Spark Advance/Retard Mechanism

[Diastole]

I am new to Model As.
I am curious as to what is the mechanism for how the lever changes the spark timing?
The distributor is mechanically controlled by the cam. The rotor is set in timing the engine. Neither are adjustable once set. The points gap is fixed, as well.

What action does the lever do to advance or retard the spark?

I understand how to set the lever and when to change it. But, how does it work? What does the lever change in the distributor or the engine to modify the time the coil discharges to create the spark and ignite the compressed fuel/air mixture?

Thanks in advance for help with this illusion mechanism for me

[40 Deluxe]

The timing lever moves the points plate in the distributor. Pulling the lever down moves the points plate in the opposite direction of distributor rotation, which causes the points to open sooner, thus advancing the timing for higher RPM operation.
This is necessary because, while fuel burns at a certain rate (it does not just explode), at higher RPM there is less time for it to burn so it needs to start burning sooner. Thus the need to advance the timing (pull the lever down).

[nkaminar]

Welcome to the Barn, where all things are answered. Just remember to feed the cows.

You can take the distributor cap off and move the lever to see how the plate that the point are attached to moves.

[Bob Bidonde]

These slides may help you see what goes on when you operate the spark advance lever.

[Diastole]

Very helpful and explanatory. This, therefore, explains why the upper plate is not fixed and is allowed to move. It is the movement over decades that can cause the jumper wire to fray.

Therefore, lever all the way up to full retard (moving points in direction of rotation, so the crest of the cam opens the points later in time) allowing the spark plug to fire at TDC for ignition. And, advance the time of the spark plug firing as engine RPM increases so that burning fuel gets a head start so that at ignited fuel/air the cylinder reaches TDC as the burning mixture starts and needs to expand. This “advancement” moves the points in a direction, opposite of rotation, so the crest of the cam opens the points earlier in time.

This is a great forum and great hobby. I hope to get a Model A someday

Will

[rotorwrench]

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More modern cars of the late 40s through the 60s used the same principal of spark control operation but they were controlled by an MAP connected vacuum servo acting on the points plate to advance & retard spark.

In the early V8 era, Ford used both a centrifugal advance and what is referred to as a vacuum brake to hold back the points plate for load conditions on the front mounted distributors.

Page 10 of the Model A Instruction book has a brief paragraph on the operation of the spark control lever for normal driving and load control. Retard the lever for starting to prevent kick back during the start phase per page 6. This is especially important if hand cranking the engine. Ford fractures of the arm were common when hand cranking an engine with the spark control too far advanced. Proper operation will give better overall fuel mileage.

[Flathead]

That is why the contact tip on the rotor is so wide.