Courtesy the Merged Chapter and the Restorer
It's amazing how much time we put into setting breaker point gap to tune our Model A's to perfection, yet almost always ignore the gap between the rotor tip and the contact points in the distributor body.
Many times the alignment of the rotor tip and contact points are skewed and also too far apart. This results in taking more voltage for firing the spark plugs.
First, check the alignment of tip to contact point with the distributor off. Remove the spark plug wires from the distributor body so that they will not put any tension on the body, thus cocking it. The flat angled rotor tip should be horizontally straight across, not higher or lower. If the center of the rotor tip is below the contact point in the distributor body, do not try to bend the tip, as this will either cause it to break or will cause rotor tip to not be parallel to the contact point.
The solution is to cut some shims placing them inside of the rotor body between it and the cam screw until rotor tip alignment with the contact points in the distributor is correct. Remember to cut a nitch in your shims to match the inside of the rotor body. After years of polishing rotor tips and contact points, many Model A's have as much as a 0.060 gap. Try a new rotor and/or distributor body and bring the gap down to 0.035. This is where the Model A runs best at top speed. You can also build up the contact points with solder and then carefully file them back to the correct gap. If you originally had any missing or sputtering,, this may solve your problem.
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