M-Block 351M/400 Performance

Vacuum advance tuning

Copyright 2002-2003 Dave Resch
All rights reserved.


Vacuum advance enhances throttle response by adding a little ignition timing advance at the same time that the throttle plates open further. Inadequate vacuum advance causes sluggish throttle response, especially off idle (like starting moving from a stop) and when accelerating from a steady (part-throttle) speed. Excessive vacuum advance causes pinging under those same conditions.

Vacuum advance systems on Ford distributors are easily adjustable. All of them have an adjusting screw inside the vacuum canister. The adjusting screw moves a spring to apply tension against the vacuum diaphragm. Increasing spring tension on the diaphragm makes the vacuum advance less sensitive. Decreasing spring tension on the diaphragm makes the vacuum advance more sensitive.

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.

Checking vacuum advance function

Before you can tune the vacuum advance, you need to make sure your vacuum advance system is working. Start by checking the basic function of the system.

You need some the extra vacuum hose (1/8" ID) to connect to the vacuum advance diaphragm so you can apply vacuum and observe the operation of the system. A vacuum pump is nice, but if you have a clean piece of hose, you can just use your mouth to apply vacuum to the vacuum advance.

Use this procedure to check basic vacuum advance function:

Checking Basic Vacuum Advance Function

Step 1.

Remove the vacuum hose from the vacuum advance canister.

Step 2.

Attach test hose to vacuum advance canister.

This is the hose you'll use to apply a test vacuum to the diaphragm.

Step 3.

Remove the distributor cap.

Release the clips and pull the cap back so you can view the distributor workings.

You don't need to disconnect any wires from the distributor cap.

Step 4.

Apply test vacuum to the vacuum advance.

Use a vacuum pump, or just suck on a clean piece of hose attached to the vacuum advance canister.

When checking basic vacuum advance function, you want to make the following observations:

  • The diaphragm should hold a vacuum. If it does not, you need to replace the vacuum advance canister. A bad diaphragm will produce little or no advance, and it will introduce unfiltered air into the carburetor’s throttle bore.

  • From the back of the canister, an arm extends into the distributor housing and attaches to a pin on the base plate. This arm should pull on the base plate when vacuum is applied, causing the base plate to rotate clockwise around the center shaft housing.

Vacuum advance works by rotating the base plate to change the position of the magnetic pickup relative to the “stator” on the distributor shaft. By rotating the base plate in the direction opposite of distributor shaft rotation (counter-clockwise), timing is advanced because the trigger signal is generated earlier.

Testing spark port vacuum and vacuum advance timing

Once you have verified basic vacuum advance function, you need to test the carburetor’s spark port vacuum signal and the total vacuum advance timing. To do this, you need the following tools:

  • Vacuum gauge

  • Timing light

You’ll use the vacuum gauge to monitor the vacuum signal from your carburetor’s spark port. You’ll use the timing light to measure the timing advance produced by the vacuum advance system.

Use this procedure to measure spark port vacuum:

Measuring Spark Port Vacuum

Step 1.

Connect the vacuum gauge to the spark port hose.

You can use a T fitting to connect the vacuum gauge to the spark port vacuum hose and leave the vacuum hose connected to the vacuum advance canister.

Step 2.

Start and warm up the engine.

Get the engine up to normal operating temperature.

Step 3.

Work the throttle while watching the vacuum gauge.

Make a note of the vacuum readings at idle, and at every 500 rpm from idle up to 3000 rpm.

Observe and record the changes in vacuum when moving the throttle.

  • Try various throttle changes, slow, medium, and fast throttle opening.

  • Try changing the throttle positions from different rpm.

If possible, try to measure spark port vacuum both while parked and while driving. This is where it comes in handy to have a few extra feet of vacuum hose so you can connect the vacuum gauge and prop it up on the cowl for driving tests.

Make a note of both peak vacuum levels and changes in vacuum levels. Vacuum level changes as the throttle position changes.

Use this procedure to measure vacuum advance timing:

Measuring Vacuum Advance Timing

Step 1.

Disconnect the spark port hose from the vacuum advance.

Plug off the spark port hose and attach a test hose to the vacuum advance canister.

Step 2.

Connect the vacuum gauge to the vacuum advance.

You can use a T fitting to connect the vacuum gauge to the test hose, between the test vacuum source and the vacuum advance canister.

Step 3.

Hook up the timing light.

Connect the timing light power leads and clip the inductive sensor on the wire for the #1 cylinder spark plug.

Step 4.

Start and warm up the engine.

Get the engine up to normal operating temperature.

Step 5.

Apply test vacuum and monitor timing advance.

Apply a test vacuum to the vacuum advance canister and observe the ignition timing change shown by the timing light.

