Dwell angle and vacuum retard

[lorkp]

I have a stock ‘79 that runs points and a distributor with vacuum advance and retard. My question is about setting dwell: at idle, vacuum retard on the distributor decreases the dwell angle by about 7 degrees; am I supposed to set dwell to the spec (59-65) with it connected or disconnected?

To me, it would make sense to set dwell with it not connected, since vacuum retard is pretty much just for emissions at idle and not a factor when driving. But perhaps this retard “offset” was factored in by BMW. What do you think?

[epmedia]

The deviation is from wear, and or sticky components. Set the dwell while vacuum retard hose is disconnected. Consider doing some service to the distributor. Also, remove the rotor and put a couple/few drops of engine oil under the small round felt pad (assuming the felt pad exists). May even want to polish the points cam.

[lorkp]

Thanks, epmedia. I’m trying to access some rudimentary geometry in my brain—could the problem be that the rotating plate’s circular center is not the same as the shaft’s?

[epmedia]

Thanks, epmedia. I’m trying to access some rudimentary geometry in my brain—could the problem be that the rotating plate’s circular center is not the same as the shaft’s?

Yes, plausibly from sticky dirty grease in the plate's ball bearings + possibly some wear in there.