My motor now idles nicely at below 1000 rpm and at ± 1200 when warm. No more 1700 rpm that cannot be adjusted down.

From the beginning I suspected that the too-fast idle was caused by the throttle valves not closing properly. The accelerator & fast idle cables were disconnected, TPS removed – all the common reasons for not closing had been eliminated. Adjusting the between-TB linkages while running the engine did not fix this - although the TBs were in sync it seemed that they were still not closing.

I removed the TB assembly. I adjusted the linkage between TB3 & TB4 so TB3 was in its most open position relative to TB4. I repeated this for the other linkages, TB2 open relative to TB3 and TB1 open relative to TB2. In effect TB4 valve was now shut and the others in varying degrees of open.

Using a strip of audio magnetic tape as a "feeler gauge" I adjusted the center stop screw so TB4 throttle valve was just closed – a distinct drag on the tape/”feeler gauge”. Leaving the stop screw in that position I adjusted the 3 linkages so TB3, TB2 and TB1 butterfly valves offered the same resistance to pulling the "feeler gauge" out as TB4.

The throttle valves were now physically synced as good as I could get them. In order to fine tune this static sync at the closed position I cobbled together a  tester, a process to test the “vacuum” each throttle valve can hold against the same negative pressure applied. The sketch below illustrates the principle.





My vacuum source was the input end of my compressor. With the other tube from the “regulator” connected directly to my water manometer I drilled small holes in the lid of the jar until the manometer showed a difference of about 1.2 meter against atmospheric pressure.

Connecting the TB as shown in the sketch and with the bypass screw shut I could test each TB “vacuum” It took me about 4 iterations of making inter-TB linkage adjustments and they were balanced. Readings on the manometer were   between 215mm and 225mm.

After installing the TB assembly I did final adjustments of the bypass screws(using another water manometer) with the engine running. They are balanced and the motor idles below 1000 rpm.

I'll show photos of the actual process in subsequent posts

Testing TB 2 vacuum







Top view of my "regulator" showing 4 very small holes to limit the vacuum to about 1000mm of water




If you enlarge this image you can see marks on the wall of the TB where the throttle valve touches. All TBs were like this. I don't know if this is sufficient wear to have caused the fast idle.




Here is the fine adjustment on the bike

Have you checked that all the butterflys are in sync when you come off idle because #3 TB is the base reference for the TBs because it is the one that is directly connected to the cable and adjustment screw. I recall that one of the guys on Motobrick.com used the choke/fast idle to set the TBs and had #4 lagging at the initial opening because the fast idle cable pulls at #4 and the throttle cable pulls at #3. Everything was sweet at idle but as soon as the throttle opened all the sync went down the drain.

RicK G wrote:Have you checked that all the butterflys are in sync when you come off idle because #3 TB is the base reference for the TBs because it is the one that is directly connected to the cable and adjustment screw. I recall that one of the guys on Motobrick.com used the choke/fast idle to set the TBs and had #4 lagging at the initial opening because the fast idle cable pulls at #4 and the throttle cable pulls at #3. Everything was sweet at idle but as soon as the throttle opened all the sync went down the drain.

Hello Rick

That's why I did the dynamic balance after all the other stuff - see last photo in my earlier post. The water level in my 4-jar "manometer" remains constant on acceleration. I took it up to just past 3000 rpm. I stopped because of the noise I'm happy with in sync up to 3000 rpm. In fact I'm amazed at in sync up to 3000 rpm on a 30 year old motor.

Kafflut wrote:In fact I'm amazed at in sync up to 3000 rpm on a 30 year old motor.

That's why we like BMWs. My K1100 and BMW car just turned 25 and they still take a lot of beating for putting up the miles.
The right amount of TLC and they go forever. Balanced with a darn good thrashing every now and then of course.
Thought I had best say something because the guy on Motobrick.com did a huge write up and had quite a few copy what he did with disastrous results and they wanted to lynch him. All in good humour.

Balanced with a darn good thrashing every now and then of course.

We refer to that as an Italian tune up. And yes, absolutely essential, that's why the autostrada was invented.

I'm in awe of the home built instrumentation. Isn't there a saying along the lines of - 'die Boer maak a plan'.

K100 Forum at its inspirational best - great stuff Mate.

RicK G wrote:Thought I had best say something because the guy on Motobrick.com did a huge write up and had quite a few copy what he did with disastrous results and they wanted to lynch him. All in good humour.

You will notice I included "What-Worked-For-Me" in the title of the thread

If you can remember the post in Motorbrick.com please send me a link. I would like to read it. There has been so much said about TB balancing and Too-Fast Idle and I believe the subject is not yet exhausted.

We read over and over about the thing hanging on the throttle cable or the fast idle cable or the TPS is keeping stuff open. Oh yes, and the hose clamps interfere. Here's one I've not read about but experienced(not on this bike): The "choke" indicator switch! The one behind No 4 TB, screwed in so the plunger gets activated when you pull the fast idle lever. I've seen that sucker screwed in so tight that instead of being activated by the lever it pushed down on the lever ever so slightly. Result - raised idle revs. Maybe it's supposed to have a washer/spacer to limit the depth it can screw in.

Send me the link if you can find it.

I've done a little bit more and I think it further improved how my bike now works; I opened the AFM and cleaned the contact points. I turned that notched wheel 3 clicks CW(leaner) I now have to use the fast idle when starting - that's okay, but idle revs seem to be down just a bit. I'll ride it a few 100 km and see.

Thanks for your input

rawdonball wrote:I'm in awe of the home built instrumentation. Isn't there a saying along the lines of - 'die Boer maak a plan'.

K100 Forum at its inspirational best - great stuff Mate.

Indeed, there is a saying

Your knowing it tells me you have more than just a casual knowledge of Afrikaans and of South Africa

I'm glad you liked the post. My mates at the bike club often call me MacGyver and tell me I waste time by doing stuff myself and looking for workarounds. But I figure would the time I "waste" on workarounds be better used had I been watching TV? I'm retired, so it's not like I'm neglecting my day job

Here's another workaround I did

http://bmwk100k75.weebly.com/shims.html

Surface grinding shims. I could buy new but I figured what for, the old ones are too THICK not too THIN!?

I couldn't find that on the Motobrick forum it's been 4-5 years since I was there and I am no longer a member.
You have done some very good work there and using the magnetic tape is one of those things that is so simple. You have inspired me to start working on the K1100 TBs for my K75.