[Shiny]

Originally Posted by Kenth

Causes for erratic timing at idle speed is most often due to too weak centrifugal timing springs and/or a vacuum advance connected to manifold with a too weak vacuum signal to fully employ the vacuum advance unit. Move to ported source.

Thank you again Kenth

Seems my springs may indeed be contributing and I will try to find originals as a start.

As to erratic idle because of vac advance "partially" employed, that should only happen with manifold vacuum, correct? As I interpret, ported vacuum at idle should be too low to actuate the vac advance IF I can get the carb to run off its idle circuit and keep the blades closed.

Set the timing at idle speed with the original springs removed from the centrifual advance.
Note: Look for/mount a 1/4" bushing on centrifugal stop pin, need to be there.
Use a dial-back timing light, set 30° for 3.25" mains engines and 32° for 3.00" mains engines.
Mount the original springs and check for initial timing, which usually ends up at 9°-12°.

I have a bushing that limits total mechanical to ~22 and the initial has been 9 to 11 in the past. So I'm more concerned about the stability at idle.

If you think your engine needs a faster "all in" curve, replace original springs for weaker springs, but not TOO weak. Look for steady timing mark at idle speed.
In any case, you will never exceed the total timing you sat with the springs removed.

Understood, thanks! But I have a ways to go before I get concerned about the full curve and how fast the mechanical maxes out.

Now it´s time to tune the carburetor.

Hoping to get here soon!

Must....

Stabilize...

Idle...

[Shiny]

Originally Posted by Sirrotica

https://forums.maxperformanceinc.com...r.php?u=143447

FWIW, this is a member on the board that does distributor service, he's located in Memphis TN. Many people on the board have been happy sending their distributors to him to have them curved, and repaired.

I'm certain he has parts from old distributors that he could sell you also, along with springs and weights. I've spoken to him on the phone, and he's a great Pontiac guy.

Thanks Brad

I'll reach out to him if I can't find my stock springs.

Sad these parts aren't available new, yet advance kits are and they get people like me in trouble!

Mike

[Sirrotica]

Originally Posted by Shiny

Thanks Brad

I'll reach out to him if I can't find my stock springs.

Sad these parts aren't available new, yet advance kits are and they get people like me in trouble!

Mike

He buys up a lot of NOS parts for the old iron when he finds it, he may have NOS parts too.

FWIW, I used to keep a original spring on one side and put the medium spring on the other back long before there were dial back timing lights. We used to swap the springs, and go out and run the car flat out to see what it preferred. It wasn't scientific, but we found out what worked by the butt dyno. I never was able to use 2 of the light springs in anything, they just were too light to gain anything much over stock.

[nUcLeArEnVoY]

I've turned my reman distributor into sort of a "mimic" of the original W72 distributor that came with my car, by purchasing a set of good used weights and center cam, which for my car would've been #139 weights and #389 center cam, and also found a good used #1973636 vacuum advance unit from a W72 1979 motor. I put the heavy springs out of a Moroso kit on it, since original GM springs were very heavy to give lazy advance curves so that they don't give too much timing early on, as the vacuum advance can throws a ton of advance at it (up to 25 degrees) by as little as 12" of vac. I have decent get-up-and-go with this setup, especially since I use the stock base timing setting of 18 degrees. I'm probably leaving power on the table by using the stiff/heavy springs, but I like always returning to the same idle, and knowing that there's no way I'll ever ping since I don't get full mechanical advance until I'm at near redline (and it's only like 20 degrees more), by which point vacuum advance is usually out of the equation because the throttle plates are nearly fully open.

The original weights and center cam are made better than all of today's junk, especially those weird weights that come with the recurve kits. The factory weights and center cams are hardened steel and usually don't lose their serviceability until the eye on them that fastens over the post goes out of round and so they get very loose... but even then, they can be welded and repaired.

The stiff/heavy black springs out of the moroso kit are a decently serviceable substitute for the factory heavy springs. They're the ones I use and work pretty well, but again, be ready for a very lazy curve.

As to erratic idle because of vac advance "partially" employed, that should only happen with manifold vacuum, correct? As I interpret, ported vacuum at idle should be too low to actuate the vac advance IF I can get the carb to run off its idle circuit and keep the blades closed.

Ideally, there should be NO port vacuum at idle. If there is, then your idle speed screw is turned in too high... you're idling off the primary circuit at that point, or near close it.

The idle won't be erratic as long as your vacuum signal without the advance hooked up is higher than the vacuum signal required for full deployment of the advance can you're using. I still use the factory DS-VMV (modulator valve) that limits exactly 9" of manifold vacuum to the vacuum advance can while at idle, and then once you give it gas and port vacuum exceeds 9", the vacuum going to the advance then just becomes whatever the port vacuum is. My engine without ANYTHING hooked up makes around 14-15" of vacuum at 700 RPM idle since I have a mild cam, which is well over 9", so my idle won't be erratic at all since the vacuum advance will always be fully deployed as much as the modulator valve will let it be. My vacuum advance fully deploys by 11", adding a total of 25 degrees timing stock (but I limited it to 19 since I deleted EGR).

