Maybe I can add a little to the discussion here- - - - - -Most of the stovebolt engines work best with ported vacuum to the distributor, as the pointer & ball setup on the flywheel is designed to allow the engine to be timed with the vacuum advance out of the picture. This is done by drilling the port for the distributor fitting so it enters the bore above the throttle plate, choking off the vacuum source while idling. As soon as the throttle is opened slightly, the vacuum comes in gradually to work the advance unit. The distributor will be calibrated to take into account the position of the timing port when advancing the spark. The vacuum advance is mostly used to provide better gas mileage under light-throttle cruise conditions, with the majority of the spark timing advance provided by the centrifugal advance weights which respond to engine speed. The final 10 degrees or so of advance is available under light-throttle cruise conditions by allowing vacuum to be applied to the advance unit when there's little or no heavy pulling going on. As soon as a driver steps on the gas to pass, pull a hill, etc., the vacuum drops off, and the ignition timing is retarded several degrees to prevent detonation while accelerating or pulling a hill under load. As soon as the load goes away, the timing can be advanced again for best fuel mileage.
If vacuum is available at idle, the timing gets advanced 8-10 degrees or so as soon as the vacuum line is connected. This raises the idle speed, and requires that the idle speed screw and mixture adjustments must be made with incorrect initial timing. The added initial timing also affects the engine's response to load-pulling and cruising fuel economy. I don't know of a way to correct the problem, other than to find a carburetor with the right passageways drilled to provide ported vacuum rather than manifold vacuum at idle.
Jerry
Last edited by Hotrod Lincoln; 02/26/2011 5:28 PM.
