'84 K100rs sometimes stalls when gearing down

[JourneySA]

Hi Guys,

my '84 K100rs has started stalling when gearing down sometimes (4th to 3rd / 3rd to 2nd) and then starts up immediately as if nothing happens. I've done some research and suggestions range from a basic service needed (filters, plugs etc) to idling speed being too low. Has anyone experienced this?

also - what is the correct idling speed en how do you set it?

Thank you!

[The Mighty Gryphon]

When coming to a stop after running at high speed my K100RS 4v has a tendency to stall.  This usually happens at the stop at the end of an expressway ramp.  Bike always starts right up.  Otherwise, the bike runs flawlessly, smooth with lots of power, torque, and perfect throttle response. 

I haven't any idea why it happens, but with the bike running as nice as it is, I am reluctant to screw around with anything, especially since it had not been used much before I got it, and over the past two years and 8,000 miles I have seen a noticeable improvement in engine performance.  As a result, I will just be patient and see if it corrects itself.

As far as the idle speed, I am not sure what controls it.  I just run the occasional shot of Techron through the system to clean the injectors, and keep the throttle bodies balanced with the air screws about 1 1/2 turns out from lightly closed.  That has given me an idle speed of approximately 900-950 rpm on a warm engine.

[kennybobby]

Here's something you might try:

Using a pen or marker or file, put an alignment dot/file mark on each brass throttle-body bypass screw and housing, then turn the screw in until snug and record the number and fraction of turns.  Now remove the screw and clean out the orifice with a cotton swab or towel and some carb cleaner spray, etc. (don't clean off your alignment mark).

Put the brass screws back in and align the markers, then back them out 1/4 turn and go for a test ride and observe the idle speed, stalling, etc.  If speed is still too low back the screws out another 1/4 turn and ride again.  Once it is idling too fast, turn them in 1/8 turn and repeat, adjust +/- 1/8 turns as needed to get idle where you want and no stalling.

[johann.muller]

Hi


Test what happens when you blip the throttle while standing still. If a modest blip of about 5k rpm stalls it on a quick close we probably dealing with mixture. My 83 has been stubborn about this forever and I manged to get it better, but never 100%. I blame a slow ECU and 33 year old parts. (and a stubborn home mechanic)


First thing to check. Throttle close switch. You should hear a clear click at the throttle body as the throttle closes. This switch is used by the ECU to cut fuel when throttle is closed about 2000rpm. You may get backfire if the switch is not working. Next upping the idling alleviates the problem slightly, but you may need to move the close switch I just mentioned to make sure it still clicks at throttle close. I run 1000 rpm idle when warmed up properly.


Now onto the slightly higher grade stuff, where you may need access to workshop tools. The old debugging order for a mix issue / tune-up issue remains valid. Timing, Valve clearance, then Fuel. Doing fuel system adjustments without the previous ones, will probably just be frustrating.


If you want to attempt it. Follow the haynes manual instructions for balancing the throttle bodies. The cleaning suggestion by another member is valid. Mine was covered in 30 year old muck and one of the 4 needles used to adjust the air mix was damaged. Luckily I had a junk throttle body to strip. Do not set the linkage screws to balance. Only adjust the brass mixture screws using your balancing tool. This will get things better, but you still need to have your ECU air bypass set. For that you will need access to a CO meter. (this is where I am in the process, I have done the timing, valve clearance, balancing and speed)  My current bypass is set according to the 10m highish RPM and read spark plug method.


Good luck.

[John Lang]

FWIW, Haynes, p.146, provides idle adjustment procedure.  For the K100rs, access the adjuster by removing the left knee-pad panel.

[K1300S]

need to baseline the tune for any driveablity issue.   TB balance, idle switch, vacuum leak test, mixture, idle adjust, etc.  this should be first step, always.  unless these parameters are checked and set correctly, diagnosis is a random guessing game.

[johann.muller]

The poor secondhand engine that went into my rebuild project was sadly out of the whack to begin with. 3 Valves needed shimming. 2 Exhaust, 1 inlet. The timing was pure guesswork, setting to the Haynes manual "this should at least run" setting. The pickup was missing, so I had to use the one from my busted engine.


I rode the bike for the first year, before sorting out the timing. Our local BMW agents no longer had the tooling. Finally got frustrated and found instructions to build a sensor box. Got the timing dialled in perfectly. Balancing the throttle bodies resolved the slight idle miss. I must have balanced those 3 times before with no luck.


The sudden death at down gear, especially from high RPM was with me for that first year. I can still cause it to happen by blipping the throttle to high RPM 1 out if 3 times, but it now never happens when riding. I suspect my mix is not exactly right. I think I have eliminated most of the other causes. Fuel consumption is good, power curve is smooth. So I have not really felt the need to delve further just yet.