Vintage BMW Motorcycle Owners

Hello friends. I have three BMW's. A BMW R69 from the year 1956, a BMW R50 Sport from the year 1961 and a BMW R51/3 from the year 1953. Does that make me an expert? The answer is no! And even when I go out every weekend to travel kilometers with these machines, one still has doubts. For example this: My R69, when idling, jerks from side to side more than normal. The points are regulated, and the carburetion is according to the manual. But here I have doubts, what is the usual reason, in which one of these machines shakes from side to side when it is idling? I appreciate all advice. Cheers!

Welcome! Vibration like that could be due to not a good synch between the carburetors. Additionally you might have differential timing, meaning that one cylinder fires a little before the S-mark and the other cylinder fires on or after the S-mark. One can redo the timing so that you split the difference as best as possible. I had differential timing on my R69S and talked with Richard at Bench Mark Works before they closed up. He outlined a procedure to adjust the timing to bring the firing points closer in line. That worked very well and now my bike's idle is very smooth. Maybe Vech will be along to help explain that.

Thank you so much! Differential timing makes a lot of sense. Now, how to correct that, is a good question.

maybe is your crankshaft ?????

Hmm, I don't think so, this came after adjusting points and carburetors. Now, how to fix it is what I don't know.

General concept on "fixing" differential timing. Rotate the engine so the timing of the cylinder that is farthest out is in the timing window. Take the engine timing cover off. Sit on the right side of the bike. With a blunt point punch, put it against the bolt that holds the advance unit to the camshaft. With light taps using a hammer, try and move the bolt to the other side of the bike. After a few taps, recheck the timing. If the differential timing hasn't changed, repeat. It didn't take much tapping to correct my situation.

Thank you so much! These are the situations in which a video would be appreciated (just kidding). I understand the theory, but my guessing is that it is more complex in praxis. (Certain level of ‘know how’ is clearly required). I’m pretty sure however that the origin of the trouble is differential timing. Thanks again!

put on gloves.
start engine.
Pull left spark plug cable. observe for a second to 2
pull right cable. observe.
If the engine shakes at idle, the engine should stall on one one side or the other.
Turn up the screw that raises the slide on the carb that stalled would be the first experiment.

Also look into the throat of the carbs. I had a rough idle episode when the steel insert in one carb pushed up out of the aluminum body

Idle speed too low?

Thank you so much for your responses, will try the idle experiment