This page is about all models from just after WWII until they stopped using the
advance mechanism. It applies to the R50, R60 and R69S, but was especially
a problem on the /5 models from 1970 and on, until they implemented the
Causes of a high idle
You may have one or more of these factors going on at the same time.
Test for them and then correct as needed. Don't go off buying things
without a proper diagnosis.
1. The mechanical advance not working properly. Read about the
test, springs and lubrication below.
2. Throttle cables holding the slide
up a bit. Try rerouting them, or replace with real BMW cables.
3. An intake leak can contribute to a high idle. This
page about the /2 carbs.
4. The normal procedure is to set the idle while hot, but then it may
be a bit low while warming up. That is common.
What is a timing advance mechanism unit?
As an engine goes from idle to higher rpm, the fuel in the combustion chamber
wants the "fire" to start sooner. That fire is started by an electric spark.
When the spark happens that is called the "ignition timing" and we are quite
concerned about having this happen at exactly the right time in the engine
To change when the spark happens, we need some way of sensing the speed of
the engine and then telling the spark to happen. This is done by the
timing advance mechanism.
How does the BMW motorcycle timing advance unit do this?
The spark happens when the points open up. The points are opened up by the
camshaft. The front of the camshaft has the advance mechanism mounted on it. It
has a "bump" on it that comes around and forces the points to open up. We call
that bump the cam lobe. The advance mechanism unit has two of these lobes.
The arrows point to the cam lobes
Always examine the cam surface for a smooth surface to rub against the
points. A worn surface will quickly wear out the points rubbing block.
This shows a "smooth" lobe
How do we get the "advance" aspect?
The advance unit has two flyweights mounted on the ends. As the rpm
increases, so does the force to allow the weights to pivot outwards. As the
weights "fly out" they cause the cam lobe to rotate a bit. You can demonstrate
this easily. With the advance unit off of the camshaft, pull the weights
outwards. Notice that the part with the lobes will rotate. That means that as
the rpm increases, the weights fly out and make the lobes open the points at an
earlier time. The ignition timing is advanced and the (fire) combustion starts
earlier. More power is generated because the fire is started enough in
advance to complete the burning of the fuel.
A simple test for the high idle
A high idle that creeps up during and after the warm up time may be due to
weak centrifugal advance unit springs. (These are the springs that are always
trying to return the fly weights to the resting position.) In this case, no
amount of carburetor adjustment will return it to normal. Here is a very easy
test for this. While the engine is running at the "too high" RPM, gently pull up
on the ignition key. Don't allow it to jump out, lift it only enough to kill the
engine, maybe 1/8". On the /6 and later, just turn the key to kill the engine. Allow a few seconds for the RPM to drop down to below normal, almost stopped,
and then push the key back in. The ignition system will work again, the engine
will start firing again and the RPM should go back to normal. The rpm
should stay down at normal. If it stays at a normal
rpm, then your springs need to be replaced. It may creep up again in a few seconds,
but this proves that when the weights got pulled back in where they belong, the
engine runs at a normal RPM. If the rpm goes up quickly then I would suspect the
carb adjustments. You could easily have both issues, weak springs and
poorly adjusted carbs.
To be sure, remove the front cover and put your strobe light directly on the
advance mechanism. As the key is pulled to drop the rpm and then reinserted,
watch the weights carefully. They should return to the relaxed "pulled in"
position. If they return and then go out again with increasing rpm, that
shows that the advance is working and the rpm is held up by fuel, not ignition.
The advance unit carries the lobes that opens the points. When the points
open up, the spark occurs. The lobes change position on the shaft according to
RPM. The faster that the advance unit rotates, the more the weights swing
outwards. As that happens the cam lobe rotates in the direction that makes the
points open earlier in comparison to the piston. This is what it meant by
When the springs get weak they allow the weights to fly out at a lower RPM. This "advanced" timing causes the RPM to increase quite a bit. The solution is
simple, replace the springs. It is easy, but they are a bit expensive. There has
also been a heavier set of springs available for the /5 to change its advance
Care and feeding of the BMW Timing Advance Unit
The advance unit has two types of springs. One spring for each flyweight to
return it to the normal resting place, and
the flat spring to limit the maximum rotation of the lobes. That is two coil
springs and one limit spring. Both type of springs are important to the
proper operation of the engine.
