Hello all - new member here. I am riding a new-to-me 1988 K100RS Special Edition. Really enjoying this piece of German engineering. 135,000 KM on the clock.

I have been hunting down the cause of a strange idle issue. When firing the bike cold (fast idle on first click), the bike purrs perfectly. After riding for 5+ min and the bike comes up to operating temp some issues begin to arise. It seems that the bike is searching for idle, it hovers around 1000 RPM, only fluctuating very slighly at this speed. It also felt like it wanted to stall, and would stall occaionally while at stop lights. It needed either the fast idle on or some throttle to keep it from dying. It doesn't sound 'sewing machine' smooth as I have heard it described. I have done a few things from the recommedations in this forum which seemed to have helped.

1) Replaced fuel filter (installed correctly)
2) Replaced spark plugs (NGK non-resistive with proper screw on terminals)
3) Replaced plug leads (5k ohm ones from EME)
4) Cleaned all visible electrical connections under tank and grounds for corrosion with DeoxIT.
5) Checked for vac leaks around TB boots, TB ports, FPR hose, crankcase hose, oil filler, hose from air intake to plenum. Used propane and carb cleaner and can't find any noticable issues.
6) Fuel is returning to fuel tank as there are many ripples in tank when running and applying throttle.
7) A few new tanks of 91+ octane gas and a can of sea foam has been ran through the engine.

EDIT:: 8 ) checked valve clearances and they are within spec, none too tight.

This has made a big difference in overall performance but the surging, or unsteady idle still persists.

I then tried following Ned's advice running the bike with the TPS disconnected and I much prefer the ride with it disconnected. I then re-connected and adjusted it so it remains 'open'.
I also have set up a fuel pressure gauge on the tank to the fuel rail and measured fuel pressure. It was bouncing around like crazy around 20-40 PSI. I removed the FPR vacuum hose from TB#4 and capped everything off and now the pressure holds at a steady 34-36 PSI, at idle and over 3k RPM. 

It seems removing the vacuum line from the FPR has eliminated the bike wanting to stall and really dip down when idling. It still tries to search and hunt at idle but it is MUCH better.

Something interesting I've noticed is when idling it holds a rock steady 36PSI but when I turn the bike off (kill switch to OFF positon and key out) the fuel pressure immedialty drops to 0PSI with the gauge positioned between the tank and fuel rail.

Is this instant loss of pressure considered normal if I have the FPR disconnected from the TB vacuum port? Is this a sign of a failing FPR?

I would appreciate any insight or ideas as to how to even out the idle. While it is completely fine off idle and up to redline, I would like to have the idle steady and even.

Thanks guys, you've helped me out so much already!

Sorry guys, like a true noob I posted this to the wrong section. Could a mod please move this to the 'engine' section please?

There is no mention of balancing the throttle bodies.  It certainly wouldn't hurt.  It might also be worthwhile to check valve clearances.  Both procedures have an effect on the idle on my  bikes.

Forgot to mention I have checked valve clearances and they are all within spec (intake an exhaust).

Do you think I can balance throttle bodies given the slightly irratic idle? 

Thanks .75!

note about the fuel; pressure dropping instantly
if an after market fuel pump has been fitted , it may not have the reverse flow valve integral with the pump , like the original has
which may cause this condition

but not to discount that the fuel pressure regulator is not working properly , the seat may have some issues ?
or there maybe a slight leak at the hose unions inside the tank ? which might be worth investigating in any case if original hoses are still fitted .

the next thing I would test is the injectors , for flow and spray pattern

good luck

Welcome.

If the problems are appearing after the engine is warmed up, there could be two potential cuases:
- the Hall Effect sensors under the timing cover may becoming defective
- the temperature sensor in the stand pipe may becoming defective.

My bets are on the HES. Remove the timing cover on the front of the engine to expose the HES plate. Start the bike cold and use a hair drier to quickly warm up the HES by blowing warm air over it. If the engine falters, the diagnosis points to faulty HES.

Ok so on my to-do list is to check the HES and look into balancing throttle bodies. I will pull the fuel rail and check the spray pattern of my injectors as well. I will look at the connectors in the fuel tank. Is there an explanation as to how they would be causing my fuel pressure to drop to 0 instantly when killing engine? 

I did clean the water temp sensor connector with some deoxit and a toothbrush. I measured it's resistance from pin 10 to ground on the ECU computer. 2.5k ohm cold and ~200 ohm at running temp - seems it is within spec. I also have no issue with the cooling fan on the radiator powering on either.

Thanks for the input, will look into those two pointers soon.

Regarding the injectors, how long did the bike sit before you got it?  You mention running a couple tanks through it with Seafoaam and seeing an improvement.  That makes me suspect some varnish in the injectors.  My K100RS sat for a couple years and took about 5,000 miles of running to finally run perfectly.

Normally, varnish screws up the performance at wider throttle settings, but it is entirely possible to be preventing the injectors from closing completely.  This could give a rich mixture which would give a smooth idle when cold, but upset things when the engine is warm.  Also, a leaky injector will bleed off the pressure when the pump is shut down. 

When you pull the injectors and check the spray patterns, pay close attention to any leaking between pulses and when the engine is shut down.  While you are down there, you might want to pull the spark plugs to take a look at them.  Do they all look the same?  Are the electrode insulators sooty or white?

I commend you on the job you are doing in chasing this problem.  When you are done your bike will be at a state of maintenance that will serve you well.  Many times the tendency is to just change the oil and maybe the plugs and start riding.  This approach often overlooks issues that will cause more serious problems down the road, no pun intended.

BTW, as long as you are wringing out all the stuff that makes the engine run, have you checked the air filter?  While it probably doesn't bear on the problem at hand, after 135,000 km it could be pretty dirty and need replacement.

Hey guys, back with an update.

Sync'd the TBs today with a homemade 2 bottle set up (1/8 ID tubing, 2 glass water bottles, 3/8 tubing connecting both bottles)



Idle is MUCH better, so much so that i was able to pinpoint my rubber between plenum and TB#1 has a minor vac leak. Will put that on the winter project list. 

Just wanted to say thanks for the suggestions. Seems to have helped me out quite a bit. I will yank th injectors off and give them a clean (carb cleaner and 9V) and change the o-rings as well.

Two questions:

1) Is it normal that engine will drop in RPM when I spritz the vac leak with carb cleaner? I was expecting a rise in RPM!
2) I noticed I had some exhaust in the vacuum line (I think from TB #3) and I seem to burn a little oil (135,000KM on the bike and it is a BMW so I expect this is 'normal'? Any thoughts?

Edit: .75 - Bike has never sat as far as I know. I picked up in the spring so it did sit for a Canadian winter, but only a few months. I will be taking a look at the injectors as my next project. Spark plugs were a little wet when I pull them, this could be due to the low idle and rich mixture when idling. I also thank you for your kind words - I like to get my machines running well before settling down with them lol.

Nice balancer!  I use a similar setup made with Snapple bottles. 

As long as you are happy with the idle, I would put off messing with the injectors.  If it has not been sitting for an extended time you should be able to get things running properly with some Techron fuel system cleaner and some high speed riding.  Also known as an Italian tune up, it's a lot more fun than messing about the garage during your short riding season.