1985 K100RS throttle bodies tuning

[walt3022]

My low-miles K100RS has newly adjusted valves, rebuilt fuel injectors, has no air leaks in the induction system (at last!), a functioning air mass sensor (near as I can tell) and a clean air filter. At idle, the engine runs roughly at 800 rpm +/- and the exhaust smells heavily of unburnt hydrocarbons. The air bleed screws (those brass items on the throttle bodies) don't seem to have any effect on the idle. Their o-rings are in good condition, yet playing round with the screws doesn't make a difference on the vacuum meter's reading or the idle. I'm wondering what the baseline setting of those screws should be. They are all presently set at one turn out from being fully seated.
The throttle butterfly valves themselves seem to be in synchronization and show no sign of ever being moved from the original factory setting.
TIA for any insights.

 

[deilenberger]

I see you have a way of checking the TB balance. Carb-sticks, or some other means?

I'm also assuming you've checked the crankcase breather hose (the Z shaped one to the rear of the back TB..) for cracks/splits.

You won't see a huge difference with the BBS (big-brass-screws), but they should make some difference in idle speed. Normal setting for them is 1.5 turns out from gently seated. If there are really no leaks in the intake, AND your valves are adjusted - they should all be very close to each other when balanced using a vacuum balance tool.

The intra-body screws (the ones with green paint on them, or on later models a blue plastic cap) are never to be adjusted. They were set on a flow bench and there is no way they can go out of adjustment if they're not mucked with.

It's also worth checking that there is free play in the "fast-idle" cable (aka "choke"), and that the switches at the rear end of the throttle body isn't holding the throttle open (these are the "choke" warning light switch, and the throttle-position-switch.) Ditto on the throttle cable - some free play must be evident.

The idle screw that is between the TBs can and should be used to adjust idle speed. The BBS are only used to balance the TBs at idle. They have no real effect above about 1,200 RPM.

It's possible if you get the screws in the right position, and then adjust the idle screws to get the right RPMs that the screws will become more responsive.

Good luck!

BTW - there is a CO adjusting screw on the air-flow meter. Look at the right/rear corner of the airbox. There should be a rubber plug in a hole. Remove the plug.. the adjustment is under that hole. Forget if it's an allen or screwdriver adjustment (on the cars it was a screwdriver).. It opens/closes a bypass passage (unmetered air) around the air-flow meter vane.

To set it correctly - engine idling, press the starter button. That causes a 10% enrichment of the mixture. Listen to the RPM:

- if RPM increase and stay increased - the mixture is lean. Adjust screw in.
- if RPM decrease and stay decreased - the mixture is rich. Adjust screw out.
- Ideally - the RPM should increase about 100RPM, then drop down to the same speed or lower as it started at.

You can also use a CO meter to make this adjustment.

 

[98lee]

A little clarification: Don mentioned "idle screws". He meant idle SCREW. There is only ONE idle screw. It is the one with the hex head with a slot in it.

The silver screws that have NO head, but have a slot in them are not to be touched!


Another possible source of a leak could be the large diameter (2") 90 degree hose between the air cleaner box and the intake plenum. It could be cracked or not seated properly.

 

[walt3022]

Thanks for the prompt and insightful responses guys. I use a Harmonizing manometer - probably the most sensitive such device on the market. It's a wonderful tool.
I opened the brass screws about 1/4 turn and the idle improved in the sense that the engine didn't seem to be running so rich. I turned the idle screw between TBs 1 and 2 just a tad and that helped too. I'll run the bike a bit more then check with the vacuum device again. Noticing that improvement, I didn't get involved with adjusting the air metering device. But let me get some clarification on the recommended test. One can push the start button while the engine is idling? That doesn't produce a horrific squawk and gnashing of teeth?
I'm thinking I probably should splurge for a new throttle cable, as the present one doesn't really return so smartly when the twist grip is released.

 

[hhshort]

Hey Walt There will be no gnashing and crashing as the starter is electronically locked out as soon as the motor starts. The starter relay ground goes through the ignition black box and is opened when started. Harold in Kansas

 

[walt3022]

1985 K100RS TB tune

Thanks for all the advice and tips. I now have a very sweet-running K100RS

 

[42906]

There is some misinformation here. The "idle screw" should never be used to adjust idle speed. Ever! Here is why. Moving that screw moves the whole throttle shaft. What is down on the end of that shaf, at the rear of the throttle rack? The Throttle Position Switch, or TPS for short. The position of the TPS on the throttle shaft is critical and once set, you do not want to, and should not have to, disturb it. The correct method to adjust idle speed is by using the brass bypass screws on each Throttle Body, or TB for short.
Using mercury manometers or "carb sticks" as most call them with the hoses attached to the spigots on each TB, you can set idle speed and adjust TB balance at the same time.
If you do disturb the positon of the throttle shaft by turning that screw, you have to check and if necessary adjust the TPS. The way to check it is to listen very carefully, as you crack the throttle open you should hear a faint click inside the TPS. If you don't, you need to release the two phillips head (or filister head for our European brothers and yes they are a royal beyatch to get at, oh well) screws holding the TPS in position and move it gently until you position it such that just opening the throttle causes a click to be heard.
Have you ever wondered why some K bikes make hellacious backfires? This is from adjusting idle speed using the screw in the middle of the throttle rack. Don't fool with it, use the bypass screws to set idle speed and TB balance.

