Jim Smart
April 1, 2009

Your Mustang's distributor performs one heck of a job. It has to deliver a timed spark to each spark plug at precisely the right moment in sync with piston travel and crankshaft, camshaft, and valve timing events. And it has to do it quickly and under extreme conditions. Because your Mustang's distributor has such an overwhelming task, it's important to understand how to rebuild and properly tune it for optimum performance.

Spark timing is everything to performance and engine life. We're not talking just for trailblazing, dragstrip performance; it's also required for normal driving where you count on precise distributor function. You need pep to get onto the freeway. You need a smooth idle and an engine that will not stall at the traffic light. And you need just the right amount of spark advance as rpm increases. Poor performance happens when there isn't enough spark advance (retarded timing). It also happens when there is too much (advanced timing). Engine damage can occur when there's too much spark advance.

We needed a distributor for a '64½ Mustang with a D-code 289 4V engine. Garrett Marks at Mustangs Etc. pulled this core from his inventory. It's a C4AF-12127-U single-point Autolite distributor, exactly what we need for a daily driver. Garrett was also able to provide us with the correct B8QH-12120-A bushing to complement our rebuild. PerTronix set us up with an Ignitor II module, a Flamethrower ignition coil, and ignition wires for improved driveability.

As antiquated as old Ford distributors may seem, you can fine-tune them if you think of them as precision components. Vacuum advance and centrifugal advance must work hand in hand for a seamless transition from idle to power. You can fine-tune a distributor with a distributor machine, but they are in short supply these days because distributor curving has become a lost art. You can also tune your distributor in a running engine using a timing light and dwell meter.

The vacuum advance unit is a small chamber that attaches to the distributor with two machine screws. Inside you'll find a rubber diaphragm and a linkage connected to the breaker plate inside the distributor. The vacuum advance unit moves the breaker plate to advance spark timing as we come off the carburetor's idle circuit and begin accelerating. The centrifugal (mechanical) advance is tied to the distributor shaft beneath the breaker plate and is there to advance or retard rotor position. As the shaft spins faster, it throws the flyweights outward (centrifugal force) to advance spark timing. Spring tension against the flyweights is what limits spark advance. The more spring tension, the higher the rpm as the mechanical advance is being applied. The less spring tension, the lower the rpm range when mechanical spark advance comes into play. It all depends on spring thickness and how much tension we place on the spring during curving. This is where a distributor machine comes in handy.

We managed to unearth a serviceable Ford vacuum advance unit for this rebuild.

An important footnote to this article is the distributor we've chosen. Our C4AF distributor, as found in early '64½ Mustangs, is fundamentally the same as C5AF (1965) and later distributors. However, the mechanical advance unit is different with smaller flyweights and a different approach to adjustment.

Precision Tuning
Ignition tuning isn't black magic. It's the simple physics of spark timing as it relates to piston position during the compression/ignition stroke. You want to set initial spark timing around 6-12 degrees BTDC at idle (650-750 rpm) with the vacuum advance disconnected and the vacuum hose plugged to get your baseline ignition timing.

Make sure the vacuum hose is getting metering vacuum, not constant intake manifold vacuum. Metered vacuum is above the throttle plates where vacuum occurs only when the throttle is opened. There should be little or no vacuum at idle.

The objective is to have vacuum and mechanical spark advance that is seamless. When you lean on the throttle, vacuum advance goes to work right away, handing off to mechanical advance as rpm and vehicle speed increases. To get there, vacuum and mechanical advance needs to come into play at appropriate times, rpm-wise.

Disassembly begins with breaker point and condenser removal. Next, disconnect and remove the vacuum advance unit. Don't lose the tiny C-clip, which is easy to misplace.

Once you have the initial timing dialed in at around 6-12 degrees BTDC at idle, you're ready to work on total timing and how quickly it arrives. Aim a timing light at the balancer and watch what happens to timing as you increase engine speed. Total timing, around 34-36 degrees BTDC, should be in by 3,000-3,500 rpm. Avoid going beyond 36 degrees BTDC. Depending on how your engine is configured, 34-36 degrees BTDC should be optimum. This means holding the throttle at 3,500 rpm and watching total timing with the vacuum advance connected. Total timing should be in by 3,500 rpm. The trick is adjusting both vacuum advance and mechanical advance so timing follows rpm as you open the throttle. When you goose the throttle and rev the engine quickly to 3,500 rpm, timing should follow the rpm without pre-ignition (pinging or spark knock). If there is spark knock as you jab the throttle or under hard acceleration, you have too much timing (timing too far advanced). If your engine falls on its face, you don't have enough timing.

There are some who will insist that you can take total ignition timing to 38-42 degrees BTDC. But do you want to risk your engine? Excessive timing may make more power, but you run the risk of engine damage because spark knock can melt pistons and break ring lands. Forty years ago, you might have been able to get away with 38-42 degrees BTDC total timing with 100-octane gasoline. Today, it's risky business with 87-92-octane pump gas.

Once you have mechanical advance timing dialed in, vacuum advance adjustment is easy. Aftermarket vacuum advance units adjust with an Allen wrench via the vacuum port. Watch the rate of advance when you goose the throttle to 3,500 rpm. How quickly does the timing mark move? If it moves quickly to total timing before the engine arrives at 3,500 rpm, you need to slow down the advance rate by turning the Allen wrench clockwise. This places more tension on the advance spring, which slows advance rate. Counterclockwise reduces tension, which in turn speeds up the advance rate.

If you're using a Ford vacuum advance unit, it works the same way with a different approach. Add shims to the advance spring if you want to slow down the advance rate. Eliminate or reduce the number of shims if you want to speed up the advance rate. When mechanical advance and vacuum advance work together, as they should, you get total timing, which is the most important timing element of all.

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