Unless you have radically altered the centrifugal advance curve in the distributor, 20 degrees is way too much advance at idle speed. Disconnect the vacuum advance and plug the hose, and set the throttle to 2500 RPM. Set the advance to 36 or 37 degrees at 2500, and let the idle advance fall wherever it will. If there's a hesitation on tip-in, you'll need to alter the advance slots so you can use 12 to 15 degrees of initial advance without exceeding 37 degrees at 2500. With the vacuum advance functional, look for about 42 to 45 degrees total advance at light-throttle cruise.
Jerry
