M-Block 351M/400 Performance

Correcting retarded ignition timing

Copyright 2002-2003 Dave Resch
All rights reserved.


Ignition timing and fuel/air mixture are interrelated parameters. Since excessive ignition timing advance and excessively lean fuel/air mixture will both contribute to pinging problems, adjusting these parameters for optimum performance is an iterative, trial-and-error process.

You might advance ignition timing a little, then enrich the fuel/air mixture, then advance ignition timing a little more, etc.

Understanding ignition timing

Ignition timing refers to the timing of the spark plug firing, relative to the crankshaft position. Ignition timing is measured on the spark plug of the #1 cylinder.

There are two elements of ignition timing:

  • Base timing, and

  • Advance curve.

Ignition base timing is what you measure with a timing light and adjust by rotating the distributor body.

Timing advance is the change in ignition timing (from the base timing) produced by a timing advance mechanism. The term “advance curve” refers to the rate at which the timing advance changes.

Most Ford distributors built since 1968 have both mechanical advance and vacuum advance systems.

  • The mechanical advance system consists of weighted cams and springs that provide ignition timing advance as engine speed increases. Centrifugal force causes the weighted cams to move and advance the timing, so mechanical advance is sometimes referred to as centrifugal advance.

  • The vacuum advance system consists of a vacuum motor connected to the base plate of the distributor. It provides ignition timing advance in response to a vacuum signal. The source of the vacuum advance signal is usually the carburetor’s “spark port.”

Ignition base timing and vacuum advance can be easily adjusted without removing or disassembling the distributor. Adjusting the mechanical advance requires at least partial disassembly of the distributor.

Tools for adjusting ignition timing

Adjusting ignition timing requires just a few special tools:

  • Timing light

  • Vacuum gauge

  • Tachometer

I recommend a decent timing light with an inductive pickup and a xenon flash bulb. The xenon bulb produces enough light that you can see the flash during daylight, which makes it easier to work on your engine. You don’t need a fancy timing light with advance/delay adjustments.

You need a decent (reasonably accurate) vacuum gauge for tuning the ignition vacuum advance and adjusting the idle speed.

You also need an accurate tachometer, ideally one that can show rpm variations down to 25 rpm or so. You’ll use the tachometer to adjust idle speed and to measure rpm when checking the ignition advance.

Adjusting ignition base timing

Use this procedure to adjust ignition base timing for maximum practical advance:

M-Block Ignition Base Timing Adjustment

Step 1.

Check and record the current base timing.

  • Remove and plug the vacuum source from the vacuum advance diaphragm port.

  • Make sure the idle speed is adjusted to the calibration specification (50 rpm). If you don't have a calibration label, set the idle to 650-700 rpm.

Recording the current base timing gives you a known starting point. Keeping records makes it easier to back-track if you go too far in your adjustments.

Step 2.

Set vacuum advance to maximum sensitivity.

  1. Insert a 1/8" Allen wrench into the vacuum advance port and engage the adjustment screw.

  2. Turn the screw clockwise until it stops. That is maximum sensitivity for the vacuum advance.

Make a note of the vacuum advance adjustment.

Step 3.

Test drive.

Take a test drive with your current ignition settings. Warm up the engine to normal operating temperature before you start driving.

  • Try to drive over a variety of conditions that are typical for your usage – moderate to brisk acceleration, steady high speed (freeway), steady mid-speed (30-45 mph city driving), climbing hills (the longer the better), etc.

  • Try accelerating both mildly and aggressively from various steady-state conditions.

Make a note of any conditions under which the engine pings, and how pronounced the pinging is.

  • No pinging at all means you can advance the timing more.

  • Very faint and/or infrequent pinging means you are close to the threshold of maximum practical timing advance.

Step 4.

Reset the timing.

If you encountered either no pinging or very slight pinging, advance the base timing by 2 degrees.

Step 5.

Test drive again.

Repeat the previous test drive and try to duplicate the test conditions.

Make a note of any changes in pinging or engine performance.

  • If pinging worsens, either drop the base timing back to the previous setting, or try adjusting the vacuum advance sensitivity.

  • If you still encounter either no pinging or very slight pinging, advance the base timing by another 2 degrees.

Step 6.

Repeat steps 4 and 5 as necessary.

Keep repeating the cycle until you get to the maximum practical base timing advance.

Determining the right vacuum advance sensitivity for your engine requires a trial-and-error procedure similar to the procedure for setting the ignition base timing.

Next: Vacuum advance tuning

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