I thought I might do a little cross posting...I had discussed this on the Vintage BMW forum. It might be useful/interesting to others.

A few months ago, I had noticed that the R69S was running rough and had little power. I began to check the usual suspects. Fuel flow, health of the new plastic floats (replaced the brass ones years ago), air filter, valve clearances, etc. Finally, it looked like I needed to check the timing. I hooked up my voltmeter to the points. Note that to use a voltmeter, one has to remove the lead wire to the magneto coil as this is a path to ground. This is a fiddly process as the nut/screw are pretty teeny...not something you want to drop or loose. So the initial checks revealed the the points opened well past TDC, so I was running a very retarded ignition. I believe this is probably due to the points rubbing block wearing down. I added some cam lube to the advance unit cam. I decided to change the points gap and was able to bring the S-mark back into the window. Figured I was good to go, so I buttoned things up.

A few weeks later, I got out on the bike. But within a few miles, I could hear pinging and it seemed to be coming from the right side. I limped back home. I have known that I have differential timing...I've seen it with a timing light where the S-mark is a blur because the left and right side points open at different times. I had also roughly measured it as about 1/4" by watching the position of the timing mark in the window for each of cylinder points opening. I thought that maybe this was contributing to the pinging on one side with the known differential timing...one cylinder was OK in timing, the other cylinder was advanced.

This was getting to be too fiddly, so I bought the magneto timing tool from Bench Mark Works. It works on induction and you don't have to remove all those small wires! Here's a picture I captured from their website:


I hooked it up...the ignition has to be on whereas using an ohm meter doesn't need that. Anyway, I removed the plugs and put the transmission into 4th gear so I could easily turn the engine over. I could see that my S-marks were advance and I could also see the difference from side to side. Richard at Bench Mark Works gave me instructions to reduce the timing difference. Once I had rotated the engine to the point where the most advanced side points was opening, I should take a punch and "love tap" the bolt that holds the advance unit on. I did that and rechecked...my S-marks appeared in the window almost in the same spot. The theory here is that for the cylinder too advanced, the points were running up on the cam too soon. By tapping the bolt from the right to the left, I'm driving the cam away from the points rubbing block, thus retarding the ignition. I modified the gap to get the static timing near the center of the window.

But my concern is mostly with where the timing is at full advance. So, I got a toothpick and propped out the advance weights to simulate full advance. I can do this because I have removed the small spring that was originally connected to soften the rate of advance. I have the later advance unit with the spring on its side, not the breakage prone "question mark" spring. In place of this spring, I installed what has been deemed the Caro stop screw. I don't have a good picture of it, but it acts as a hard stop for the advance unit weight movement. Years ago, I bought this screw from Bench Mark Works...not real sure of the size or dimensions of the screw. If you don't have this configuration, then this way of setting the timing won't work. It looks like this configuration but in my case, I removed the spring. Below are pictures (from Duane Ausherman's site) of two styles of advance units...the one on the left is the older style and the limit spring was prone to breaking...the one on the right (which I have) has the limit spring placed on its side. I removed this spring and put a screw where the brass colored head is...this is the hard stop to my advance.


With the advance unit propped into full advance, I turned the engine again for each cylinder and noted where the F-mark showed up. Because I had "love tapped" the advance unit bolt, the marks were in the same place but I was still advanced. I closed the points a little bit more and ended up with the F-mark slightly low in the window when the points open. This is 2-3 degrees of retardation. I can live with that...I'm more concerned about getting too advanced on the R69S plus being slightly retarded in timing gives me some room to move if I'm a little bit off. I removed the toothpicks and rechecked static timing...it was retarded as well, but the S-mark was near the bottom of the window. Shouldn't be a problem...I'm sure the bike will still start fine.