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Centrifugal Mechanical Advance - How does it work?
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Author:  bwhitejr [ Thu Jan 12, 2006 4:10 am ]
Post subject:  Centrifugal Mechanical Advance - How does it work?
I took my distributor apart and saw that the vacuum advance is connected to the advance plate mechanism, but the centrifugal advance part isn't physically attached. How does the centrifugal part of the advance mechanism actually advance the timing. I am sure it has something to do with the slot in the governor being forced to rotate in a counter-clockwise direction (advance), but how is that rotation translated to the advance mechanism?

bwhitejr
Author:  SlantSixDan [ Thu Jan 12, 2006 7:36 am ]
Post subject: 
Hold the distributor drive pinion fixed (either by wrapping a shop towel around it and holding it in your right hand, or by having the distributor installed in the engine). Grasp the rotor and turn it clockwise. You'll notice you can do so, against spring tension, and when you let go, the rotor snaps back to its original position. Do it again while watching the weight guide pins in the cam slots. See how they move? As the distributor spins faster and faster, the weights tend to swing outward, which, via the guide pins in the cam slots, rotates the cam clockwise. Since the distributor also turns clockwise, and the rotor (and point cam or reluctor) is solidly connected to the cam, this extra clockwise rotation causes the points to open (or reluctor spline to pass the pickup coil) "ahead" of when it normally would if there weren't any centrifugal advance. Hence, earlier spark. The breaker/pickup plate isn't involved at all in the centrifugal advance. It works the opposite way: The vacuum can, via its arm, pulls the pickup coil or breaker points counterclockwise. Remember, the shaft is rotating clockwise, so that counterclockwise rotation causes the points to open (or reluctor spline to pass the pickup coil) "ahead" of when it normally would if there weren't any vacuum advance.

Clear as mud?
Author:  Reed [ Thu Jan 12, 2006 7:36 am ]
Post subject: 
Sorry, double post. :oops:
Author:  Reed [ Thu Jan 12, 2006 7:36 am ]
Post subject: 
THe pegs on the weights move in the slots in the governor and make the governor turn clockwise, the same rotation as the rotor. Because the reluctor is mounted on the governor and not the pickup, this has the effect of advancing the timing. The vacuum advance moves the pickup but the mechanical advance moves the reluctor. The vacuum advance moves the pickup counter clockwise and the mechanical advance moves the pickup clockwise.

Because one end of the weights is fixed near the center of the distributor, when the weights move outward with the increase in RPM, the peg which engages the governor will naturally describe an arc in its movement. Since the governor has straight slots for the peg to fit it, the arc movement of the weights forces the governor to rotate, thus advancing the timing.
Author:  Jeb [ Fri Jan 13, 2006 3:50 pm ]
Post subject: 
Huh, I always wondered how it worked.
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