Greetings, Can anyone explain briefly, why the dist. on the B-20 retards the spark at idle via the vacuum and then advances the spark mechanically when engine speed increases? Since the vacuum/retard effect is ongoing while the engine is running, it seems that the mech. advance would be trying to overcome the vacuum control. The vacuum control on this B-20 retards the spark about 12 degrees from the 10 BTDC setting when the vacuum line is reconnected to the dist. after timing is adjusted. I think ignition timing is causing some hesitation in the acceleration at low speed, but I don't know how the vacuum control and the mech. advance are supposed to work together nor do I understand how to interpret the green book specs for this. Best, Mike