Background:
'66 396 360HP with slightly larger than original cam. Just rebuilt. Original distributor.

I am trying to check/optimize my distributor mechanical advance. I have read and re-read many posts and studied many papers written on this subject. What I have run into is this:
The mechanical advance starts to come in around 900 RPM and continues to increase (~20 degrees) until about 2000 RPM. Then it stops. When I rev to around 4K-5K RPM the remaining 10 degress come in. Looking at specification section of my '66 book it shows a mechanical advance curve of: (I think I remember what is shows, I'm at the office..)
900 8 degrees
1275 14 degrees
1600 19 degrees
5000 30 degrees
The important thing to note is the last RPM number of 5000. This is contrary to everything I have read from this site from many very experienced individuals. Most people, if not all, have expressed a desire to have all of the mechanical advance in by 3000-3200 RPM.

If I install a lighter set of springs the advance begins ~500RPM and is all in by 1800?? It seems that the weights, while engine off, do not want to "snap" back to initial position with anything but the heaviest set of springs I have.

Some questions:
1) I have examined the weights and verified their cleanlinest but cannot "see" what might cause the delay in the last 10 degrees. Any ideas here?
2) If I use a lighter set of springs which allows the advance to begin to early, the mechanical advance is fighting the idle speed. Generally speaking the car just does not want to run well below 600RPM (cam).
3) Is it normal for the weights to not "snap" back into place with engine off?

Any thoughts, experience, testing ideas would be extremely beneficial.



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Paul
TC Member #1657
'66 SS 396/360

You may want to send you dist to Dave Ray, his website is; www.davessmallbodyheis.com (http://www.davessmallbodyheis.com) he can set you dist to your engine specs and convert it to his HEI set up if you want to do it that way. Give him a look.

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Leo Paugh
Maryland Chevelle Club #017
A.C.E.S.#3731
progress has little to do with speed, but lots to do with direction.
Maryland Chevelle Club (http://www.chevelles.net/mcc/mcc.htm)

Thanks for the Link. I may do this over the winter. I still have a few shows I can attend this fall.

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Paul
TC Member #1657
'66 SS 396/360

Paul. No offence meant, but you are speaking about changing a SET of springs. In many posts I have read (and tried this) to use 2 different springs, one a bit stronger than the other to, amongst other things, assure a proper return of the weights.

I also noticed on my old weights that the inner side (or the opposite site on the middle part on top of the shaft) is worn in such a way that the outward motion will not be smooth or continuous anymore.

Hope this helps.

Rob

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www.si.hhs.nl/~rob (http://www.si.hhs.nl/~rob)

Have you tried some aftermarket weights? I've noticed that problem on some with original factory weights and springs. It seems to be mainly caused by the shape of the inner edge of the weights and the "cam" attached to the shaft that they pivot off of. I've had success by using weights from a Mr Gasket kit. You may also be able to cure it by grinding the right profile on the stock weights. I haven't tried that yet on points type dist. Of course the sticking could be at least partially caused by age related things too. You just have to experiment with weight profiles and springs to get it right. It seems no two are the same.

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von '69 300 Dlx SS TC #15 ACES #1575 HSCA #5
My '69 SS (http://mywebpage.netscape.com/jerryacheson/vons69.html)

Thanks for all the advice guys. I have checked and rechecked for worn parts and it does not appear to be an issue. And yes I have tried different springs as a pair. I think I will purchase a new set of weights and springs and start over.

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Paul
TC Member #1657
'66 SS 396/360