[many birds]

Trying to tune the ignition on my setup. My distributor is an aftermarket HEI—Pertronix. Purchased by previous owner. I have already changed the module for a GM version and welded a limiter on the vac can. The vac is limited to about 12 degrees. However, my mechanical advance only adds 10 degrees at the crank. What’s going on here? And how do I fix it to get the recommended 20+ degrees?

Thanks!

[mgarblik]

There is some information missing here that could help nail the issue down. First, how are you measuring the mechanical advance? Are you using a dial-back timing light? Using a degreed balancer? Using a distributor machine? Using timing tape? Regardless of how you are measuring the 10 degrees, if you are using a distributor machine the 10 degrees is 20 at the crankshaft. If you are measuring on a running engine in the car, you could have very weak or very strong advance springs. Very weak springs may be giving you considerable advance at idle you are not accounting for. You didn't mention idle speed for your starting number. Very strong springs may only give you 10 degrees at 4000-4500 RPM,s or whatever speed you are assuming full advance is happening at. Fill-in some of the blanks and people will be able to help you. It's always possible, there is something mechanical wrong with the aftermarket distributor, wrong weight shape, advance slot and so forth.

[many birds]

I have a dial back timing light —Old sunpro. I didn’t trust it, so I made my own timing tape. Got the same results. Idle is set to 700 in drive, 850ish in neutral/park. I have summit springs on it, I didn’t use their weights. I’m currently using one heavy spring and one medium spring. I have tried various combos on the springs, but the results are the same. Advance comes in all of a sudden, around 1600 rpm. I have cleaned and lubricated the weights and still no matter what combo of springs, it throws the advance suddenly and stops at a total 10 at the crank. I have taken it up to 4K rpm, but only maybe a degree of change.

[mgarblik]

Thank you for the additional information. Any chance the weights you have installed are for a clockwise rotation distributor? Do the Summit weights and the Petronix weights look basically the same? Happen to have some stock Pontiac weights handy to look at the profile or just try them? With the weights and springs off, does the shaft and rotor advance movement seem smooth and enough movement like a stock HEI? Hope this give you a few ideas to check out.

[many birds]

Tried monkeying with it again this morning. Bottom line, no luck.

I bumped the base timing to 17 degrees. Reset the idle and mixture screws. Idled ok, about 12 inches of vacuum and 9 in drive. Excellent throttle response off the line, but mild pinging on moderate throttle (top of the primaries, not into the secondaries on the q-jet). Hot starts were ok, no worse than before, but I don’t want any pinging.

I have backed it off to 13 degrees now. It really likes something closer to about 10 degrees at idle, but that makes it sluggish everywhere.

[MescaBug]

I'm having a similar problem. Following this thread...

[many birds]

The distributor shaft moves freely. The weights looks pretty similar, except that the pertronix weights are coated black and the summit ones are chrome. They weighed the same also. I checked with a kitchen scale. The summit kit was for Pontiac. So, I’m assuming it’s for counterclockwise rotation. I did not remove the center-plate to look at it carefully. Could it be upside down?

[PAUL K]

Originally Posted by many birds

I did not remove the center-plate to look at it carefully. Could it be upside down?

Recently I was trying to limit the amount of advance on an aftermarket HEI. I flipped the center plate upside down and it cut the advance almost in half. You might want to give that a try.

[David Ray]

The weights you want are 41, center plate is 375, straight out of the ZZ Chevy large HEI.

Unfortunately, the large HEI uses a center and weights package easily changed to reset curves, but, for EMISSIONS applications. Yes, some of the curve packages do hit on target, very few of them. The Zz package is the ONLY performance package GM ever did of the 437 different combos for their large HEI setups.

To go further on the curves for the large HEI's, unlike the points distributors with set start and limit abilities, the large HEI has those start and limit points integrated into the center and weights package, that is why the slots in the rotor mounting plate are so large, to make it easy to install ANY weights/center set to meet any and all emissions curves.

Please remember, until the ZZ crate engines, there were NO performance large HEI's that GM had, only grocery getter emissions stuff.

Aftermarket "kits" are, for the most part totally useless on the street, as they are designed to remove the vacuum advance completely, and run insanely high rates of initial advance, FOR DRAG RACING ONLY.

As far as the vacuum advances, the ONLY reasonable ones are the Crane/FAST 99600-1 for large HEI, but, there is a problem, their degrees stop plate is the best in the industry, although it is mounted on the WRONG side of the pull pin. Crane method defeats the ability to set degrees differently without adversely effecting vacuum rate.

EASY to revise/fix, send email and ask for the pictures and info package to remount the scroll plate correctly, and set the advance to give 10 crankshaft degrees, correct to full manifold vacuum, and then you should be able to run your 10 initial

gmvacuumadvancemodifications@gmail.com

The package is totally free of charge, and will help the vacuum advance curving immensely. The Crane/FAST 99619-1 stop plate will fit both adjustable advances, and stock advance canisters. Easy mounting with ONE 8/32 screw and #8 flat washer, one self drilled and tapped hole.

As far as the mechanical, find the 41/375 parts, and that should help the mechanical curve.. MOST of the aftermarket large HEI producers have gone to this combo for their distributors, I'd suggest asking a manufacturer if you could get those parts separately, as the GM cost of those parts is roughly $151.00 (includes center shaft, reluctor, sprigs, weights, center plate). These parts are also used in different stock emissions applications but not as a set, weights for one combo, center for another, with NO parts numbers for any of it.

