Throttle
& clutch cables; confusing throttle gears/cams assembly
etc.
©
controlcables.htm-7B
First...information on "why" the BMW throttle assembly design.
From a posting by me, edited here, from the Airheads Mailing LIST...slightly edited here..
I had an off-LIST inquiry about some
things, and in the original query, he was complaining (considerably), about certain parts of the airhead
design. He had a nice list of 'bad design items'.
BMW is not perfect, witness the transmission circlip problems, clutch lever
clips, valve seats, etc. 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 many
others. Whether it is the single-cable type (I prefer that one
slightly), or the dual-cable type.
The cam gear was designed to be NON-linear. That is, as the throttle is
initially, from off, moved a certain amount, the cam moves slowly, and as the throttle is more
rotated, as when much more power is needed, the cam moves much faster, pulling the
cable faster for any particular number of degrees of throttle movement.
The advantage is that as one takes off gently from a stop, there is more and
smoother control over the carburetors.
Further, 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 parts, they will wear very
slowly. There is some complication involved if
you ever have to replace the gears, they are not the same
from early and late models, and it is a crap-shoot with some
dealership parts department, if you will get the right ones,
so bring along the old ones.
Do not tie the throttle cables down with wire-ties/wire-wraps where they were
not supposed to be. 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...it has an adjustment...for the upper cable free play.
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 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 result in a 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 over the years, as BMW has changed the
design of some of the parts (especially for the SINGLE throttle cable
versions) in the twist-grip throttle assembly, and some earlier parts are NO
LONGER AVAILABLE. BMalso 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. Many 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