Here's a simple Autolite single-point distributor. The vacuum-advance unit moves the breaker plate as the throttle is opened, and the vacuum advance receives a vacuum signal. The downside to contact points is irregularity. Point gap and dwell angle change constantly under the best of conditions, affecting performance regardless of how often you tune the distributor. |
Marvin recommends installing a PerTronix Ignitor II electronic-ignition retrofit kit, which eliminates the points and condenser, making a vintage Autolite distributor maintenance free. Marvin tunes mechanical advance first by using the right combination of springs and tension. Then, he tunes the vacuum advance. A vacuum advance should be tuned in a running engine while you're setting timing. |
It's the little things that can create performance issues. Marvin strongly recommends checking every place there's current flow for proper resistance. Distributor rotors, for example, are notorious for resistance issues that can cause a weak spark or misfire. Check the resistance across the rotor, primary wire, ground wire, rotor cap, and ignition wires with a digital ohmmeter, even when your engine is running well. |
Aftermarket vacuum-advance units are adjusted with an Allen wrench. Turn clockwise to slow the rate of advance; turn counterclockwise to speed up the rate of advance. |
This is the mechanical advance in an MSD distributor. Flyweights are affected by rotational speed and centrifugal force. As shaft speed increases, flyweights swing outward against spring pressure to advance the rotor timing. Spring tension is what determines advance curve. MSD offers different spring tensions depending on the desired curve. Ford did the same for Autolite and Motorcraft distributors. |
McAfee suggests an arsenal of top-quality ignition parts for your old Autolite distributor. A PerTronix Ignitor II retrofit is only the beginning. Opt for the gray cap because it offers the best resistance against crossfire. Go with either the factory, adjustable vacuum advance or this aftermarket, adjustable vacuum advance for precision tuning. |
There are times when no matter what we do with a carburetor, performance will never improve because of invisible flaws. Carburetors are aluminum castings that don't always have clear passages and smooth machining tolerances. Heat and use cause distortion and core shift, making a good carburetor turn bad over time.
We sometimes blame jet size, a faulty power valve, a blown accelerator-pump diaphragm, clogged idle passages, or poor idle adjustment when it's simply a distorted or closed passage. When you have exhausted all efforts and checked everything thoroughly, sometimes you've no other choice but to replace the carburetor. Doing a temporary carburetor swap can solve a lot of mysteries. If idle quality or overall performance improves, you've solved the mystery. If nothing changes, your problem likely isn't the carburetor.
Troubleshooting should include elements around the carburetor; base gaskets and spacers, intake manifold gaskets, vacuum lines and hoses, and throttle linkage adjustment. For example, did you know that a vacuum leak just about anywhere can cause poor performance? A torn power-braken booster diaphragm, bad vacuum modulator (automatic transmission), or ruptured air-conditioningnvacuum motor can cause vacuum leakage and poor performance. Trouble-shooting should include eliminating these as problem sources before looking any further.
Ignition Tuning
Performance can be improved dramatically by fine-tuning a classic Autolite or Motorcraft ignition system. First, have your distributor rebuilt by a qualified professional who can install new parts; bushings and shaft, cam, breaker plate, electronic-ignition retrofit, high-quality rotor and cap, and drive gear. Ideally, your builder will know how to properly curve the mechanical advance for your application. A seasoned distributor-tuning pro knows how to dial in the mechanical advance (affected by engine speed), then tune the vacuum-advance unit to work hand-in-hand with the mechanical advance. The mechanical advance can be dialed in accurately on a distributor machine. However, vacuum advance needs to be tuned in a running engine for best results.
At idle, there should be no vacuum to the vacuum advance because the throttle plates are closed. Vacuum-advance units get ported vacuum, which means only when the throttle opens. As the throttle opens, a working vacuum advance begins to get ported vacuum, which enables it to move (advance) the breaker plate. Theoretically, as engine rpm increases, the vacuum advance should hand off its job to the mechanical advance, which is affected by increased engine speed. Spark advance is needed because the fuel/air mixture doesn't explode when the piston reaches top dead center (TDC). The fuel/air mix needs time to ignite in a quick-fire before the piston reaches TDC. When the spark plug fires before TDC, the fuel has time to ignite and get fully underway before the piston reaches TDC. When the piston reaches TDC, it can make full use of the heat and energy created by the spark/air/fuel combo.
To get it right, dial in mechanical advance first, then fine-tune the vacuum-advance unit's rate of advance. Ford was smart about this with its original vacuum-advance units. Inside were shims and a spring designed to control the advance rate. Add shims to slow the advance rate; use fewer shims to quicken the advance rate.
Aftermarket vacuum-advance units typically employ an Allen screw inside to control advance rate. Turn the screw clockwise to slow the advance rate; turn it counterclockwise to quicken advance rate. To understand the effect this tuning has, use a timing light and watch the timing mark as you open the throttle. Jon tells us if you're seasoned at this, you won't even need a timing light. He does it by ear and feel.
Ignition timing needs to be checked and tuned at least two different ways. First, static ignition timing should be checked with the vacuum-advance hose disconnected. Follow factory specifications for initial timing. Small-block Fords are typically anywhere from 6-12 degrees before top dead center (BTDC). Six-cylinder engines are 5-6 degrees BTDC.