My 85 K100RT (22K mi) runs pretty well but I thought to adjust the vacuum screws to balance the cylinders for smoother running.

I own a set of Carbstix that use the mercury level method.

I warmed up the engine , connected the 4 tubes to the proper
connectors. (I used a "T" for the last cylinder that had the tube attached for the computer)

The mercury columns showed that the levels of all 4 cylinders were pretty close so I just tweaked what cylinders that needed small adjustment to get them all in sync.

But, while doing that I noticed that I could raise the vacuum farther up the carbstix by more adjustment of the screw.

My question is, should those adjustment screw be set for MAXIMUM vacuum and still balance? Even though all columns on the carbstix were matched evenly, if I could raise the levels by adjusting the screws, should I?

What is the desired goal using the method I did?

about what reading are you getting as it stands at the moment ?

a few of us have aimed for about 22-24

On the carbstix, there are no numerical values.

Just a series of horizontal lines evenly spaced up the carbstix.

I think the carbstix were designed for carburettors, not FI

As I said, all the tubes matched pretty well and a little tweaking evened them out.
There was smoother running of the bike afterwards.

Is there some other tool I need to get 22-24 (22-24 whats?)

i use  the bypass screws  for syncro of tb´s and at the same time set the idle speed at 950-1000 rpm.
is it even possible to define some kind of value ,since it is not possible to reach that value without changing the idle speed ???
im a Little confused? 
but still looking forward to my trip with my friends later today:tu1:

as you get more vacuum ....(less supply of air through the bypass valves) it is often necessary to wind up the idle screw a little  to open the butterflys ...just a little ( I mean a real small amount, maybe just a quarter of a turn ) when you should see  a slight fall in vacuume  

there is a trade off though  as you wind up the butterflys then the vacuum drops off  a small amount ..  with a little juggling you will find a good compromise of idle adjust screw  so that the butterflys are almost right , or on full closed,  to the choke idle bar with minimal adjustment  and sufficient vaccume for the bypass across all the cylinders  and at about 850-  revs then use the idle adjust to get to 900 .
doing it this way you will find that the idle is more stable between mid to low  hot and heat soak ...( hot motor )  so will get an even more equalised idle setting
 then you can adjust the cold idle on position 2 of the choke position properly at next cold start ....at the cable to the choke adjust nuts (on the back end of the throttle bodies near tb 4 )

having said this before ...quite often we see that the idle adjust screw is no where near the choke idle bar (could they have used the main idle to adjust the choke idle position revs ?)  ....klompy and rossko did this recently and they were already on the bar ....but I have seen a few round that the idle was adjusted by the air bypass only ...which then makes a cold start ...harder or more unpredictable revs  and a slight adjustment of idle almost impossible without going back to the bypass screws and doing it all again . (besides the vacuume being a really low value )


well that what I have experienced
 I think the value we have seen was 24mhg   but I get confused on this translation to imperial  inches and stuff

OK, you totally lost me on those last posts.
What I guess I need to do is get some real vacuum guages that show numerical values and set up the bike system for 22-24 somethings.

I don't understand why the K100 has to idle at 900 RPM. Seems kind of high but OK

I'll see what happens.

Gerry

charlie99 wrote:as you get more vacuum ....(less supply of air through the bypass valves) it is often necessary to wind up the idle screw a little  to open the butterflys ...just a little ( I mean a real small amount, maybe just a quarter of a turn ) when you should see  a slight fall in vacuume  

there is a trade off though  as you wind up the butterflys then the vacuum drops off  a small amount ..  with a little juggling you will find a good compromise of idle adjust screw  so that the butterflys are almost right , or on full closed,  to the choke idle bar with minimal adjustment  and sufficient vaccume for the bypass across all the cylinders  and at about 850-  revs then use the idle adjust to get to 900 .
doing it this way you will find that the idle is more stable between mid to low  hot and heat soak ...( hot motor )  so will get an even more equalised idle setting
 then you can adjust the cold idle on position 2 of the choke position properly at next cold start ....at the cable to the choke adjust nuts (on the back end of the throttle bodies near tb 4 )

having said this before ...quite often we see that the idle adjust screw is no where near the choke idle bar (could they have used the main idle to adjust the choke idle position revs ?)  ....klompy and rossko did this recently and they were already on the bar ....but I have seen a few round that the idle was adjusted by the air bypass only ...which then makes a cold start ...harder or more unpredictable revs  and a slight adjustment of idle almost impossible without going back to the bypass screws and doing it all again . (besides the vacuume being a really low value )


well that what I have experienced
 I think the value we have seen was 24mhg   but I get confused on this translation to imperial  inches and stuff

ok , this sounds interresting, i have experienced that the engine almost stops when i close the throttle and pull the clutch lever after running at freeway speed ,maybe this Little trick will cure it .

Puccini wrote:OK, you totally lost me on those last posts.
What I guess I need to do is get some real vacuum guages that show numerical values and set up the bike system for 22-24 somethings.

I don't understand why the K100 has to idle at 900 RPM. Seems kind of high but OK

I'll see what happens.

Gerry

the idle is fairly high because of the big overlap in the valves ....not like my old duke that would idle quite happily about 350 - 500 revs

does it hurt the bike to slow the idle down to sat: 750-800rpm?

I've never seen the specs on the K100 cams, but for as long as I've been familiar with BMW m/c - and that's quite a while now - the design parameters have provided for an engine with generous torque at low revs. If there's a lot of valve overlap, how is it that the K100 seems to pull so well off idle?  What's the secret?

a lot to do with small port dimensions ...which creates high velocity .....I have heard for many years that the bmw theory was to always have high flow ..... or something like that ....for example ..  some of their car product exhaust pipes were regarded as particularly small for capacity ...but they worked  well ....some folks decided to take this onboard and now understand the theory .

I  have been doing a little research ...and found that huge port dimensions for max hp aren't always the best choice for good overall performance across all the rev range ...

again tuning, design and length of the inlet manifold including ram stacks, help broaden the overall power band on the old 8 valve motors ...even the 16 valve has some great designs ...as proof of the extra horsepower improvements for the same capacity


 the manual says the cams are 284 degrees ...but if you look a bit closer ...most tuners rate their cams at 50 thou opening heights ..(industry standard ) .( helps to tell what the rise of the cam is in duration for working flow of intake etc .) 
someone here did a measurement ..at 50 thou ...and the working duration worked out to be about 234 degrees or something ...which is still a fair bit of overlap for a 2 valve engine .

does that help ...Im no pro (far from it.... but prepared to blunder through experimentation ) ...but we do have a great old machine huh ?