I was not able to sync the rear throttle body and found an air leak. When I first saw the intake manifolds, I thought there was an o-ring but I was mistaken. I haven’t found a source for these other than over priced NOS or used ones more dried out and cracked than mine. Any ideas?

Are you talking about the TB bushings that are screwed to the engine block? Is this where the leak is occurring? If so, refit the bushings and use RTV gasket maker between the engine block and bushing.

It’s that simple? Alright, no problem. I’ll refit them with RTV. Thanks!

whilst doing that , the head mount bushes harden at the head interface . and often distort especially at one end where the castings detour around the surface

smooth off the head mount surface of the rubbers  by rubbing them over a wet and dry covered firm flat surface until you get an even appearance

good luck

Right, I can clearly see where the old surface is squished over. Thanks for the tip!

That worked out well. I used 600 grit wet/dry sandpaper on a flat surface to clean up the mounting surface if the TB intake boots and a little RTV. Clean up the throttle bodies and everything bolted up. Thanks again for the advice!

However, the air box boots are dried up and cracked. Found a source for new ones:
https://www.ascycles.com/BMW-Motorcycle-Parts-Fiche/K100RS-1985-89?id=51752&catID=11&catname=Engine#PartsTable

She fired right up on first attempt. Next week, sync throttle bodies, adjust throttle position, etc.

Everything works great except I can't get the idle to rest on the idle stop screw. Idling around 1200 to 1400 rpm. It seems (after taking it apart for the 5th time) that the choke/fast idle is preventing the throttle from settling all the way down on the idle screw. The choke mechanism appears to be resting on the its stops, but the throttle shaft components are still resting on the fast idle cam just enough to crack it open up to 1200+ rpm. Easier to explain with some pictures, but I can barely get my hand in there... all this stuff sits between the throttle position switch and pressure regulator. At least I know what the problem is the next time I tear it apart.

Try loosening and adjusting the throttle position sensor. Sometimes that will hold the throttle open. Also check the positioning of the hose clamps below the TBs.

TB clamps should be positioned like this:



One thing I had to do on my actuator was this:

Thanks for the tips. Triple checked all that over the last couple weeks. Clamps, Throttle Positions switch, etc. Quadruple checked it! Made sure the fast idle/choke wasn't interfering. Cables all have plenty of play. Re-sync'd the throttle bodies again and got it to idle a bit lower, but still idling around 1200 to 1300 rpm. 


When it's cold, the bike idles just fine, but as it warms up the idle goes up. Could be temperature related??? I ran into another K-bike enthusiast at the motorcycle club and he suggested the air intake sensor. Check connections, etc. Temperature gauge works well. Fan comes on while sitting in traffic. It all appears to be working as it should just idling too high.

I now have the tank off again, so I'll clean and dielectic grease all connections.

jahearne wrote:When it's cold, the bike idles just fine, but as it warms up the idle goes up. Could be temperature related???

Check the coolant temperature sensor. A test to check its operating range can be found within the Tech section of the site's Portal.

It will aid those who are helping you to troubleshoot if you take the time to put your bike's model and year of manufacture within the bike* box in your Profile. If you do that, your bike's model and year will show up with every post you make. Click the Profile tab at the top of this page. Parts and function can differ among models and model years.

When you balanced the throttle bodies did you check how many turns out the air screws are from being lightly bottomed? 

I have never found a spec for the correct setting so I start with the reference throttle body air screw backed out 1 1/2 turns and balance the others to it.  If the idle is higher than I like when I'm done, I go back and set it for 1 1/4 or 1 turn and rebalance.  That usually gets the idle down to where I want it.

There is also an idle air bypass on the airflow sensor, but I leave it alone, mostly because it's a pain in the butt to get at.

Laitch wrote:

jahearne wrote:When it's cold, the bike idles just fine, but as it warms up the idle goes up. Could be temperature related???

Check the coolant temperature sensor. A test to check its operating range can be found within the Tech section of the site's Portal.

It will aid those who are helping you to troubleshoot if you take the time to put your bike's model and year of manufacture within the bike* box in your Profile. If you do that, your bike's model and year will show up with every post you make. Click the Profile tab at the top of this page. Parts and function can differ among models and model years.

Thanks, I'll check the temp sensor. My bike 89 K100RS SE is listed in the bike box, but not showing up.

Point-Seven-five wrote:When you balanced the throttle bodies did you check how many turns out the air screws are from being lightly bottomed? 

I have never found a spec for the correct setting so I start with the reference throttle body air screw backed out 1 1/2 turns and balance the others to it.  If the idle is higher than I like when I'm done, I go back and set it for 1 1/4 or 1 turn and rebalance.  That usually gets the idle down to where I want it.

There is also an idle air bypass on the airflow sensor, but I leave it alone, mostly because it's a pain in the butt to get at.

I started out at about 1 1/2 turns and ended up to right around 1 turn. I'll look into the air bypass, but do I really want to mess with it? Probably not since it's a pain to get to. Have to get to it while the bike is running I assume.