How I solved the differential timing issue

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laurent31
Posts: 14
Joined: Wed Oct 14, 2020 12:55 pm

How I solved the differential timing issue

Post by laurent31 »

Actually not really an issue because the bike was running OK and the differential timing was quite low. But I dislike things that are not what they should be. About 5 or 8 mm on the flywheel after all basics checks (taper clean and lapped, rotor spindle end face trued).

I read carefully the topic from the experts : Duane, Snowbum and Peter Ardron. Thanks much to them for their help. Duane's method is a good idea but it needs improvement. Peter is right when saying that hammering the rotor can do disasters.

I added 2 ideas to the global scheme :
- The straightening should not be made in the dual lobe plane of the advance plate but in the plane it is actually distorted which could be different.
- The rotor and the camshaft should be straightened separately.

I measured the rotor in a lathe with a dial gauge : it was bent : 28/100 mm. See measurement setup.
measurement setup
measurement setup
First of all, I bought a second used rotor. Also bent, but at least, I can destroy one.

Why are they bent ? My hypothesis is that someone, not making the simple tool to extract the rotor did it the hard way. A side-way whack on the rotor or worse on the shaft end. This method is still a common (wrong) method for removing the fan of a 2CV :
https://www.youtube.com/watch?v=jVaI554DZ2g

So measurement, a piece of chalk and drill press on the right place and I eventually got less than 1/100 mm at the end of the shaft. 5 steps and low pressure on the press each time : 400 to 800 kg. Notice the zinc sheets between rotor shaft and steel blocks.
rotor straightening
rotor straightening
Back to the engine now : no or little improvement. I must admit that the camshaft tip is also bent.
Difficult to measure but when amplified with the length of the rotor shaft, now straightened, it shows 5/100.
The forward end of the camshaft that protrudes beyond the bearing has a smaller diameter and is hollow. This part is weak and it is probable that the inner part is not bent and won't be bent by the following straightening process.

So out the stator, keeping only the rotor with the dial gauge fixed on the engine stud.
I kept the rotor centre bolt tightened with a washer to protect the shaft end. No centrifugal unit for the moment.
With a 2 mm thick flat iron, it is possible to hammer the larger shank of the rotor shaft between the engine block and the rotor rear flange.
The flat iron punch
The flat iron punch
This corresponds roughly to the camshaft end. This way, no risk to bend the rotor shaft. 2 or three light hammer blows on the flat iron with measurements between. and I got 2/100 mm at the shaft end. I said light hammering: this part is very weak !
I then installed the advance plate and did the measurement on the lobes. A last hammer blow on the flat iron and I eventually got 0 run-out. Now everything is fitted and there is less than 1 mm on the flywheel. Yeah, got it!

Sorry for the European units but after all, these bikes are German!
Laurent.

olebmw
Posts: 30
Joined: Fri Oct 27, 2006 3:00 am

Re: How I solved the differential timing issue

Post by olebmw »

How did you determine that your 3 jaw chuck is clamped on the rotor with zero run out?

laurent31
Posts: 14
Joined: Wed Oct 14, 2020 12:55 pm

Re: How I solved the differential timing issue

Post by laurent31 »

A three jaw chuck is not as acurate as can be a 4 independant jaw. But I trust it. Anyway, it can be tested with as check bar of approximately the same diameter. And mine is quite good, less than 1/100 mm of runout.

When positioning the rotor into the chuck, I avoided the small grove of the advance plate.

Laurent.

olebmw
Posts: 30
Joined: Fri Oct 27, 2006 3:00 am

Re: How I solved the differential timing issue

Post by olebmw »

Not quite the approach I would use on checking the run out on the rotor, but if the end results get you where you want to be ,
that's good.
Are you calling the flywheel, the large diameter of the rotor?

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malmac
Posts: 473
Joined: Sun Jun 29, 2014 2:10 am
Location: Toowoomba, Australia.

Re: How I solved the differential timing issue

Post by malmac »

Interesting post. Thank you for showing us what you did.

These are the adjustments that promise to allow your bike to run the way it was designed to run.


Mal
mal - R69s
Toowoomba- Australia

laurent31
Posts: 14
Joined: Wed Oct 14, 2020 12:55 pm

Re: How I solved the differential timing issue

Post by laurent31 »

The flywheel is the engine flywheel. Output of the crankshaft. On the flywheel the distance (mm) between the ignition locations of both cylinders represents the run out of the magneto rotor. This is the ignition timing difference between cylinders. It should be minimum.

Laurent

808Airhead
Posts: 1016
Joined: Wed Aug 05, 2009 9:06 pm

Re: How I solved the differential timing issue

Post by 808Airhead »

I tap the allen bolt on the advancer with a brass drift and small hammer from the side that is "advanced" and it gets within 1mm. That is Duanes method and it works.
Thomas M.
R69s - R60/2 - R67/2 - R51/3

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