What causes a distributor rotor center tab to burn through?

[dmjlambert]

OK here's what I have: 1969 CST/10 with rebuilt 350 engine and TH400 transmission. The distributor is a GM HEI from 76-80 and I'm supplying it full voltage. The Quadrajet is one from National Carburetors and 1985-86 newer design with electric choke. The engine has worked fine for years and is my daily driver. I use it about 5000 miles per year.

Late 2024 it started running bad so I decide to just give it a tune up which was new spark plugs gapped to about .040, new AC Delco cap and rotor, and I sent the carburetor to National Carburetor to have them fix a couple of problems such as Quadrajet bog due to probably accelerator pump going bad and it was not holding fuel in the bowl more than 1 day, and the electric choke was finicky. So, they got it going and sent it back to me.

I put it all back together and it had idle speed not steady and general running bad. Base timing is at 7 BTDC which is about right for this engine with the vacuum can I have (it has a lot of vacuum advance range with that vacuum advance can). I've only driven a few miles since it doesn't run well.

I found #6 spark plug wire had fallen off, apparently I didn't click it onto the distributor cap sufficiently, and I put it back on and it ran a little bit better. I found probably the more significant problem, the vacuum advance can went bad and would not hold a vacuum nor move the advance. The can being bad was a vacuum leak (through the bad vacuum advance diaphragm). So now I have a new vacuum can that I installed. I have not yet set the timing, but will do so when I get it all back together. It will probably like a little more advance because I got a new vacuum can with less range.

When putting it in I noticed the rotor had carbon buildup and the center tab was burned through. What the heck? Would going just a few miles with 1 spark plug wire loose cause something like that? Or the vacuum advance can being bad cause that? Is my assembly order wrong or missing parts or do I have a bad coil, or what? In the cap I put the rotor contact button, then the spring with fat side down on the button, then the rubber gasket, then the coil. The stuff is in the order shown in the picture. Is that right and am I missing parts or is that it? Are there any tests to do before I get a new rotor and put it back together?

[kwmech]

Old parts/lack of maintenance/ lots of miles. Sounds like you got the correct orientation. For some reason ( I don't remember) I started using dielectric grease on the larger rubber insulator on both sides (not heavily) kinda like a silicone sealer. Don't get it on the spring. I think I saw it from the factory like that one time. The factory uses a grease -white in color- like they put underneath the modules. What is really important is that you get the ground strap back into the new cap. It'll be a bent piece of this metal or a wire. Missing that piece can cause wayward voltage spikes that have no place to go

[Rust_never_sleeps]

Almost has to be bad contact at the spring, so it arcs across that gap, no?

MMMMaybe a bad points condenser, but IDK if it would do that. You'd see erosion across the points gap if so.
NM, clearly HEI and I'm blind

[dmjlambert]

On this distributor there is no ground strap to put under the coil. I understand some distributors have that. The ground is through the wires on top like in this picture from the top of the coil. Can't be Old parts/lack of maintenance/ lots of miles, I'm talking about having just replaced the rotor cap, button, etc. a few miles ago (maybe 50 or 100). So trying to figure out what went wrong. Hasn't given me this trouble before, it is a sudden malfunction.

[Steeveedee]

OK, I re-read your first post. That plug gap is fine, but #6 plug wire being loose caused the spark to go to the next best ground. HEIs will do that right through the rotor to the distributor shaft. That's very common.

[1970cstblazer]

I would get new old stock Delco or Standard cap/rotor, which are easily found on ebay. The new issued parts are all junk.

[RichardJ]

>>The factory uses a grease -white in color- like they put underneath the modules<<

Thermal Heat Sink Grease or Thermal Compound is designed to conduct heat efficiently, while Dielectric Grease is designed to insulate against electricity, meaning it prevents the flow of electrical current
A dielectric is an insulator and can reduce heat transfer.

di– A prefix that means “two,” “twice,” or “double.” di- can refer to two polarities.
A capacitor is made up with two plates, separated by a dielectric.

The 4-pin HEI module isn't actually four pins. It has five connections. The hollow rivets provide the ground when the module is bolted to a heat sink.

[dmjlambert]

I would really like to get this truck running, so I think I will get a Standard rotor and brush set from O'Reilly. I'll use dielectric grease on the brush rubber gasket. And attach all the spark plug wires this time. I will look into ordering tune up parts online so I have some stuff in stock and can get the truck back running quicker.

I'm not taking the ignition module out and it's been working great for years, so not messing with that right now. But when I do I'm going to use computer CPU heat sink compound, and not going to use anything that looks or feels like grease for it even if it comes with the module. Thermal compound is not similar to grease at all. I have 0% confidence parts distributors are packaging the correct stuff with the modules or that car parts stores know what heat sink compound is. Parts distributors and parts stores benefit by selling dielectric grease to go under the modules, because then I would be back for another module right after the warranty is up.

[LockDoc]

Originally Posted by dmjlambert

I would really like to get this truck running, so I think I will get a Standard rotor and brush set from O'Reilly. I'll use dielectric grease on the brush rubber gasket. And attach all the spark plug wires this time. I will look into ordering tune up parts online so I have some stuff in stock and can get the truck back running quicker.

I'm not taking the ignition module out and it's been working great for years, so not messing with that right now. But when I do I'm going to use computer CPU heat sink compound, and not going to use anything that looks or feels like grease for it even if it comes with the module. Thermal compound is not similar to grease at all. I have 0% confidence parts distributors are packaging the correct stuff with the modules or that car parts stores know what heat sink compound is. Parts distributors and parts stores benefit by selling dielectric grease to go under the modules, because then I would be back for another module right after the warranty is up.

I use Arctic Silver computer heat sink compound on all of mine.
.

[pjmoreland]

Do you have the retainer that snaps onto the top of the cap to prevent the spark plug wires from coming loose?

https://www.amazon.com/Allstar-Perfo...=ATVPDKIKX0DER

[Rust_never_sleeps]

Originally Posted by Steeveedee

OK, I re-read your first post. That plug gap is fine, but #6 plug wire being loose caused the spark to go to the next best ground. HEIs will do that right through the rotor to the distributor shaft. That's very common.

Very cool piece of info. Does this tend to burn through the rotor contact as well?

[Steeveedee]

Originally Posted by Rust_never_sleeps

Very cool piece of info. Does this tend to burn through the rotor contact as well?

If you mean the carbon electrode, not usually, since it is in contact with the electrode on the rotor. Note that it is possible to damage the carbon electrode with the rotor electrode when installing the cap. Not what happened here, though. I graduated uni in '84 but was a mechanic for many years before that, and have seen dozens of burnt rotors. Sometimes when a vehicle wasn't running right, we would pull the rotor and put gasoline in the "well" Gas will wick through even if you can't see light.

[tdangle]

Ive had that happen to a stock 78 Camaro years ago. Just replaced cap and rotor and put another 150K on before I sold it.

[Cali72K20]

Here is an older site that has a lot of info related to the problems you are having with your HEI. Scroll about halfway down and it has some test and solutions to hopefully fix it.

https://garage.grumpysperformance.co...gnitions.2798/

[dmjlambert]

Originally Posted by pjmoreland

Do you have the retainer that snaps onto the top of the cap to prevent the spark plug wires from coming loose?

https://www.amazon.com/Allstar-Perfo...=ATVPDKIKX0DER

No, I don't but I have it on my wish list. It looks nifty. I have one spark plug wire I think for plug #8 that I replaced, and it's the only one that doesn't have the little retaining knob that fits into the holes of that thing. It would probably bug me having that one wire kind of stand out glaring at me. But I'm still thinking about it.