How -To Rebuild Autolite/Motorcraft Distributors
Mustangs Etc. And Pertronix Show How To Make Your Vintage Ford Distributor Like New
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.