ignition advance centrifigual unit

Hi everyone,

so the "D" shaped mounting point of my centrifugal advance unit is damaged by a previous owner
obviously the end of the cam needs some clean up also

I see the part number as: 12 11 1 356 286

the bike is a 1971 R75/5

it looks like the factory changed the unit and part number around 1973 to a different style
I found something on the internet that stated the earlier plate had a quicker advance for better fuel, and that the later plate was preferred due to fuel quality....

hmmmm.. maybe?

I was thinking that I would find a new unit, that had a correct "D" shaped mounting point and try to clean up my mounting and hope for a better fit
I assumed I should buy the one for my year and the early characteristics.

but thought I would still ask... to see... if it is recommended to upgrade to a slower curve?
is that a recommended improvement?

thanks for everyone's time
brant

What is the extent of the damage? Is it possible to get the advance unit to engage the nose slot and come to a solid position? No matter, you will always have to engine the slot and turn the advance unit until it stops and then tighten things down...that way you have a known starting point. Since the engine turns clockwise (I think) when looking at the front of the engine, you would rotate the advance unit counter clockwise to set it in position.

Not sure the new advance units are all that well built. I know on the /2 side, someone has been pretty disappointed with what is being offered...they might have been non OEM too. The advance rate, etc., is controlled by the stiffness of the springs which can be replaced.

If the current advance unit isn't going to work, I might pursue a replacement system, one that doesn't rely on the cam detent. Such as the Boyer Brandson system. There are better systems that are crank-triggered so there isn't the variation side to side. The Wedgetail system out of Australia is getting good coverage and seems to be worth considering.

If you chose to go down this path, only you would know as it's basically out of the way. And you'll likely get better performance.

good advice... and an aftermarket unit is bound to be cheaper than the 300 factory replacement

my damage seems contained only to the flat "D" mounting point.. I can only guess that someone in the past didn't have the unit seated correctly and then overtightened the nut causing the deformation of the mounting area

It holds timing.... once the center nut is tightened. but does not sink in and lock to the flat D shape of the cam....
with the center nut loose... you can rotate the unit pretty significantly (almost 90 degrees)

I was hoping that a replacement might improve the engagement and reduce the rotation
but again, If I tighten the nut on the end of the camshaft... it seems to hold well enough to not change timing... I just have to play guess a few extra times to get the right starting point and timing point.

probably wrong.. but I found a comment on a different forum about using the 1973 (later unit) on the earlier bikes and changing the curve
the person commenting... said the factory did this in 73 due to poor fuel quality
I did not know if that was bunk... or actually good advice

so if the springs are what control the weights.. and control the curve... then it would stand to reason that the 1973style just used different springs... hmmm I wonder if the actual base plate those weights and springs are mounted on would be the same.... just thinking out loud. I might look on ebay for a clean base and then see if I can transfer my weights and springs over to it.

edit... I just looked on ebay... looks like the weights are made on.. not changeable... so back to buying new, buying used... or buying and upgraded ignition system



brant

Looking at the boyer and autstralian one...
they both have a small ignition box that needs to be put somewhere....

I assume not under the front cover? is under the tank the commonly used mounting point for the module?

I'd spend some time on Snowbum's site for a description of ATUs or advance units:

https://bmwmotorcycletech.info/ignitionsingleplug.htm

If you can get a repeatable timing, that might work out. I think some of the issue with repeatability is that you usually have to remove the advance unit in order to adjust the point gap due to the location of the lock screw. Then you put the advance back on to test...did it get back in the same spot?? There is a "tool" that replaces the advance unit for the purposes of setting the gap perfectly. Note that the gap on the points can be a pretty wide range, not the 0.016" that is called out. That is an initial setting and then if you need to change the actual timing of the spark, you have to rotate the points plate. If you run out of plate rotation, you change the gap a little bit (it doesn't take much) and retry timing.

As for the "brains" of the units, I have the Dyna III installed on my /7. It relies on the original advance unit so it doesn't help your situation. But it has a box which I have bolted to the cross tube on the frame downtubes...this gets it in the wind for cooling as needed. The /5 doesn't have the cross tube but it could still be located on the downtube. I think one needs to follow their directions for cooling. If they say it's important, better not put it under the tank. If there's enough wiring, maybe a mounting system behind a side cover (if you have them) might work.

Thread Drift Light On..
I've got a 1978 R/80.
I've asked this question before on this forum and cannot get my head around this question:
How Does The Ignition Advance?

The Points are on a fixed plate; they don't move, doesn't/ can't change position.
The Points Cam lobes are on a fixed point on the shaft, doesn't/can't change position.

With out one of the above "moving" I cannot figure out how the spark timing advances...
I see the spring weights spread out with RPM increase and that's it.
How do the weights spreading out effect the time the points open?