[many birds]

Finally got my hands on a set of stock weights and center cam. The weights are #045 and the center plate is 346. I used the medium heavy springs. I am getting about 23 degrees of advance. All in by about 2800. It advances a little further by about 1 or 2 degrees if I take it to 4000+ rpm. My vacuum can is adding 14 degrees. I set the base timing to 10. It idles steady, no surging, and rpms drop about 150 going into drive. However, it has a mild ping under load and moderate throttle at about 2000 rpm.

So, I went back and tuned the adjustable vacuum can to about 10 degrees and retarded the base timing to 9 degrees. Still intermittently pings, but very slightly on moderate acceleration. No ping on full throttle—though it’s kind of hard to hear over the q-jet music.

Im running AC Delco R45 plugs.

Should I retard The timing further back? Is any amount of ping acceptable? I ask because I’ve noticed even modern cars sometimes ping intermittently under heavy load or or right off line at a stop light.

[Schurkey]

1. Aftermarket weights are generally junk. Total scrap metal. If the Pertronix stuff is different, I don't know about it. I'd rather use GM weights.

2. Weights from GM are not specific to direction of rotation. You want to use weights from one direction on a distributor that turns the other way? Flip the weights upside down. Flip the center-piece upside-down, too.

3. The way I remember it--and someone should verify, 'cause I'm old 'n' senile--is that "Chevy" has the stamped numbers facing "up", and distributors that turn backwards from Chevy have them "down".

4. 10 degrees at the crank is something lots of guys would happily take...'cause then they can run too much initial timing, about 22--25 degrees. Given a choice, I'd like to see somewhere in the 16--22 range, depending on the vehicle. Enough centrifugal to keep the initial low so as to avoid starter motor problems.

5. If this were me, I'd be looking at the size of the slot that the limit pin goes through. On Genuine GM distributors, the slot is way too huge, some guys weld them to provide a positive-stop for the advance. Perhaps Pertronix went the other way, and made the slot too small. A grinder would easily fix that.

[mgarblik]

Schurkey has some good ideas to follow and check out. If my memory serves correct, on an HEI, the plastic rotor on the underside will actually become the advance limit due to the shape of the weights, not the slot like a point ignition. That's one reason the weight shape and installation direction is important. Not saying that's the way it should be but many years ago I remember modifying the weights to get additional advance. That's one reason it would be nice to have some factory weights to look at. I have many sets of point ignition weights and they are not all the same. Unfortunately, I don't have any HEI stuff anymore. HEI distributer is too big to fit any of my engines. I forgot about the center piece. That's an item to look at first. Hope you get it.

[pastry_chef]

This may interest some with CD boxes.
http://www.daytona-sensors.com/tcs-1...ol-system.html

[Sun Tuned]

Historically Pertronix stuff (HEI) runs around 12 distributor degrees(24 crank degrees). There may also be a "54" number stamped on one end of the center cam. At least the last ones I saw lately were done that way.

None of he aftermarket, center cams and weights are heat treated like the factory stuff or the ones I have made here. You want them heat treated. They will not last any reasonable amount of time if they aren't. Take one and drop it on the concrete floor. If it's heat treated it will ring like a crystal glass. If not it simply goes chink and that's it. Those are soft and will cause issues later I promise.

Very possible that the center cam is upside down for its intended application so I would definitely look into it. It will still work obviously, but not like it should and with a limited amount of advance at that.

[Sun Tuned]

Yep Schurkey, u got it right.
Numbers up for chevy
Down for Pontiac.

You can also draw a file across the weights or cam to see if they're heat treated. If not, the file will remove material quite easily on each stroke. If they are and done correctly, if you're using a cheap file, you might end up knocking a good portion of the file off in the process. You won't scratch the finish of the good stuff from the factory.

[many birds]

The center plate is oriented correctly. Below is a picture of the setup. Interestingly, I found a set of factory weights from my other car (75 t/a). I couldn’t find the factory center plate however. The factory weights don’t fit with the aftermarket center plate. Attached is picture of the factory weights, summit weights and the ones installed in the pertronix. The black ones are pertronix. It’s amazing how different the factory ones are compared to aftermarket.

My problem still remains—only about 10 degrees of advance. And comes in suddenly.

[many birds]

I have another aftermarket center plate, the summit one. It looks cheap compared to the pertronix one. Not sure, but I may give it a try.

The slots in the advance mechanism seem more than adequate. If I manually move the weights, it seems like I should get about 20 degrees at the crank easily. There is nothing sticking or rough movements. This is getting a little frustrating.

[Sun Tuned]

That's a Moroso/Mrgasket curve kit with a fairly stiff set of springs.
Nothing good will work out with that without a lot, and I mean a lot of work. They can be made to work, but you don't have enough lifetime left to honestly try fooling with it. It will be an excersize in futility at best.

Grab a set of stock weights, and a centercam and proceed. You'll be much better off.

[many birds]

Originally Posted by Sun Tuned

That's a Moroso/Mrgasket curve kit with a fairly stiff set of springs.
Nothing good will work out with that without a lot, and I mean a lot of work. They can be made to work, but you don't have enough lifetime left to honestly try fooling with it. It will be an excersize in futility at best.

Grab a set of stock weights, and a centercam and proceed. You'll be much better off.

Where can I get a good stock set? And how much do they usually cost? What should I be looking for in a stock set?

[Sun Tuned]

And that stock weight at the bottom of your pic is for a points distributor not for an HEI.