Mechanical advance only 10 degrees
 

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!

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.

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.

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.

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

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?

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.

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.

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.

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

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.

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.

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.

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.

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.

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

That would explain why it doesn’t fit. So...it might not be from my ‘75 t/a. That raises a whole new mystery. Since the t/a has factory HEI. Maybe for another thread?

Does the rest of the setup look ok for a counterclockwise distributor?

I tried the summit center plate...total piece of crap. So poorly drilled that I had to use a little impact engineering to get it on the posts. But it is still limited to exactly 10 degrees. Interestingly, putting it on with the summit weights, retarded the initial timing to 0 degrees.

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?

Might be easier to latch on to a core. Anywhere from 15.00 to 40.00 for a core. Doesn't really matter if it's s chevy olds or Pontiac Buick either as the weights and center cam are 100% interchangeable between each other.

Absent that option, pm me for option 2.

Not really your issue ... but I would remind people when comparing stock and after market weights .... that how much they weigh is not the whole story ... WHERE the weight is also matters. The shape of the lobe can change the behavior of the weight, even if the total weight is the same.