'k.

Now, if your retard function was working and retarding the ignition, setting the timing to 5 or 6 btdc without any vacuum signal at 700 rpm....then putting the vacuum back to the distributor, would pull the set advance off and it would read about 2 atdc....this would reduce tail pipe emissions c/o% as the retard function is secondary to the advance vacuum signal....the canister is coupled inside to allow the advance vacuum to overpower the retard vacuum when any vacuum is applied.
When the engine lost vacuum as the rpm's dropped, the vacuum retard would drop out and the added advance would pick the rpm's up again...(.a self correcting feature.)

When the throttle is opened, the induced throttle venturi vacuum is sent from the throttle body port to the advance canister and immediately the retards drops out and advance begins, mechanical advance starts around 1200 rpm. Total vacuum and mechanical advance would be around 42 to 46 degrees and would drop to mechanical only when opening the throttle for power, as also the manifold vacuum drops low so there is still no vacuum retard signal.



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