Throttle
cables; confusing throttle gears/cams assembly
etc.
©
controlcables.htm-7B
First...information on "why" the BMW throttle assembly design:
From a posting by me on the Airheads Mailing LIST...slightly edited here...
I had an off-LIST inquiry...he had a list of 'bad design items'.
BMW is not perfect, witness the 1985+ transmission circlip problems,
early clutch lever (at transmission)
clips, 1981-1984 valve seats. But, one of his complaints was
about the throttle at the handlebars, with its requirement for cleaning and
re-lubrication every few years, and its complexity, including the need to
synchronize the marks on the cam gear and throttle sleeve.
The BMW airhead throttle design is far better, NOT WORSE!, than
MOST
others. Whether it is the single-cable type (I slightly prefer that one), or the dual-cable type.
The throttle cam gear was designed to be NON-linear. As the throttle is
initially, from off, moved a small amount, the cam moves slowly
as far as amount of cable pulled; and, as the throttle is more
rotated (as when much more power is needed), the cam moves much
faster, pulling the cable more, in proportion, for any particular number of degrees of throttle movement.
The advantage is that as one takes off gently from a stop, there is smoother control over the carburetors;
there is certainly none of the super-critical throttle that
one finds on many other bikes.
IN ADDITION....the design is such that the throttle cable is a straight-pull,
and there is no bending of the inner cable as in many other types of designs;
and, the throttle cable at the bars-end can be expected to last a very long
time, without fraying or breakage.
If you clean and re-lubricate the throttle gears/chain assembly
now and then, and the cable barrel ends, they will wear very
slowly.
There is some complication involved if
you ever have to replace the gear-cam and/or cover, they are not the same
from early and late models, and some parts are no longer
available, and you need to know which later parts are correct
ones for your bike......and it is a crap-shoot!! with some
dealership parts department, if you will get the right ones,
so bring along the old ones; and, know what is going on,
because JUST having the old ones along is NOT NECESSARILY
going to tell you all you need to know, depending on your
bike's throttle parts. Information and part
numbers are below in this article.
HINTS:
Do not tie the throttle cables down with wire-ties/wire-wraps where they were
not supposed to be tied down; generally there is ONE. Do not route cables wrongly, including with sharp
bends. Don't lubricate cables,
BUT.. DO lubricate the end barrels (for
SURE at the carburetor ends, after every bike bath).
If you have the type of throttle on the bars with ONE cable
coming out of it, you have (unless you are riding a BMW
single!) a tubular junction under the tank...that has an adjustment...for this
single upper cable free play; to take out most, but not all,
of the free play. Be sure the rubber sleeve on the nose
of this T connector is in good waterproof condition.
Do NOT bend the cable at the left carburetor when checking your oil.
There is no need for that oil dipstick to be overly tightened. Bending
the left cable is a
prime cause for that left cable to have increased friction, possibly spread
some coils on the wrapped sheath (making that carb fun and games to
synchronize, if bad enough), and eventually you might break an inner strand...usually
where you can see it between the throttle lever on the carburetor, and the sheath.
A single strand found broken (You DO inspect these cables regularly, don't
you?), will usually cause other strands to break from why the
first strand broke....this will result in total cable failure in a few hundred miles or
so.
In your on-bike tools and parts, you
should have a spare clutch cable, and a
spare throttle cable(s). There are all
sorts of lengths of throttle cables, be sure you have the correct one for your
model, your carburetors, and your type of bars.
Throttle gears and cams:
This has caused a LOT of confusion,
as BMW has changed the design of some of the cam and the cam
cover (especially for the SINGLE throttle cable
versions) in the twist-grip throttle assembly, and some earlier parts are NO
LONGER AVAILABLE (so combinations of later ones must be used).
BMW also modified the top
cover and gasket of the on-bars rectangular brake master
cylinder, information on that is in my brakes article. BMW in its infinite wisdom, does NOT list the
throttle assembly in the carburetion section of Katalogs....no, it is in
Section 32, which is STEERING.
The throttle cam is not the same
for 32 and 40 mm carburetors. Some original cam gears are NLA.
The
proper cover plate to use with the
later cam assembly
is 32-72-1-457-050.
The cover plate,
32-72-1-242-561 UNDERSIDE, was flatter at the screw hole. The newer plate
has a round protrusion at the center, and that is about 1/2" diameter and
maybe 1/4" deep.
The throttle cover was 32-72-1-233-538 for the earliest models.
The new cams are thinner, don't fit the old assembly cover.
The cam assembly for the 40 mm carburetors is now 32-72-1-457-081.
The cam assembly for the 32 mm carburetors is now 32-72-1-457-080.
The throttle TUBE did NOT change.
The cam gear for the R65 series is 32-72-1-238-378.
Be SURE what you purchase will fit!!
This throttle assembly has been a PIA, and it may be best for you to contact
knowledgeable Airhead folks, such as Ted Porter's Beemershop, if you don't
understand what is what.
Revisions:
04/26/2007: add
information on throttle gears and cams
01/14/2010: Remove clutch cable from title, and minor
changes in article, to clear up grammar and ensure clarity.