I'm assuming you don't use a modulator valve, so let's say your engine has a stock cam and makes around 18"-20" of idle vacuum without vacuum advance hooked up, and you install vacuum advance can that is FULLY DEPLOYED by 11" of vacuum and you hook it to manifold vacuum. In doing so, you'll notice your idle shoot up because you're giving more advance, allowing you to close your idle speed screw on the carb more, and you won't have an erratic idle since your vacuum even before the vac advance was hooked up was higher than when it's fully deployed, so therefore the vacuum advance is always fully deployed and is not moving while at idle, and so it won't pull or add timing erratically. It's stationary, and won't move until the throttle plates open wide enough that engine vacuum decreases.

On the opposite side of the scale, let's say there is somebody with a very low LSA cam with a ton of lift installed... makes around 9"-10" of vacuum at idle, and he tosses on a vacuum advance can that isn't fully deployed until 12". Putting on the vacuum advance can may raise his idle speed enough that the engine will exceed 12" of vacuum, but then as he draws back the idle screw, the vacuum will decrease again, and go UNDER the maximum deployment signal needed for the vacuum advance, and so the idle will fluctuate as the overlap of the cam sort of "flutters" the vacuum advance diaphragm, since for a second it may be fully deployed, then a second later it may be only getting 11 or 10 inches and pull timing... there's your fluctuating idle.

Make sense? That's when your advance is hooked to a manifold vacuum port.

With your advance hooked to port vacuum, you don't get ANY vacuum advance at idle, since port vacuum should not be present at idle. At the end of the day, port vacuum and manifold vacuum are the same thing - the only difference is that a port vacuum orifice is located ABOVE the throttle plate, but will eventually blend and become one with manifold vacuum once you open the throttle plates enough. You can see this yourself by hooking a vac gauge to a port vacuum and manifold vacuum port and rev up the engine. You'll see a manifold vacuum signal at idle, then as you give gas, you'll see the port vacuum signal raise as the throttle plates open and eventually meet the manifold vacuum signal and they become the same, before eventually leveling off and going down to zero as the throttle plates fully open. All engine vacuum is, is a measure of the RESISTANCE the closed throttle plates create against the negative pressure of the engine sucking in the air. As the throttle plates open, that pressure differential equalizes with atmospheric pressure, and thus the vacuum signal is lost.

[Formulas]

visually original points weights have a deeper gray and smoothe surface
aftermarket weights have like a flash chrome surface

the bottom set in my pick is one original and one aftermarket weight and you can see the different angles in the armpit of the weight that affects the curve plus aftermarket weights are not heat treated for hardness

[dataway]

If you find out what Part number you need for weights ( or the code number like 37) let me know, I have some OEM weights laying around that I collected during the search for the correct parts for my distributor.

Somewhere I found, or a forum member provided me with, the correct part numbers and code numbers for my distributor ... springs, weights, advance plate and shaft/cam.

[ftwmlp]

Originally Posted by Sirrotica

He buys up a lot of NOS parts for the old iron when he finds it, he may have NOS parts too.

FWIW, I used to keep a original spring on one side and put the medium spring on the other back long before there were dial back timing lights. We used to swap the springs, and go out and run the car flat out to see what it preferred. It wasn't scientific, but we found out what worked by the butt dyno. I never was able to use 2 of the light springs in anything, they just were too light to gain anything much over stock.

It is probably this gentleman who worked on mine several years ago:
Bob Davis Distributors
901-412-4414

was(still is?) member SunTuned
Mike Pearson

[Shiny]

Originally Posted by Formulas

post a picture if your weights it will be easy to tell if you have aftermarket ones or not

I pulled the distributor and don't like the weights and springs (which is why I opened the thread).

Here is what I have. These weights rattle around, not making good contact with the cam and don't feel "retracted" well by the springs. The silver spring seems too weak. I think I put these in from a Moroso kit but it has been way too long.



This is a used distributor (same 1112008) I bought off eBay a while back. Are these original weights? They are held in place better, contact the cam, and the springs look heavier. I hope these are stock and I can use them. The weights have "sharper" tips than the ones in my car.


This is one currently listed on eBay. It looks like my used one.


Just to illustrate, here are links to a couple videos comparing my "current" with my used:

In Car, Sloppy

Used, Tight

Mike

[Shiny]

Originally Posted by Formulas

visually original points weights have a deeper gray and smoothe surface
aftermarket weights have like a flash chrome surface

the bottom set in my pick is one original and one aftermarket weight and you can see the different angles in the armpit of the weight that affects the curve plus aftermarket weights are not heat treated for hardness

Both of yours look different than the aftermarket weights in mine, but probably just a different brand.

