From: johnsw@wsuvm1.csc.wsu.edu (William E. Johns)
Newsgroups: rec.motorcycles
Subject: BMW carbs
Keywords: Boxer twins, Bing carbs
Date: 12 Aug 92 15:26:56 GMT
Organization: Washington State University
Originator: bill@wsuaix.csc.wsu.edu


 
>From: gn@cray.com (Gary L. Nelson)
Subject: Balancing carbs
 
>When using a mercury vacuum gauge (manometer?) to balance the
>carbs on my '91 BMW R100RT with 18,000 miles, I can balance
>them fine at idle, but when I increase the RPM to 3,000, the
>mercury level rises at a different rate and is not the same
>at 3,000 RPM. As far as I can tell, the throttle cable slack
>is taken up at the same time and actuates the throttles evenly.
 
>Questions:
 
>Is this normal?
 
Well, it's not normal, it is a simple indication of a job not
yet finished.  You adjust out this difference.
 
>If not, is the difference at the higher RPM due to dirty carbs?
 
NO
 
>Do I have to adjust the cables so they actuate the throttles
>at different rates so the vacuum levels match?
 
Yes you have to adjust the cables, but not for the reason stated.
 
Your carbs work at three different levels.  The lowest rpm is controlled
by the idle screw and your richness setting.  It sounds like you
got them more or less ok.  The midrange power comes from a combination
of your throttle pull and the vacuum that pull provides to the needles.
The more the butterfly valve opens, the more vacuum and the higher the
needles pull up.  This is an adjustable factor.  How to below.  The
third level is controlled by your main jet.  This is just a small piece
of brass with a hole in it.  We are dealing with a midrange adjustment.
 
Having the carbs synched at low rpm the next step is to let the thing
cool down for a bit, go have a glass of buttermilk or something.
 
OK it's a little cooler, so let's adjust the cable setting. Pick one of
the carbs and loosen the lock nut that holds the throttle cable in
position.  Tighten your cruise control or have a friend hold the revs up
to 3k. You will see that the Hg in the manometer is running smoothly and
the two columns are not at the same level.  Now gently turn the knurled
part of the throttle cable and get the two columns the same.  As soon as
they are the same, without releasing the knurling on the throttle cable,
tighten the lock nut.  Invariably as you develop your technique the
columns will shift a bit, just keep doing it until the lock nut is tight
and the columns are the same.  Back off the revs and make sure you
didn't change anything.
 
When properly set up, the adjustment on the cable should have no effect
on the idle balance.  You will be delighted with the smoothness of the
bike as you accelerate.
 
As long as we're talking Beemer tuning, I want to echo something bob
pasker mentioned some time back.  I had my Yamaha mechanic tune my FJ
with the hydrocarbon/CO monitor and the FJ now runs so well it's scary.
I brought the PD down to his shop, put up with the several smart-assed
cracks about inferior bikes in a Yamaha shop (I put up with this because
the guys down there know what they are doing) and had him do the same
number to the PD.
 
What he did was drill a small hole in each header downstream of the
crossover pipe and inserted a permanent fitting which seals with a screw
and accepts the little valve fitting on his tuning machine. (Note: my FJ
and I am told that all Yamahas already have such taps in the headers, do
other bikes have them as well?) Anyway, after the fittings were in, he
did the adjusting.  The difference is incredible.  It is now smoother,
quieter and more powerful on acceleration.  I was getting 41 mpg with all
luggage at highway speeds before, now I'm getting 45.  I _like_ a good
tuning job.  (Let's see, at 45 mpg and 7.6 gallons before reserve, I get
342 miles per tank before reserve and another 77 miles on reserve--geezzz
I love a big tank.  :-)  )
 
For the record, it cost $30 for the FJ, and $45 for the Beemer, the
extra cost on the Beemer coming from the one time need to insert the
fittings.
 
If you have a mechanic you trust and he has the equipment, this is a
good option to consider.
 
I'd love to find CO/hydrocarbon measuring devices for mere mortals but
they are expensive.  The closest I found to reasonable was at a NAPA
shop for about $400, a small hand-held unit.
 
>Am I doing something wrong (besides leaving myself open to
>*witty repartee* from rec.motorcycles)?
 
Nah, it is refreshing to see a simple question dealing with motorcycles
come over the nets.  I have battled with BMW carbs a bunch and have a
reasonable understanding of them.  Glad to help.
 
Ride Free, and smoooth!
 
Bill
________________________________________________________________
bill@wsuaix.csc.wsu.edu if you feel compelled to send email
johnsw@wsuvm1.csc.wsu.edu if you seriously expect an answer
AMA #500924   DoD #00314   WMTC #0002   SR= 2.125
FJ1200 R100GS/PD 740GLE  KotV KotRR KotD #0001  :-)
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When the ordinary man attains knowledge, he is a sage.  When the
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Newsgroups: rec.motorcycles