I can plainly see with a timing light the advance happening with RPM increase.

What am I missing?

All other ignition systems I have worked on the base the points are attached to rotates, from vacuum or centrifugal forces.

Am I just a retard?
Nick

advance

First, i would want to see if the wear on the advance unit was on the unit itself or the end of the cam. If it is on the cam then a new advance unit or anything aftermarket isn't going to cure the problem. If that is the case then a cam replacement may be the best choice.
As for the method of advancing the ignition, you can see the actual lobe section turn when you pull out the weights, thus advancing or retarding the timing.

nickrides said:

Thread Drift Light On..
I've got a 1978 R/80.
I've asked this question before on this forum and cannot get my head around this question:
How Does The Ignition Advance?

The Points are on a fixed plate; they don't move, doesn't/ can't change position.
The Points Cam lobes are on a fixed point on the shaft, doesn't/can't change position.

With out one of the above "moving" I cannot figure out how the spark timing advances...
I see the spring weights spread out with RPM increase and that's it.
How do the weights spreading out effect the time the points open?

I can plainly see with a timing light the advance happening with RPM increase.

What am I missing?

All other ignition systems I have worked on the base the points are attached to rotates, from vacuum or centrifugal forces.

Am I just a retard?
Nick

Click to expand...

The "points cam lobes" are actually on a hollow tube which fits over the tip of the cam shaft, and they are rotated by the weight. Take a close look and pull on the weights and you will see this movement.

1074 said:

First, i would want to see if the wear on the advance unit was on the unit itself or the end of the cam. If it is on the cam then a new advance unit or anything aftermarket isn't going to cure the problem. If that is the case then a cam replacement may be the best choice.
As for the method of advancing the ignition, you can see the actual lobe section turn when you pull out the weights, thus advancing or retarding the timing.

Click to expand...

I got busy with other chores yesterday and did not pull the advance unit yet.
I will get the numbers off of the actual unit and try to find a used part that matches...

I think as much of an improvement that a modern ignition system would be (I have them on all of my vintage cars)
this bike is pretty stock and I kind of like it that way
so for now... will hold off and fight the urge to mount an auxiliary ignition box anywhere that could show
and just find a cleaner mounting stock centrifugal ignition unit...

I'll get pics of the cam
my recollection was it is still salvageable...
and I'm not loosing timing once the nut holding it is tightened...
so I'm hoping for a cleaner unit, will only make things better.

brant

Switching

Switching from points to non in my case made perfect sense. I put a lot of miles on and hate adjusting points or as has been my experience having points going out of adjustment at inconvenient times.

I guess if I didn't ride so much or the bike in question was little used, I might be tempted to keep the points intact.

The cheap way to eliminate points is the Dyna system, but the best way I have found so far is the Wedgetail system. Very easy to install and simple to set up. St.

nickrides said:

Thread Drift Light On..
I've got a 1978 R/80.
I've asked this question before on this forum and cannot get my head around this question:
How Does The Ignition Advance?

The Points are on a fixed plate; they don't move, doesn't/ can't change position.
The Points Cam lobes are on a fixed point on the shaft, doesn't/can't change position.

With out one of the above "moving" I cannot figure out how the spark timing advances...
I see the spring weights spread out with RPM increase and that's it.
How do the weights spreading out effect the time the points open?

I can plainly see with a timing light the advance happening with RPM increase.

What am I missing?

All other ignition systems I have worked on the base the points are attached to rotates, from vacuum or centrifugal forces.

Am I just a retard?
Nick

Click to expand...

The points cam lobe is actually part of the advance mechanism. Take the advance unit off the camshaft and you'll see there are two "high" sides, that engage the points block and therefore open the points. As engine rpm increases, the advance weights move out and move the cam lobes to advance the timing, by opening the points earlier.

nickrides said:

Thread Drift Light On..
I've got a 1978 R/80.
I've asked this question before on this forum and cannot get my head around this question:
How Does The Ignition Advance?

The Points are on a fixed plate; they don't move, doesn't/ can't change position.
The Points Cam lobes are on a fixed point on the shaft, doesn't/can't change position.

With out one of the above "moving" I cannot figure out how the spark timing advances...
I see the spring weights spread out with RPM increase and that's it.
How do the weights spreading out effect the time the points open?

I can plainly see with a timing light the advance happening with RPM increase.

What am I missing?

All other ignition systems I have worked on the base the points are attached to rotates, from vacuum or centrifugal forces.

Am I just a retard?
Nick

Click to expand...

There’s a pretty good explanation and visualization of a centrifugal advance assembly here: https://vehicletech1.blogspot.com/2019/05/centrifugal-Vacuum-advance.html They are addressing this from an automotive standpoint, but the operation of the centrifugal advance mechanism is the same for both car and MC.

Best,
DeVern