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1985 K100RS, rough running


New member
1985 K100RS- 260k miles, but in great shape, except...

I love this bike and have devoted a lot of time in trying to keep it running:
- Did a complete engine tear down to track engine rattle. Separated tranny from clutch, removed bell housing from engine, opened engine to inspect output shaft (6 rivet). Shaft was in great shape, as was the rest of the power train. Clutch friction plate was above 5mm. All splines were in excellent shape (all got lubed with Staburags). You might have seen my posts on K100 Facebook groups.

- Bike is running poorly. Cold and warm starts are rough and engine sputters. It does run but just not well (this was the case before my tear down). Here are the things I did to specifically address this issue:
- new Z Hose
- new fuel filter
- swapped Mass Airflow Sensor
- new plugs, new plug wires
- measured coil resistance. All 4 measurements indicated that coils were not dead
- measured resistance of injectors. All were at 16 ohms
- injectors were cleaned 8k miles ago by Mr. Injector
- spray pattern test showed even spray patterns for all four injectors
- idle adjustment procedure doesn't change anything
- throttle body rubber intake hoses were closely inspected and cleaned. No cracks were visible.
- new HES Hall Effects sensor. Timing was set with HES LED sensor device from EME.
- new timing chain installed
- valves were checked and adjusted as needed
- fuel pressure was checked. 36 psi on left fuel hose but needle on the pressure meter was bouncing erratically. Is this an alert?

The insides of the engine were like new. Amazing. I have ridden this bike for the last 10k miles. Original owner did meticulous maintenance (I have all the records since new). It shows!

What am I missing here? I am pretty stumped...
Thanks for your answers.
RE the rattle you were chasing—was it the classic “can o’ rocks” rattle at idle? And did you reshim the gears inside the intermediate housing to eliminate the side slap that so often causes that rattle? Always wanted to dig into that on both my brick-k bikes but it’s a huge project just to chase a rattle at idle. Kudos to you for being willing to go after it!

On the rough running, it may be time to have a long hard look at the throttle bodies. O-rings on the air screws and throttle plate shaft can wear or crack, etc. And, it’s possible that a previous owner has tweaked screws and left things unbalanced. I don’t know if anyone offers a rebuild service on the TBs and there are few parts listed as available from BMW, but I do think they are a possible source of the rough running.


PS: You can check the TB shaft seals by spraying some propane or other flammable hydrocarbon near the TB shafts at idle, with all the intake plumbing intact, and see if the motor reacts…