My previous column discussed the basics of the operation of breaker point- type distributors. In that column, I listed the many problems that could exist with distributors, but I did not address the details of actually setting the distributor to provide correct ignition timing.
The timing of when the distributor releases the charge from the ignition coil to the spark plug varies across the range of engine revolutions. The total timing of the distributor, reported in degrees, is comprised of that provided by the initial timing, the centrifugal advance mechanism, and the vacuum advance unit.
Distributor or crankshaft degrees
When reading timing specifications it is critical that the type of degrees reported in the specifications consulted is determined. Distributors turn at one-half the speed of the crankshaft. Therefore, degrees of timing can be reported in distributor degrees or crankshaft degrees. Typically, distributor specifications are reported in distributor degrees whereas typical engine tune-up specifications are reported in crankshaft degrees. However, variations on this pattern can occur so it is important to verify the type of degrees (distributor or crankshaft) reported.
Distributor testing machines, which test distributors across a range of revolutions, are of necessity based on distributor degrees, which are one-half of the crankshaft degrees. Accordingly, if the distributor testing machine shows the maximum centrifugal advance to be 14 degrees this translates to 28 degrees at the crankshaft.
Figure 1 presents the distributor specifications for 1941 Buick engines. In this instance, all degrees are reported in crankshaft degrees. Therefore, if these particular specifications were used to compare with the results obtained on a distributor testing machine they would have to be divided by two.
Setting the timing
The centrifugal timing is a function of the centrifugal mechanism. If it is not correct, it can be fixed by cleaning accumulated lubricant and dirt and repairing worn components. Fine tuning of the centrifugal component can be accomplished with the tension of the springs restraining the centrifugal weights. A distributor testing machine is necessary to accurately check centrifugal advance timing and make all adjustments.
Vacuum timing is a function of the vacuum advance mechanism and is also tested using a distributor testing machine. It either performs as intended or it does not. However, there are adjustable vacuum advance mechanisms available for some distributors, but not all.
The initial timing is the timing component that is set when an engine tune-up is performed. It is accomplished by setting the engine position to the specified timing mark found on the flywheel or crankshaft damper when the number 1 piston is in its top dead center position on its compression stroke. Because each piston makes two complete revolutions for each complete cycle – intake, compression, power and exhaust – the timing mark can appear when the piston is not on its compression stroke. The compression stroke can be determined by holding a thumb over the spark plug hole and feeling the pressure buildup on the compression stroke. There are also devices that be can installed in the spark plug hole that whistle on the compression stroke as air is forced through the device.
With the number 1 piston in its correct position indicated by the timing mark on the compression stroke, the initial timing can be set close to the required specification with the engine stopped by using a test lamp (see Figure 2). Connect one lead of the test lamp to the distributor primary terminal and the other to ground. Then, with the ignition switch on, slightly rotate the distributor clockwise or counterclockwise until the lamp just lights. If a test lamp is not available, remove the high tension lead from the center of the distributor cap. Hold this terminal about one-eighth inch from the closest ground (the distributor case or block). Again, turn the distributor clockwise or counterclockwise until a spark jumps from the high-tension lead to the ground. The distributor is now set in the firing position at specified idle speeds and is sufficiently accurate to start the engine. This assumes that the rotor is pointed to the number 1 cylinder spark plug wire connection.
Fine-tuning the initial timing requires that the engine be fully warmed and the carburetor linkage off the fast idle cam and set to the correct idle speed. After these conditions are reached, shut off the engine and connect a stroboscopic timing light in accordance with manufacturer’s directions. Also, disconnect and plug the vacuum connections to the distributor vacuum advance mechanism. Then, re-start the engine, verify that the idle speed is correct, and point the light at the timing mark. The timing light will emit a flash of light every time the number 1 plug fires and the timing mark will appear stationary opposite its pointer. If the correct timing specification is not observed, move the distributor clockwise or counterclockwise until the mark is at its proper position. Then, tighten the clamp on the distributor and double-check the reading.
William C. “Bill” Anderson, P.E., has been involved with the automotive hobby for more than 30 years with experience ranging from hot rods, to sports cars, to sports car racing, and to restoration of vehicles from the 1930s through the `80s. He is an author, magazine editor, car show judge and professional engineer. A member of several car clubs and a leader in some, through Anderson Automotive Enterprises he restores and appraises cars.