General Category > Quadrajet Carb Talk and Tips

HEI Distributor mechanical advance curve options

<< < (4/4)

70GS455:

--- Quote from: Cadman-iac on March 21, 2026, 06:48:16 PM ---    So I've found some more information about these centerplates and weights that is helpful. Mainly that the stamped numbers go up for Chevrolet, Buick, and Cadillac, and numbers down for Oldsmobile and Pontiac.

 In other words, if your distributor rotates clockwise, the numbers face upwards.

 If your distributor rotates counterclockwise, then the numbers face downward.

 That's the way the factory designed it, and after testing each centerplate/weight combination, with numbers up and down, it's obvious that you get much better results with everything set up like the factory designed it.
 
 Almost every one of the combinations performed poorly when I set it up with the numbers down, or opposite of what the factory position is. Most every time the weights did not retract fully into position, leaving them flopping around, so there's no way the advance would return to the minimum setting using just the springs.


  I'll post the results of my tests in the next couple of days. It's not a lot, I only have 7 centerplates and 6 different weight numbers, but it gave me a good idea of which set I need to use to get my timing dialed in to the optimum setting, that being a WOT setting of 36° maximum centrifugal advance, and a cruise timing of 52° all in with vacuum and centrifugal advance.
 Im gonna go with manifold vacuum, and advance unit with 16°, and an initial timing of 10°. The vacuum advance starts pulling at 4", and is all in at 10".

  Something else I noticed is that depending upon which centerplate you use, they can change the position of the rotor in relation to the distributor shaft by a few degrees. I saw this when changing the centerplate and would have to reset my gage to compensate for the change.
 

 I still will need to run the engine to see what the advance curve will be, and I can adjust with springs to get it into the rpm range I need.

 On the vacuum advance units, I figured out how to find out how many degrees it will give you. Most of the GM originals have the degrees of advance stamped into the mounting bracket. Using this, I measured the distance from the edge of the shaft and the edge of the opening where the shaft stops. Each .013" is equal to 1° of advance, so if you've got an aftermarket unit that doesn't have any stamping on it, and you want to know how much advance it will provide, you can measure the gap and divide by .013". So for example, if it'll move  .250", then it will advance the timing by approximately 19°, give or take a degree.

  Hope this helps some.
     Rick

--- End quote ---
Coil + goes to 12v supply. Coil - goes to the HEI module terminal that doesn't connect to ground. Coil HT to a plug. Plug strap to ground

77cruiser:
I had a stone once, don't ever want another, didn't end up in the hospital, but I had a spasm in my back for a few weeks.

Cadman-iac:
 
Quote:
Coil + goes to 12v supply. Coil - goes to the HEI module terminal that doesn't connect to ground. Coil HT to a plug. Plug strap to ground.
 Quote:

 That looks like it would work just fine on an open HEI.

 It kinda reminds me of something I did on a 50 year old single cylinder 15hp cast iron Briggs engine after the original coil ended up toast.
 I took an old Mallory coil and wired it into the points. Now it just takes a bump on the starter switch and it's running like a scaled ape, well, for a 50 year old ape anyway.

 Thank you very much for your help on this, im getting out of the hospital in the morning,  so I'll be giving this a try.

 Rick

Cadman-iac:

--- Quote from: 77cruiser on April 03, 2026, 10:27:18 AM ---I had a stone once, don't ever want another, didn't end up in the hospital, but I had a spasm in my back for a few weeks.

--- End quote ---

 Hey cruise,

 Yeah, unfortunately this wasn't my first go round with kidney stones. Had an episode about 2 years ago and passed almost a dozen, thankfully very small ones.
 This time I wasn't so lucky. I went to the ER on the 25th, and they shipped me off to Tucson in an ambulance, and by the time they had me stripped and wired, i had quit talking altogether, and my wife says that they put in a tent between kidney and bladder.  Gotta get that out in the next week or two.
 This thing kicked my butt! Supposedly I'm going to be extremely tired for a few months from what the doc says.

 I just told the wife that I was getting fat anyway, so decided to develop a stone to lose weight. I don't think she bought it, but its my story and I'm sticking with it,  lol!!

Navigation

[0] Message Index

[*] Previous page