Had an issue with this 77 dizzy with having the timing maxed out in the advanced settings just to get to stock specs . Come to find out that the vacuum advance arm was not attached to the breaker plate. So the screw was in place but not attached to anything. This put the breaker plate in the wrong position - retarded in regards to the pick-up position.

can someone tell me what this other adjustable plate does- see pic. It was adjusted all the way to one side. The horseshoe pick up or whatever it is moves with it,  it I can’t find any info in the FSM about it. What does it do? 

To me, it looks like it's for adjusting the phase difference in the dual pickup distributor. I don't know what the page is for the 77 FSM, but I believe it's described on page EE-29 of the 76 FSM.

Check pages EE28 and 29 in the 77'  FSM.

I believe that is the "contactor". We discussed it some in this thread a number of years ago:
https://www.classiczcars.com/forums/topic/42650-distributor-question-the-contactor/?tab=comments#comment-384319

Unfortunately the pics are gone, but I can reload them if it would help. I don't know if the page numbers are the same in everyone's FSM, but in mine, the contactor get's mentioned on pages EE-25 and EE-26.

It only needs to touch.  I think it's there to make sure that the gap that was just set stays the same while in operation.  It's opposite the air gap.  It's an interesting thing.

Very interesting . Thanks guys. I don’t actually own a manual but can search on line like I did earlier. 

If it's not on here in the Downloads section (see the link in my signature), try http://www.xenonzcar.com.

 

The manual says it exists to  prevent hysteresis in the advance mechanisms. No idea... Makes me think they were having some small issue in the field and management went to the engineering department and said "Don't care how, but fix it".

No idea if there really was a problem, but that's what it smells like to me.

1 hour ago, Captain Obvious said:

The manual says it exists to  prevent hysteresis in the advance mechanisms.

I've read that the shaft has rotational play that can cause the "trigger" to bounce forward and backward, causing small timing changes.  That spring might be a drag on the shaft right before the trigger passes the sensor.  John Coffey wrote about shimming the notch and tang at the bottom of the shaft to remove some of that play.

I know it does give the distributor a nice mechanical buzz when you spin it.  Makes it sound like it's doing something, and set up right.

I'll bet its something electrical. Isn't that a wire attached to it in the picture? An extra insurance to get a ground connection to the rotating shaft, fixing an erratic spark problem? Guessing.... Have to dig one up and have a look

I think that you're looking at the inner raise edge of the breaker plate.  There's no wire.  It just rubs.  And sounds like a playing card in the spokes.

 

I don't know if there is any truth to it, but I have read somewhere that it stabilises timing when the mechanical advance is just starting to move. I would have thought the springs would have the same effect, but maybe theory is different in the field.