By the way, it is indeed possible to set throttle plate position on the throttle rack using the small screws with the locknuts and the paint. With enough miles, we are talking six figure miles, accumulated wear in the throttle rack will allow throttle plate position to wander, affecting TB balance and drivability. You will notice that over time you have to unscrew some brass bypass screws more than others to get the mercury colums of your carb sticks to equalize. This is your clue the actual throttle plates and shaft have enough wear to allow throttle plate position to wander. If you want I can post a procedure to adjust the throttle plates themselves. It is simple and straightforward to do, especially if you have balanced carbs on some Japanese multis. Your engine makes a darn fine "flow bench" if you know how to use it!

 

[42906]

There is a cheap gas analyzer called "Gunson Gasester Digital" that takes the guess work out of setting the bypass screw in the air flow meter. This device measures CO only and is accurate enough to set the idle mixture within half a percent of what the meter is reading. This beats the living daylights out of guessing.

http://www.gunson.co.uk/item.aspx?item=1835

It used to be available in the US from Northern Tool but is apparently discontinued. If you try Google Shopping you will find plenty are available in the US from other sources at prices ranging from the high $150's to under $230 which is a bargain, so happy shopping and happy tuning.

 

[deilenberger]

Sorry - I'll have to completely disagree, as does the factory manual (from memory)..

There is some misinformation here. The "idle screw" should never be used to adjust idle speed. Ever! Here is why. Moving that screw moves the whole throttle shaft. What is down on the end of that shaf, at the rear of the throttle rack? The Throttle Position Switch, or TPS for short. The position of the TPS on the throttle shaft is critical and once set, you do not want to, and should not have to, disturb it. The correct method to adjust idle speed is by using the brass bypass screws on each Throttle Body, or TB for short.
Using mercury manometers or "carb sticks" as most call them with the hoses attached to the spigots on each TB, you can set idle speed and adjust TB balance at the same time.

In cleaning and adjusting a LOT of K bikes, the one observation I've made is that the BBS (big-brass-screws) are usually out WAY too far. It is not at all uncommon to find them 2-3 turns out from seated... and sometimes more. People tend to balance the TBs by turning the screws out, instead of doing the factory procedure of gently screwing them in, and then starting 1-1.5 turns out. A K bike with correctly adjusted valves, where the inter-trottle-body screws haven't been dicked with, and there are no intake leaks, will typically have the screws within 1/4-1/2 turn of each other.

it is true that you have to readjust the TPS after setting idle with the idle adjustment screw, but that's really trivial to do. Two allens, listen, click, tighten. The one that catches some people is the fast-idle setting, which has it's own adjustment. That also should be checked after setting the idle to make certain it's fully off when the "choke" lever is in the off position, and if you have an early model with a "choke warning light" - make certain the switch for it (located on the rearmost TB) isn't holding the throttle open.

If you do disturb the positon of the throttle shaft by turning that screw, you have to check and if necessary adjust the TPS. The way to check it is to listen very carefully, as you crack the throttle open you should hear a faint click inside the TPS. If you don't, you need to release the two phillips head (or filister head for our European brothers and yes they are a royal beyatch to get at, oh well) screws holding the TPS in position and move it gently until you position it such that just opening the throttle causes a click to be heard.
Have you ever wondered why some K bikes make hellacious backfires? This is from adjusting idle speed using the screw in the middle of the throttle rack. Don't fool with it, use the bypass screws to set idle speed and TB balance.

By the way, it is indeed possible to set throttle plate position on the throttle rack using the small screws with the locknuts and the paint. With enough miles, we are talking six figure miles, accumulated wear in the throttle rack will allow throttle plate position to wander, affecting TB balance and drivability. You will notice that over time you have to unscrew some brass bypass screws more than others to get the mercury colums of your carb sticks to equalize. This is your clue the actual throttle plates and shaft have enough wear to allow throttle plate position to wander. If you want I can post a procedure to adjust the throttle plates themselves. It is simple and straightforward to do, especially if you have balanced carbs on some Japanese multis. Your engine makes a darn fine "flow bench" if you know how to use it!

Actually - it's not a good flow bench unless it has very carefully adjusted valves, and you're certain there are no intake leaks, and all cylinders are equally carboned up.

FWIW - (and I've worked/tuned a LOT of K bikes for local club members and IBMWR members,) once the valves are adjusted, and all leaks are fixed, the TB's track each other just fine on a set of CarbStix. And the brass screws often end up in exactly the same position relative to each other. That includes bikes with well over 100,000 miles on them.

If someone wants to diddle with the inter-body screws - it's really at their own risk, and they should be aware they can make things lots worse, especially if they're doing this as part of troubleshooting for a problem engine.

Small Edit: You're correct on the Phillips for the TPS, but a short ratcheting right-angle screwdriver should be in everyone's toolbox.

