The spark advance mechanism of the Holley pressure type distributor is controlled by a vacuum operated diaphragm operating against the tension of calibrated springs. The degree of spark advance is primarily regulated by the strength of the vacuum acting on the diaphragm. The vacuum is transmitted from two interconnected passages within the carburetor through tubing to the distributor diaphragm chamber. One of the passages is open at the venturi throat of the carburetor and the other opens into the throttle bore. The two passages are calibrated to supply vacuum of the proper strength for the required spark advance during operation under all engine speed and load conditions. Any time the vacuum from the venturi becomes stronger than the vacuum from the throttle bore, the distributor passage ball in the vertical passage from the throttle bore will be drawn up to its seat, closing off the lower passage. This prevents any weakening of venturi vacuum due to air bleeding from the lower passage. Care should always be exercised when performing any service operations on a carburetor to prevent damage or distortion of the distributor vacuum passages.
