R90S - engine timing

[tiktok]

I may struggle to describe this well - bear with me.

1975, 80k miles.

When running points the base plate was fully advanced and the view from the inspection window at 2800rpm was this:

I took the bike out for a run, 50 miles, no problem. Next day I covered 8 miles before it started missing, then cutting out entirely. The timing was out.

I took a punt on a Boyer Bransden module paired with the original coils and a new battery. But with the plate fully advanced (anticlockwise) the F mark is just north of the inspection window at 3500-4000rpm. There is still the double image appearing (letter F).

QUESTIONS
Does the split image in the video indicate chain or sprocket wear worth addressing at 80k?

With the Boyer, what would cause the stator to run out of adjustment before F is reached? Chain slack, cam-nose runout, something else?

Is elongating the points plate slots a reasonable fix?



Work so far:

• Stripped and rebuilt carbs (twice)
• New plugs
• Swapped leads and coils
• Set valve gaps
• New battery

 

[Gelandestrasse]

The F mark is for fully advanced running timing (3000rpm). You need to back it off (ie retard your ignition). This would also relieve your starter symptoms (neck stuck out there).

On tickover, at below 1000rpm (800 ideally but difficult to measure) the timing mark S should be in the middle of the hole, lined up with the mark on the edge of the hole.



The blurred/split image is likely a bent camshaft tip. I think Snowbum has the best advice there https://bmwmotorcycletech.info/timingchain.htm