  • Make a note of the vacuum readings as the timing changes.

  • Note the vacuum reading at maximum timing advance.

You can also experiment with changing the vacuum advance sensitivity and observing the affect that has on vacuum advance timing.

  • Try turning the vacuum advance adjusting screw all the way clockwise (maximum sensitivity) and re-test for maximum advance timing and vacuum level at maximum advance.

  • Try turning the adjusting screw all the way counter-clockwise (minimum sensitivity) and re-test for the maximum advance timing and vacuum level at maximum advance.

Testing vacuum advance timing at both maximum and minimum sensitivity will give you an idea of just how adjustable the vacuum advance system is, and how much affect adjusting the vacuum advance will have on total ignition timing advance.

Setting vacuum advance sensitivity

Before you set the vacuum advance sensitivity, you should adjust ignition base timing using the procedure on this page . In that procedure, you adjust ignition base timing with the vacuum advance set to maximum sensitivity.

Much like ignition base timing, the more vacuum advance timing you have, the better your performance. The limiting factor for vacuum advance is the same as it is for ignition base timing — predetonation or pinging. The ideal vacuum advance is the maximum possible without excessive pinging.

Use this procedure to set vacuum advance sensitivity:

Setting M-Block Vacuum Advance Sensitivity

Step 1.

Determine the range of vacuum advance adjustment.

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

  2. Turn the adjustment screw all the way clockwise, until it stops turning.

  3. Turn the adjustment screw all the way counter-clockwise, counting each full revolution, until the last screw thread "clicks" when it exits the nut on the diaphragm spring.

  4. Make a note of the total number of turns from maximum to minimum sensitivity.

  5. Make sure you re-connect the vacuum hose from the carburetor spark port to the vacuum advance canister.

Step 2.

Set vacuum advance to maximum sensitivity.

  • Turn the adjusting screw clockwise until it stops.

  • Make a note of the vacuum advance adjustment.

Step 3.

Test drive.

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

  • Try to drive over a variety of conditions typical for your usage – moderate to brisk acceleration from a variety of steady speeds, acceleration while climbing hills, 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 ignition base timing more. In that case, refer to the procedure for setting ignition base timing on this page .

  • Significant pinging when the throttle is opened means you should reduce the vacuum advance sensitivity.

Step 4.

Re-adjust vacuum advance sensitivity.

Set the vacuum advance to the middle of its sensitivity adjustment range.

  • Use the 1/8" Allen wrench and turn the adjustment screw counter-clockwise by half the total number of turns from all the way in to all the way out.

    For example, if the total range is 16 turns, turn the adjustment screw counter-clockwise by 8 turns.

  • Make a note of the new vacuum advance adjustment.

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 significant part-throttle, throttle-opening pinging continues, try reducing vacuum advance sensitivity further.

  • If there is no throttle-opening pinging, try increasing the vacuum advance sensitivity.

Step 6.

Re-adjust vacuum advance sensitivity.

Adjust vacuum advance sensitivity (increase or decrease it) by half of the remaining adjustment range.

For example, if the total range is 16 turns and the current setting is in the middle (8 turns out), turn the adjustment screw clockwise by 4 turns to increase sensitivity, or counter-clockwise by 4 turns to decrease sensitivity.

Make a note of the new vacuum advance adjustment.

Step 7.

Repeat steps 5 and 6 as necessary.

Keep repeating the test drive and adjustment cycle until you get to the maximum vacuum advance timing without excessive pinging.

Each time you make further adjustments in the vacuum advance sensitivity, reduce the size of the adjustment by half of the previous adjustment.

For example, if you adjusted the vacuum advance sensitivity by 4 turns last time, make the next adjustment by 2 turns.

Remember, the fuel/air mixture can also affect pinging. Excessively lean fuel/air mixtures cause higher combustion temperatures, which can promote or aggravate pinging. After you adjust the carburetor to correct an overlean fuel/air mixture, you will need to re-adjust the ignition timing to optimize engine performance.

Vacuum delay valves

The vacuum signal from the carburetor spark port can momentarily spike when the throttle plates begin to move. Generally, this is more likely to occur when the throttle plates are closer to being closed (at or near idle).

To prevent pinging problems caused by such a vacuum signal spike, Ford used vacuum delay valves inline between the carburetor spark port and the vacuum advance canister on many engines. The vacuum delay valve is just a metered orifice that slows down the transmission of the vacuum signal, effectively buffering out any spikes in the signal.

You may need a vacuum delay valve if your engine has persistent problems with off-idle pinging, or momentary pinging at part-throttle acceleration, even when vacuum advance sensitivity is reduced to its minimum.

Check out this page for more information about Ford vacuum delay valves.

Next: Correcting overlean fuel/air mixture

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