Distributor for '76 SDV

[mgbeda]

Hi Folks,

I'm going to have to replace the distributor on my '76 DeVille (carbureted).  In the 90's I replaced the original one (because of a seized centrifugal advance bearing) with a junkyard one from a 425.  That centrifugal advance bearing has seized as well.

I also have what seems to be a properly functioning one from another '76, but with EFI.  I'm pretty sure that is a different distributor, right?

The rebuilt ones seem to be the same unit for all 74-80 with a carburetor and electronic ignition, but the factory ones seemed to have a different part number every year, just about.  Is there a difference between the distributors in these years?  Along those lines if I get another junkyard one do I have to find a '76 or would other years work properly as well?

Thanks,

-mB

[Dave Shepherd]

No good on the efi dist for you, the different part numbers refer to the mechanical and vacuum advance curves, they will still work for you though.

[TJ Hopland]

The EFI part of the EFI distributors from that era was an extra 'pod' on the stalk of the distributor.  This contained sensors that triggered the injectors for the EFI system.  The rest was the same as the non efi cars so other than a few extra parts it should work fine.

The difference in factory part numbers was likely primarily a different 'tune'.  Could have been the vacuum or mechanical advance that was slightly different.  Aftermarket has always tended to adopt the one size more or less fits all theory.  Mechanical advance did not seem to vary as much even in the aftermarket stuff but where the aftermarket seems to fail miserably is in the vacuum.  Most aftermarket vacuum cans seem to have at least twice the travel than they should have.  This tends to make the engines 'ping' which makes people retard the base timing which tends to kill or at least flatten the performance.   Some of the aftermarket cans have an optional little do dad that limits or adjusts the travel so you can get it closer to what it should be.   I have found that a little more mechanical advance than stock and fairly close to stock vacuum advance seem to wake up stock engines.  You don't need lighter springs because stock advance came in fairly quickly since these tended to be fairly low rpm motors and cars. 

The earliest HEI distributors did seem to be very prone to getting good and stuck and I am guessing that is why they changed the design fairly quickly.  I think I have only ever see 2 of that early style.   The rest usually were not too difficult to get unstuck and you can buy replacement weights, springs, and bushings.  I think Mr Gasket brand were the decent quality kits.   

[mgbeda]

Thanks for the replies and knowledge.

I was lucky enough to find a '76 SDV in my favorite junkyard last weekend with the engine still in it.  And the mechanical advance even works in it.  So I'm good for now.  And when this one fails at least I'll have a correct original to rebuild.

-mB

[waterzap]

You can still get them at Rock Auto

[mgbeda]

Still fiddling with the distributor.  The one I got at the junkyard has a good mechanical advance, but a bad vacuum advance.  I tried the vacuum advance off the parts car and that's bad too.  The one I had in the car before works, but it's from a '78.

TJ, you said that "Most aftermarket vacuum cans seem to have at least twice the travel than they should have.".  Are there any that don't?  Where can I find the one with the optional adjusting do dad?
The only replacement I've found out there is the "Standard Motor Products VC-206".  Will that work right?  (I'm guessing no, because of what TJ said.)

Of course if I could find a GM part that would fix everything.  I found out the part number of the original is 1973564, for a 75-76 500, except CA cars.  (The one working one I have is 1973669, which seems to be for 78-79 425's).

Any suggestions?

Thanks,

-mB

[35-709]

My factory shop manuals for my '73, and one for a '74 I no longer have, both have the specs on what the mechanical and vacuum advance should be at a certain RPM in the Engine Electrical section.  I have to assume that the '76 shop manual also lists those values.  An advance timing light (mine is from Sears) can tell you exactly what the mechanical and vacuum advance are at any RPM (tested separately, follow the directions), you should also have a temporary tach. hooked up.  If your aftermarket vacuum advance is allowing too much advance I would modify it to shorten the amount it can pull (to increase) the advance.  You can buy an "adjustable" vacuum advance, but I have found for a 472/500/425 Cadillac engine an adjustable vacuum advance needs to have the adjustment turned all the way out to approximate the factory specified maximum advance.

I would add that the vacuum advance in my '73 that I purchased from Rock Auto is pretty close to right on. 

[TJ Hopland]

Here is one of the adjustable ones.    It has that black cam piece that sets the overall travel and using the allen key you can adjust how fast it reacts.  With this, tach, timing, and a timing light you should be able to get it set to factory specs. 

http://www.summitracing.com/parts/crn-99600-1

There are other brands and vendors, this is just a popular brand from a popular online vendor. 

[waterzap]

Measure your vacuum at idle, vacuum can disconnected.
Then get a can that is fully “in” at less than 2 inches of vacuum less than what your idle vacuum is.
Use manifold vacuum, not port.
I had so many problems with my Toronado until I got the timing right. Used to get around 15.5 Hg at 700 rpm, now get around 19. Engine just purrs along.

Found this attached article.

The basic rule for vacuum advance control (VAC) selection (henceforth referred to as THE RULE):
THE VAC SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE
VACUUM AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING.

Issue I see with the adjustible cans are two fold. They seem to break more often than the regular ones, and the timing comes in too fast. From charts it looks like most of them are all in at around 10 Hg. Which according to the article is too soon.