 

[42906]

My question is why disturb the throttle rack and have to reset the TPS when idle is correctly set using the bypass screws? My other point is that most home tuners make this adjustment without realizing you have to adjust the TPS afterward, and then curse the K bike for the awful backfires that are the result. There is no need to disturb the TPS by turning this screw, set idle speed with the bypass screws.

My bike didn't need any adjustment of the throttle plates until over 200,000 miles were behind it. In my case, the engine was freshly built and so were all the rubber parts, but the TB's were original. There was something like a turn difference between bypass screws and we still couldn't get one cylinder to balance. The gent who taught me the technique built and tested the original K's when he worked at BMW, so I trust his techniques are sound. TBs develop leaks around the throttle shaft openings that cannot be repaired. They are minor and you balance around these but use takes it's toll on these parts. Other parts wear and allow throttle plates to move relative to one another and this is what you can compensate for. There is a great fear of doing this that is unjustified in my opinion.

I have the small racheting phillips you describe and find it harder to use than a regular screw driver angled in at the screws. Allen head screws are the cure.

 

[98lee]

I have the small racheting phillips you describe and find it harder to use than a regular screw driver angled in at the screws.

Remove coil cover. Then a stubby screwdriver works fine.

 

[42906]

Remove coil cover. Then a stubby screwdriver works fine.

Once I do that I can get at the screws with a normal screw driver just fine.

 

[jwetering]

Just one point of clarification please...

When I adjust the position of the TPS and listen for that click - is it with the engine on or off, ignition on or off?

And remind me again what the click means...is it the switch that shuts off the fuel when the throttle is closed and revs are above 2500 rpm?

Thanks

 

[timtyler]

When I adjust the position of the TPS and listen for that click - is it with the engine on or off, ignition on or off?

Doesn't matter, but it sure is easier to hear the click with the engine off.

 

[timtyler]

Just one point of clarification please...
And remind me again what the click means..

The switch just tells the bike's electronics that you've stopped turning the throttle.

 

[jwetering]

Thanks guys - I got that all figured out.

 

[98lee]

Just one point of clarification please...

When I adjust the position of the TPS and listen for that click - is it with the engine on or off, ignition on or off?

And remind me again what the click means...is it the switch that shuts off the fuel when the throttle is closed and revs are above 2500 rpm?

Thanks

Do it with the engine off. You are listening for the click of a microswitch inside the throttle position switch. It is a VERY faint click.

Yes. The click means that the switch is telling the computer that you are coming off fully closed throttle. The switch does two things; tells the computer when the throttle is fully closed and there is another microswitch inside that tells the computer when you are near wide open throttle. The second one will be properly adjusted when the fully closed throttle switch is correct (no separate adjustment needed).

 

[jwetering]

Hi Lee - you may have noticed the thread I started called "injector service indicated". In that thread I tell the story of the mix adjustment I did using the lean drop method. Mixed results - but overall better than before.

Before I started the adjustment I checked that the TPS clicked where it was supposed to. I did but it sure looked like a dead easy adjustment to make if it didn't.

Thanks for the help here guys. I've been maintaining my cars for years using advice from the excellent forums available for my brand of choice - and now that I've become obsessed with motorcycles in general and the BMW in particular this site has become a very important part of my life. I say that because I'm the kind of guy that doesn't sleep well if/when my equipment isn't working exactly as it should, so quick access to good technical advice is actually important for my health.

I also don't like working with shop manuals. Sure they're great for tables of data like torque specs or fluid capacities, but I find the procedures are often not written that well, and the photographs are too dark and too tightly cropped. Of course you can't ask specific questions to clarify either.

Anyway - thanks to Al Gore for enabling the technology to allow forums like these to exist and thanks again to you all for taking the time to answer questions and post useful information.

 

[42906]

Hi Lee - you may have noticed the thread I started called "injector service indicated". In that thread I tell the story of the mix adjustment I did using the lean drop method. Mixed results - but overall better than before.

Before I started the adjustment I checked that the TPS clicked where it was supposed to. I did but it sure looked like a dead easy adjustment to make if it didn't.

Thanks for the help here guys. I've been maintaining my cars for years using advice from the excellent forums available for my brand of choice - and now that I've become obsessed with motorcycles in general and the BMW in particular this site has become a very important part of my life. I say that because I'm the kind of guy that doesn't sleep well if/when my equipment isn't working exactly as it should, so quick access to good technical advice is actually important for my health.

I also don't like working with shop manuals. Sure they're great for tables of data like torque specs or fluid capacities, but I find the procedures are often not written that well, and the photographs are too dark and too tightly cropped. Of course you can't ask specific questions to clarify either.

Anyway - thanks to Al Gore for enabling the technology to allow forums like these to exist and thanks again to you all for taking the time to answer questions and post useful information.

Actually, thank DARPA. Your defense tax dollar at work.

 

[walt3022]

wrenching and good mental health

"I've become obsessed with motorcycles in general and the BMW in particular this site has become a very important part of my life. I say that because I'm the kind of guy that doesn't sleep well if/when my equipment isn't working exactly as it should, so quick access to good technical advice is actually important for my health."

LOL! So there are more loonies like myself around here!