[Shiny]

nUcLeArEnVoY - Thanks for the detailed post. That is helpful and appreciated!

[Shiny]

Originally Posted by dataway

If you find out what Part number you need for weights ( or the code number like 37) let me know, I have some OEM weights laying around that I collected during the search for the correct parts for my distributor.

Somewhere I found, or a forum member provided me with, the correct part numbers and code numbers for my distributor ... springs, weights, advance plate and shaft/cam.

Thanks! I still haven't found PN/codes for my weights. I'm optimistic the ones on my crusty used disty are "correct" and can be used.

Mike

[Shiny]

ftwmlp - thanks for the contact info. Hopefully I have what I need but appreciate your help.

Mike

[Kenth]

This may help some:

[dataway]

Here's a photo of the correct OEM parts in my 1111270 dist. (68 GTO, auto trans).

I traced my erratic idle timing to floppy weights and the advance plate not moving freely enough. Basically if you hold the gear, the rotor should not move without trying to move the weights, and the weights should not have any slop, the springs should be keeping them from having any slop.

I got new OEM springs from Sun Tuned I'm pretty sure. Look at the loop in the old springs ... are they spread open compared to new? Is the inside of the loop worn where it contacts the posts?

After getting mine sorted the idle timing was rock solid. (using manifold vacuum).

In this photo I have one new OEM spring installed and one "black" Moroso spring. The weights typically, in my experience don't bottom out against the center of the cam, they make contact with the cam tang underneath the spring.

[Shiny]

Originally Posted by Kenth

This may help some:

Kenth, thank you!

I had a note from one member that said the weights in points distributors were not stamped with numbers or codes. This obviously makes it harder to tell what's what!

[Shiny]

Originally Posted by dataway

Here's a photo of the correct OEM parts in my 1111270 dist. (68 GTO, auto trans).

I traced my erratic idle timing to floppy weights and the advance plate not moving freely enough. Basically if you hold the gear, the rotor should not move without trying to move the weights, and the weights should not have any slop, the springs should be keeping them from having any slop.

I got new OEM springs from Sun Tuned I'm pretty sure. Look at the loop in the old springs ... are they spread open compared to new? Is the inside of the loop worn where it contacts the posts?

After getting mine sorted the idle timing was rock solid. (using manifold vacuum).

In this photo I have one new OEM spring installed and one "black" Moroso spring. The weights typically, in my experience don't bottom out against the center of the cam, they make contact with the cam tang underneath the spring.

Thank you!

Did you look at this video of mine?

Link to Video - Floppy With Aftermarket Weights

Clearly not right and floppy as you described. I will replace the weights with those from my used distributor. They are "tight" and the points on the tangs look like yours. The springs look original too so even if I can't "prove it", I think the weights are "correct".

Other than me stupidly putting in bad weights, my distributor is in good shape.

Progress!

Mike

[Shiny]

Thanks to Dataway, I checked for free movement of the advance plate.

It moved freely, but...

It hit a stop that was preventing the springs from pulling the weights into the cam!

Following lots of articles and guidance, I had installed a brass sleeve over the pin to limit max mechanical advance to achieve a total of 32 to 36 degrees.

My method was BAD. The sleeve worked well to limit max but also prevented return!

Doh!

I am humiliated as usual but grateful to all for making me think!



Mike

[Kenth]

I have found that a piece of 1/4" O.D. brake tube suitable for a bushing mounted with a drop of Loctite bearing mounting fluid and an E-clip.

[Shiny]

Thanks Kenth

I have yet to look. I think my brass sleeve worked fine for limiting but the slot wasn't big enough in the "home" direction.

And I didn't notice...

I can either grind one side of the brass sleeve or open up the slot in the "home" position.

Cliff describes brazing the slot on the "max" side, which would avoid all this. But I can't do that.

With original springs. I may never reach max. And for now, I prefer to over-index on the idle stability. I can install weaker springs later if I actually get the engine running and carb set up.

[dataway]

Yep, the bushings need to be pretty thin since when you use a bushing you are not just limiting the movement by the thickness of the bushing ... but by TWICE the thickness of the bushing because it limits both ends of the travel.

I think lots of old stock distributors have floppy weights because the rubber OEM bushing wore out and fell off ... which leaves too much room in the slot.

Also depends on what advance plate is being used, AND what you want to limit it too.

Here is a photo of a 524 advance plate, and a 536. As you can see .. takes very little change in slot length to make up 12 degrees (36 - 24)

BTW ... I have seen weights with a code on them ... I'll have to check my stash for an example.