Older style /2 advance unit with the "question mark (?)" shaped advance limit
spring. This type of spring was used up through 1967. It was replaced by the one
below. This spring failed often, as the tip of the spring would break off at the
point where the arrow points to. This failure of the spring would allow the
ignition timing to advance too far. That would allow some overheating of the
pistons. That was a common factor in piston
Newer style /2 advance unit with the "U" shaped advance limit spring. Be very
careful if you try to adjust the spring to change the limit. They break
easily, don't ask how I know that.
A few more great advance pictures from Bernd can be seen at
What is a worn out /2 advance unit?
Photo by Rico Carballo, thanks
This is the old style advance unit with the ? mark advance limit spring. On
the left is one where the weights are out too far. The one on the right are far
better. Hold the weights out and see how you advance unit looks. You can also
see the wear marks from the attachment bolt on the left spring. The one on the
right shows little wear. What I can't show here is how the weights on the left
unit are also floppy. The one on the left will work, but one would need to make
sure that the full advance doesn't go beyond the "F" mark at high rpm. The
timing light flashing directly on a worn advance mechanism may also show a less
than smooth movement of the weights while changing rpm. That isn't very
important. More important is that it must not stick open while the rpm is
coming back to idle.
The /5 and later BMW motorcycle mechanical advance units
This shows the /5 engine with the advance unit at the position where it opens
the points, or where it fires the plugs. The nut is the one that so many over
energetic owners over tighten and strip the threads off of the camshaft. That
amount of oil leak is common and not important for proper operation. It is
only a cosmetic problem.
The 10 mm nut that holds the advance unit onto the camshaft can be over
tightened very easily. The usual result it to strip
the threads from the camshaft. You don't want to even think about replacing
the camshaft. It only takes one finger on that short wrench to very gently
tighten that nut. It is only slightly more than finger tight. I have
never seen one fall off from under tightness, or any other reason, except
If your threads are stripped, you still have an easy option. The cam can be
drilled out and a screw used to hold the advance. To get the tool to drill it in
alignment, contact Paul at
This is the /5 advance unit. The red paint is just to mark the advance
unit for easy recognition in aligning the tip of the camshaft.
Here is the same unit with the weights held out in the full advanced
position. Ignore the (temp stops) two round light colored things that are used
to hold the weights out just for this photo. It can't be seen in the photo, but
the lobes are now rotated (advanced) with respect to the face of the unit. The
locating flat can get worn, but usually the camshaft has a flat that wears more. Between the combined wear on each part, it makes a lot of "slop" to contend
with. Be very gentle when working with this unit to protect the flats.
I like to hold the advance mechanism in the "clockwise part of the slop" when
tightening it down.
The weights can eventually become sloppy on the posts and flop around quite a
bit. In that case the timing is harder to get it to repeat itself.
Probably most of us must replace the whole thing.
Altering the advance curve
The advance curve can be altered. One would only want to do this for some
special reason. Pinging (pinking for the Brits) isn't a reason. If you remove
weight from the two flyweights, it will advance even later in the rpm range. If
you make them heavier, it will advance at a lower rpm.
It is very important to lubricate the unit. The most common fault is for the
cam lobes of the advance unit to get stuck on the camshaft. Clean off the old
cruddy grease on the camshaft. Clean out the inside of the unit very well. Use a
high temperature grease on that part of the cam. Put some grease on the cam
itself and on the felt of the points plate that rubs against the cam. Try
to get some light grease, or oil, into the place where the weights pivot.
Testing the BMW timing advance mechanism
The first test is just after tightening the unit back onto the camshaft. With
your fingers, open the fly weights and let them go. They should open easily and
"snap back" smoothly and quickly, without any sticking. When one uses the timing
light to check the timing marks, also put the light on the advance unit and rev
it up and allow it to come back to an idle. The strobe light will reveal
any sticking or uneven operation.
More BMW motorcycle tuning info