66 L79 Distributor Centrifugal Advance

[William G.]

I finally have the engine on my new 66 roadster running well with the exception of the distributor. It is (to my knowledge) a correct original distributor that I have gone through mechanically so that I knew that it is in good condition with minimal bearing clearance and .005 endplay. The engine is running beautifully but not up to it's maximum performance capabilities.

The problem I am having is that I can only get 10 degrees of mechanical advance which when added to an initial setting of 10 degrees gives me a total of only 20 degrees... all in by 2500 rpm depending on the springs I use. I should have about 26-28 degrees of centrifugal advance so the mechanism is coming up short somehow.I assume that the problem is that I have the wrong weights and/or limiting slot.

1. The problem doesn't change by changing springs.

2.I have the correct high performance shaft profile.

3.I assume that that every degree of distributor rotation equals 2 degrees of crank rotation.

4.When I measure the max angular rotation between the shaft and the plate without weights or springs I measure about 4 degrees until the limit pin stops out.

5 When I measure the max angular rotation between the shaft and the plate with weights and springs installed I measure about 3 degrees. The weights further restrict the max advance.

My best guess is that I have a combination of incorrect weights and stop limit mechanism. The pin has a plastic bushing installed.

I'd very much appreciate any input from our experts. Thanks for your help.

[Joel F.]

Re: 66 L79 Distributor Centrifugal Advance

I wouldn't include myself with the "experts" but here goes . . .

Do you have any bushings installed limiting the centrifugal travel? Any evidence of someone shrinking up the slots by welding, etc.? This sounds to me like the issue.

[Duke W.]

Re: 66 L79 Distributor Centrifugal Advance

IIRC the OE centrifugal spec is 30 @ 5100 max.

OE weights should have a stamped three-digit number, and they are not bright plated.

Remove the weights and measure the angular displacement. It should be about 15 degrees.

The "tail" of the non-OE weights may be limiting advance.

Duke

[William C.]

Re: 66 L79 Distributor Centrifugal Advance

Definately agree with Duke on the weights being non-oem, the style shown limits the distributor curve significantly. Get a set from any unmolested 66 or earlier GM distributor and you should get your advance back.

[William G.]

Re: 66 L79 Distributor Centrifugal Advance

Thanks for the responses. I assumed that the weights were causing a problem, and I will find a set of OEM weights. However, even with correct weights the limit pin is still causing a problem. I will pull the distributor and get a good look at the slot. I assume it would be OK to remove the plastic bushing. I don't think the bushing itself is robbing that much advance but I will have a look at the slot , remove the bushing and see what maximum mechanical angle that allows. Was the bushing installed on a stock 350hp distributor? Thanks again for your comments!!

[Michael H.]

Re: 66 L79 Distributor Centrifugal Advance

Thanks for the responses. I assumed that the weights were causing a problem, and I will find a set of OEM weights. However, even with correct weights the limit pin is still causing a problem. I will pull the distributor and get a good look at the slot. I assume it would be OK to remove the plastic bushing. I don't think the bushing itself is robbing that much advance but I will have a look at the slot , remove the bushing and see what maximum mechanical angle that allows. Was the bushing installed on a stock 350hp distributor? Thanks again for your comments!!

I think you may find that once you replace the weights with a set that's close to original, you will once again have near the correct amount of cent advance. Aftermarket weights often limit the amount of travel in the RETARD direction, so the advance is starting from a point somewhere close to the center of the slot instead of near the retard end.
You can test this by observing the aprox amount that the cam will rotate with, then without, the weights installed.
I don't know of many, or any, original distributors that only had 10* cent. advance so something has to be restriction the rotational motion in one direction or the other. If the original bushing is still in place, it must be restriction motion from the other side, the retard side.

[Timothy B.]

Re: 66 L79 Distributor Centrifugal Advance

William, I did a recurve on a 65 L79 and when I tryed the weights that came in the kit, I had exactly the same problem. I ended up using the original weights with the light silver springs and a 726 weight base that came from a earlier parts distributor. Going from memory I think the curve started 0@800 - 20@2000 - 24@2800 - 26@3200 and I used the original stop bushing.

I changed the vacuum advance to VC 1765, 15 @ 12" because the car idles @750, 14" vacuum with initial set at 12. After checking the 1765 vacuum advance can, I installed the rubber stop bushing on that shaft because my memory tells me it would actually give about 17* pulled to the stop without the rubber. I actually think the car would be fine with this because idle timing would be 12 + 17 to give 29 and the vacuum can snaps back fast so no rattle. I am not really sure on the high end with a possible 55* cruse timing so I will probably just leave it alone.

[William C.]

Re: 66 L79 Distributor Centrifugal Advance

Exactly, the "kit" weights are configured in a manner that reduces the travel of the limit pin in the slot. For most applications, unless you want to run a lot of initial timing they are junk.

[William G.]

Re: 66 L79 Distributor Centrifugal Advance

It seems that this problem is a common one. Thanks for all comments. Does anyone know where I can get a set of original weights? I tried calling around yesterday to no avail.

[William C.]

Re: 66 L79 Distributor Centrifugal Advance

send me an email with your name and mailing address and I'll fix you up, I have more spares than I'm likely to ever use. Also remember that the location of the pivot pin on the top plate and the length of the slot is specific to each distributor, so best not to mix them up.

[Louie M.]

Re: 66 L79 Distributor Centrifugal Advance

The weights you show look NOTHING like the stock weights in my 67 L79.

I finally have the engine on my new 66 roadster running well with the exception of the distributor. It is (to my knowledge) a correct original distributor that I have gone through mechanically so that I knew that it is in good condition with minimal bearing clearance and .005 endplay. The engine is running beautifully but not up to it's maximum performance capabilities.