Here are pics showing the inside of the distributor (the type you are talking about: JFUR4 0231 146 078) of my 1970 B20B.
As said earlier, it forms part of a vacuum retard system, specifically for emission control purposes. In this system suction for the vacuum “pot” on the distributor is obtained from a very specifically located port on the rear carb (of the twin SUs). The port is in the region of the throttle plate, on the underside of the carb, such that when the throttle plate is closed the lower edge of the throttle plate lies just upstream of the port (and suction at the port is high). As the throttle opens, the edge of the throttle plate moves to a point downstream of the port and suction rapidly falls away.
As a consequence, I don’t think the vacuum connection in this system would have any effect on the ignition timing except when the throttle is almost closed.
I have blocked off the port, as most people seem to do.
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Jul 23rd, 2021, 21:20
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