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I am working on a buddies 1979 C3 he says I dies sometimes. I have not had it die but is definitely is having a high rpm miss and not going much above 4k RPMs. the tach flutters above 2k rpms under load or full throttle. this leads me to believe the ignition module is dropping signal. all the ignition parts look newish to me. how to I definitely diagnose which part of the ignition system is at fault. the coil ohms test seems in spec 5ohm and 1ohm respectively.
in the second pic is what appear to me a second condenser I believe that is the ground and it goes too the tach but I'm not positive.
any direction helps. its been a while since I have worked on a car I can't just hook my OBD computer to to test these things
thanks in advance
Last edited by sanjuanbum; Mar 21, 2026 at 08:28 PM.
Reason: Better pic file
Hello and welcome to the forum. I can't see your pictures. Sorry.
I can tell you that little can that looks like a condenser is the tach filter. Not likely your issue.
Have you checked the flyweights for freedom of movement? Also check the flyweight bushings. These are generally plastic and do wear. If the mechanical advance is stuck. This could be your problem. The ignition modules in these HEI distributors have a reputation for failure, particularly inexpensive replacement units. The best way I know of checking one is to replace it with a known good one.
The best replacements are original used G.M. factory units. Or one from Performance Distributors. Also known as DUI. (Davis Unified Ignitions.)
Once you have verified that the mechanical advance works smoothly. Put a dial back timing light on it and check the timing curve. Timing at idle should start at approximately 12 degrees with the vacuum advance disconnected and vacuum line plugged. Then should start advancing the timing around 12 hundred to 15 hundred RPM. continue to rev the engine noting the full advance timing. And at what rpm it takes when timing stops advancing.
Stock distributors often have full timing quite high.
ideally you would like to see 12 degrees at idle and 36 degrees of timing at full advance. And have that all in by around 3 thousand RPM. however. If it's a stock distributor. It may not have all the timing in until much higher. Not ideal. But it'll run. Then there is the vacuum advance timing. However. Your symptoms are not a vacuum advance timing issue. But if your tuning this you will need to get that right as well. A factory advance can typically puts out more advance timing than we would like. If so limiters are a available. I can get very long winded on vacuum timing alone. So start with getting the mechanical timing right. Once you do. See if the engine will now rev out properly. And report back.
I should mention, if your serious about learning about this system and how to properly set it up for modern fuels. You should contact Lars, he is a technical contributor on this forum and will send you his ignition papers if you email him. Extremely helpful information.
Also keep in mind, ignition problems are often carburetor problems. And Carburetor problems are often Ignition problems.
Timing may need adjustment but probably not the problem. The cam in the dist and the vaccum arm move freely and move when revved. I was getting out the timing light today when I ran out of time…
it’s a bummer I can’t post the pics and vid to show in the post. Not sure what I’m doing wrong there. Seeing the tach video seams like a quality piece of diagnostic info to me.
Im really leaning towards a failed ignition module but I just want to be sure and not throw parts at it.
HEI distributors are in millions of GM cars from the mid 70's to the mid 80's. Perhaps a friend has a module you could borrow for a couple hours just to test your theory.
My experience with bad modules is, when they fail. The car doesn't start, at all.
If you load a video on YouTube, you can then post a link here. But I wouldn't have a clue how to do any of that.
I have been reading the millions of posts on threads on this topic...
it seems module failure makes the car not run usually. would it be possible for intermittent failure like I'm describing to be caused by a module? is it a faulty coil?
something like this has to be common. these cars are pretty simple and have been around for almost 50 years
In my experience modules either work or they don't. I have never had one that caused the car to run poorly. Others have told me it happens, but I've never seen it. Take it for what it's worth.
Every time I have had an engine that wouldn't rev, it was plugs, wires, or coil. (Assuming the mechanical advance is working as you say.) In theory that means it could be cap and/or rotor as well.
I have been reading the millions of posts on threads on this topic...
it seems module failure makes the car not run usually. would it be possible for intermittent failure like I'm describing to be caused by a module? is it a faulty coil?
something like this has to be common. these cars are pretty simple and have been around for almost 50 years
Yes.
Also, the pickup coil aka trigger located in bottom of HEI bowl has two very fine wires coming from it. They terminate at one end of module. Then module amplifies that signal and sends it to coil & Shazam the magic fire is unbridled.
The ignition advance plate rotates and can cause those fine wires to rub against inside of HEI alum housing as advance articulates.
That can cause wires' thin insulation to wear-thru and ground those trigger wires against housing: Killing any signal. If that happens enough, the wires can (and have) broken. They're very fine and fragile. As fine wires are repeatedly bent over decades, they 'work harden' and become brittle and can/do break.
Module itself must be mounted to dist housing with a thin coating of Heat/Thermal Transfer Paste applied between bottom of module and dist. That paste typically becomes dried-up and ineffective. Module will then overheat More quickly and may fail. Get that paste from electronic/computer supply OR find Arctic Silver 5 paste on Amazombie. Do Not use dielectric grease for this; Do Use Thermal Transfer Paste.
I've seen aftermarket modules packaged with a tiny envelope of dielectric grease; garbage.
Last edited by Rebelyell; Mar 23, 2026 at 02:13 PM.
I just went thru this and found the distributor cap, did not have all the lockdown screws secured to keep the cap on tight. I spent hours, from a new
accelerator pump to timing to this and that. When I saw the 2 metal rods sticking out from under the distributor, I didn't know if I should laugh, or cry.
If you upload the movie file to YouTube, you can directly copy a link here. Do a walkaround the engine compartment if you can.
No one is going to click on a PDF or .mov link in 2026. Most of us wiill look at images, and YouTube videos, though.
^^^^THIS^^^
I'll add, and hazard a guess, that Most of us who frequent C3 forums, don't routinely speak or measure in metric terms.
I can and do convert; but prefer to not suffer any bother.
I'll add, and hazard a guess, that Most of us who frequent C3 forums, don't routinely speak or measure in metric terms.
I can and do convert; but prefer to not suffer any bother.
If you aren't using kPa and radians, you're doing timing wrong.
My ignition module stopped working when I was traveling at 40mph. Bought a SMP from an auto parts store and it was dead out of the box. Purchase a Delphi and engine started immediately. Never know what you will get.
the spade conector from the capacitor to the ignition module were splayed open and not making a good connection. I squeezed them tight to make good solid contact a presto. no more signal dropping out.
I was just about to throw in the towel when I decided to go through the whole distributor one more time and noticed the connecter was quite wiggly.
I hope this helps someone in the future. thanks for all the help guys.
high rpm miss
