tight fit but maybe not so when hot?

I have cleaned all significant connectors as again they can change considerably with heat. I need to test ride but first failure was 100km from home, second about 60 km, so an extended test is called for (though I am not permitted to leave the state  )

A 400km round trip today, including a couple of hours at motorway speeds, presented no problems.

Hopefully all my connector cleaning had chased the gremlins away. Time will tell.

Haha, nope. 150 km today without problems, then once again severe power loss. 
I had a spare injection control unit with me. Swapping it made no change so at least I know what is not the problem.

Maybe attach a fuel pressure gauge to the dash so you get a reading when the problem occurs.

There might be a pattern, like if it only happens when the fuel level is low, or a certain time after refilling, or some other pattern. I gather opening the fuel cap makes no difference, or squeezing the return line.

Fuel return flow and the vacuum hose would be worth checking. Something about hoses makes sense, like Martin mentions.

1983 K100...

Original coils still on it?  You would not be the first to have coil problems.  Coils can also have temperature related failures, failing when they get hot just like the Hall sensors and other electronic stuff. 

The spark has to overcome the pressure in the cylinder.  A plug that sparks when out of the engine may not spark when it is confronted with a fuel mixture under high pressure.


As the throttle is opened more air enters the cylinder and the pressure at the end of the compression stroke increases.  This higher pressure makes it harder for the plug to spark, igniting the mixture.  A weak coil, plug wire or spark plug will sometimes work okay at idle, but fail at wider throttle settings.

Have you got it fixed now?

Just thinking out loud, plugged fuel tank vent, pulling vacuum on longer run?

Thanks guys. To be honest I have neglected the problematic bike. I know it must be "something simple" but it is frustrating.
I have another K100 parts bike in the garage and I am working to get that running and back on the road. Then I can swap a few major components to try to isolate the original intermittent fault. (I already tried exchanging injection control unit, next the air mass sensor)

Oh, and I checked the fuel tank vent! 

Meantime I have been enjoying the old R65 Airhead... The joys of simple Bing carburettors

Try swapping the coils from the parts bike. What year is it?

I have an 84 K100RT, had same issues after doing some things to the bike. Front left tank connector plug turned out to be the cause. Poor connection causing erratic problem as you mention. Another time it returned and turned out to be the big plug connector into the FICU unit under the seat, the receiver on the unit for the end of the plug developed a crack. Changed it out and solved it, then I repaired the crack and it turned out to be good to use again.