Timing Question

[Seth]

1975 R90S
While timing with a timing light, the "S" in the window is a double image, but stable.
While revving to full advance, no double image.
my old Clymer manual says the problem is the advance unit.
Is it also possible that it is the timing chain?
A new advance unit is $250, but easy.
The timing chain is less, but much more work. Where do I start?

Thanks in advance.

[Kurt in S.A.]

I've heard it could also be the timing chain. It could also be a bent nose on the camshaft. Duane has information about what this is and how to fix it...you'll need some pelotas to do this, though! Anything less than 1/4-inch double image, and I wouldn't be worried.

Another "easy" fix is an electronic ignition system. A Dyna III uses the stock advance but has independent pickups to fine tune the firing of each cylinder. My "S" mark is not a single one after mounting the Dyna. There's also a crank-triggered system which totally eliminates this as it fires only once per crank revolution, not twice. See:

http://www.largiader.com/articles/ignition/

Kurt in S.A.

[Duane Ausherman]

Sometimes a single image is only that you see a single image, but the "other" image is out of view. Are you sure that you only have one image at the F mark? It would be very rare to have split marks at the S and not the F also.

Always paint the timing marks with white paint. Always.

Each of my mechanics had a tiny can of white paint available for this purpose. We didn't bother with a brush, just dipped a slot screwdriver into the paint a mm and touched the mark on the flywhee.

[Airbear]

Seth, I put off replacing the timing chain on my '74 90/6 for far too long. When I finally went in there this is what I found -



The chain was really stretched and had been thrashing around, gouging away at the case. Replacing the chain is not that difficult - it took me less than a day including making up some 'special BMW tools'. I have made up a step-by-step guide with pics. It's about 2.7MB. Shoot me an email or a PM with your email address and I'll attach it to a return message.