Distributor Springs. You’d think this was easy...

[zKars]

So today I’m tearing down a points distributor to do a cleanup on a 5/70 Z. Single points.  D612-52

I get it apart and there staring back at me is two advance weights and only one of them has a spring attached. Hmmmmm...

Must be missing, broken, lost sometime in the past. No sign of bits laying around.

Need to find a replacement spring. Go get a spare. Post Aug 71 points, but so what. Look inside, two springs, but they are much larger diameter and looked stretched out. Go look again, find that very last D612-52 that is identical to the one with one spring, open it up, and .....

One spring. No pieces laying around.

Ok, is it me or are these things notorious for breaking advance springs?

Surely they all came with two springs. Right? Maybe not? Out of balance with only one, doesn’t make sense to me.

Check the Fiche, for the pre Aug 71 manual dizzy, the Qty column for weights is 2, but the QTY column for springs is 1...   Holy cow batman, could it be?  

Now mind you I check the post Aug 71 manual dizzy and the spring Qty is also 1.  The pictures up top for both are showing two springs

Are two dizzy’s statistically significant enough sample to say they had only one? There should be pieces if one broke....

Any experience? I can’t remember discussing this before.

Edited October 23, 2020 by zKars

[240260280]

Jim, a great discovery.  I never looked closely at early distributors yet but your photo shows two different sized slots for the weights to move. This points to asymmetry and could explain the one spring.

All the ones I opened had equal length slots

 

vs

 

Edited October 23, 2020 by 240260280

[Zed Head]

I don't know what all this means but...  One says spring, one says set.  And the 72 FSM shows the unbalanced slots.  I alternated pictures and links, hope it makes it through.

http://www.carpartsmanual.com/datsun/Z-1969-1978/electrical/distributor/240z/for-manual/to-aug-71/6

http://www.carpartsmanual.com/datsun/Z-1969-1978/electrical/distributor/240z/for-manual/from-sep-71/5

[240dkw]

That is interesting, I am positive that I went through this when I rebuilt my D612-52. I spent the better part of a day looking around the shop for the “lost spring”. And then took one out of a spare I had. Makes me wonder if I have rebuilt it incorrectly. Seems to run well.

[zKars]

Aha! un-balanced slots! Of course. One spring for sure. Dan, that third example clinches it. One spring! 

[Zed Head]

The picture shows an A and a B spring though.  Always somethin'.

The test would be how much unhindered rotation of the shaft is there with just one spring.  The springs are there to balance the shaft rotation due to the weights moving.  No spring should mean that one weight is just flopping around.  Maybe the short slot weight doesn't move at all.

The words imply that it doesn't move, maybe, "it does not leave clearance either at start or end"., but then why they do they show a spring?  Confusing.

[jonbill]

both weights are constrained by the one spring - the unsprung weight can only move out in its slot as the advance plate rotates, and the advance plate advancing is resisted by the spring on the other weight. 

thus a single spring has a faster advance curve than 2 springs. 

[zKars]

Thanks for posting the FSM description. Who knows why both springs. 

Most important part is as I paraphrase “weight with spring engages with long slot, no spring weight with short slot” so I put it back together again correctly.

[Zed Head]

26 minutes ago, jonbill said:

both weights are constrained by the one spring - the unsprung weight can only move out in its slot as the advance plate rotates, and the advance plate advancing is resisted by the spring on the other weight. 

thus a single spring has a faster advance curve than 2 springs. 

Adding to zKars just-now post - 

That makes sense jonbill.  But still doesn't really tell if there should be one or two for a specific distributor.  

Looks like one has a steeper advance rate than the other.  7 degrees in 500 RPM versus 5.  Distributor degrees, 14 versus 10 in crankshaft.  Maybe that's one spring versus two.  

 

Edited October 23, 2020 by Zed Head

[240260280]

Hmmm, the length of the slots determines the amount of advance so the short slot should greatly limit the advance.... ie, it will set the max advance limitation. This allows the manufacturer to only have to calibrate the length of one slot rather than have two matched.

Also, without a spring, the free weight will be pulled to zero at engine stop maybe even at idle (by the sprung weight pulling its weight inward, and in turn, turning the slotted cross piece to 0)....but when the engine rpms increase, the unsprung weight will greatly speed up the advancing. It would act almost as a switch (off and on) rather than a ramp up.

Perhaps this architecture allows the advance to happen quickly and the purpose of the spring is simply  only to pull it all back to zero at very low rpms?

My guess is that this asymmetry caused unwanted rotational imbalance and shaft wear so a symmetrical  re-design was made.

Edited October 23, 2020 by 240260280

[240260280]

The 1970  L24 w/o pollution devices (middle D606-52) has 17 degrees initial advance at 700rpm and the mechanical advance is "all in" (additional 12 to make total of 29 at only 1000rpm).  So it seems that the engine (focusing on performance and ignoring emissions)  likes a lot of advance and "switches on" at low rpms!  

The one-spring distributor is a way to achieve it!  But Jim has the emission distributor (D612-52) that comes in at 1,415 rpm... but it shows a 6 degree centrifugal rather than the 12 degree shown below ?

 

 

Edited October 24, 2020 by 240260280

[240dkw]

Sorry, I miss spoke, I have the non-emissions dizzy, 606-52. I saw the date in the first post and got it in my mind that it was a UN