Starting a new thread, since I figured out my distributor issue from my last thread. This could be the same issue, but I'm not sure and wanted to post it here.

Last week I installed a DUI distributor and took the car for a test drive. 10 or 20 minutes in, the control module failed. I spoke with DUI and they told me that based on the ID number, the distributor was build during a brief time that they had an order of modules that were overheating. They sent me a new module and I installed it with heat seek cream. I did a short test drive and it ran great.

Today I drove the car to work and it ran fantastic, until about 15 miles in when I gave it about 3 seconds of WOT. It ran great under WOT, but after that when I accelerate it stumbles bad. I put it in neutral and it revs perfect with no stumble. Back under load, it stumbled. I have to feather it to get the RPMs up enough to drive without stumbling.

I made it to my office and am hoping once it cools that it runs better to get home. I suspect the control module is going bad again. I know my fuel system is good and I don't think it is a coincidence that I'm having this issue (that I have never had) until I put this distributor in.

What are you thoughts? I know I'll sit here a my office all day running ideas through my head of what the issue could be, so I thought I would ask you guys to ease my mind until I can start troubleshooting. I've emailed DUI already. I may just pull this distributor and send it back at this point.

Thanks
Keith

 

1972 Corvettes came with ignition points distributors. If it were mine, I would put it back to a points distributor. Points distributors are very dependable. A points distributor will get you home. Engine may run crappy, but it'll get you home. GM and other auto manufacturers came up with electronic distributors as a way to reduce customer maintenance cost. No more replacing ignition points and condenser.

 

I had an issue with blowing multiple ignition modules in a row. Their quality probably wasn’t helping, but the thing that actually fixed it was replacing the engine ground cable. These electronic ignitions need a good ground.

 

Sometimes, bad modules damage coils; and vice versa.
Perhaps a fresh (NIB) pair is in order ?

 

I recall my ground cable being in good condition when I installed the engine, although that doesn't mean it is. Hmmmm.

Maybe the coil is damaged now, I don't know. It is odd to me that it was running great until I got the rpms up really high on WOT. That 3 seconds shouldn't have increased the heat at the module to damage it. So strange.

When I installed this distributor, I gapped my plugs to .55, as suggested by the manufacturer. That is the only setup difference between this distributor and the one I replaced.

Keith

 

I'm not 100% sure, but I think the points distributors had 12v during starting and then 6v during run the run cycle. The DUI probably needs 12v all the time, so have you changed your wiring so it's 12v all the time?

 

Yes. I updated to a cheap HEI a few years ago and ran a dedicated 12v wire from the ignition. The DUI was supposed to be an upgrade for me to get a better curve and performance. The car ran with the cheap HEI, but I was hoping for better performance with an upgrade.

 

What is your timing set to?

The DUI won't perform "better" than a cheap HEI. It simply comes set up with the "correct" advance, and was (I thought) higher quality than anything else. Spark is spark, at least at street RPMs.

Can you switch back to the cheap HEI, and see if you have issues? I'm guessing coil at this point, but retarded timing will make your engine run hot. More heat in an already crowded C3 engine compartment could help cook some marginal parts.

I have no idea how heat proof they are, but I'd look into this distributor instead. Perhaps DUI will accept a return after all of the problems you've had.

https://progressionignition.com/

 

Timing is set to 12 initial and 36 all in. The engine temp never exceeded 180.

I just went out at lunch to see if it ran differently now that the engine is much cooler. I wouldn't idle and loped like I had a huge cam in it before it eventually died and won't start. Acting the same as it did with I didn't have spark, so assuming the control module or coil went out.

Good thing it is at my office and not on the side of the road. Bad thing is I don't have a ride home now.

 

That's what mine does when the (manifold) vacuum advance gets disconnected.

Post some photos if you can, perhaps someone will spot something obvious that you can fix easily. Could the distributor clamp have slipped?

 

DUI is shipping a new module and coil for me to try, since it's currently stranded at my office. Changing those in the parking lot should be easy enough, if I can just get a ride home today.

Keith

 

The two worst conditions/stresses on the module are WOT and large plug gaps. Each of those conditions raises the flyback voltage on the coil C- terminal, which is connected to the module power/switching transistor. Doing both of these conditions really beats up that transistor. High flyback voltages can damage/break-down the transistor, shorting out the coil primary windings, which kills the secondary voltage, causing misfires.

There's no free lunch in life. When you open up the gap, you automatically shorten the arc time (and stress the module). There's certainly a possibility that the coil is defective, but I would reduce the plug gap back to .035" first before running the new module.

 

I had them at .45 with the previous HEI. I always heard that .45 for HEi and .35 for non-HEI. You certainly sound like you know what you are talking about. Can you help me understand why .35 instead of .45 with an HEI?

Thank you,
Keith

 

I suggest that 69427 has as much or more direct knowledge of GM's Delco-Remy HEI than any other CF member; and for good reason.

 

I'm about 15 miles from home. If I replace the coil and module at the office and gently drive home before changing the spark plug gaps back, will I cause issues? Pulling the plugs in the parking lot is really not an option and I don't want to tow it home.

Keith

 

It all comes down to cylinder pressure and air/fuel ratios. Remember, HEI means high energy ignition system, not high voltage ignition system. HEI was introduced when engines were low compression and tuned to be mixture "lean" to minimize emissions. Lean mixtures are more difficult to light off, so the strategy for good combustion performance was to open the plug gap substantially (from the common .035 to .055-.060) to expose more air and fuel molecules to the gap arc, and then provide more energy to that gap. More molecules and energy in the gap, the better chance of combustion lite-off. But, the problem of larger gaps is that it shortens the gap arc time (the coil delivers energy to the plug, with that energy consisting of/= plug voltage X plug current X arc duration). To ensure that there will be sufficient energy at the plug gap to last the 50k miles that was required by law IIRC, "excess" energy is available from the system to compensate for plug gap increases during that duration.
Larger gaps require more voltage to arc over, and from the equation above, something has to give, and that's the arc duration. Additional system energy from the coil restores the desired arc duration.
The reason the system isn't a labeled a high voltage ignition (an HVI, as it were), is due to the lower compression ratios in the engines of that time that didn't need high voltage to arc the gap. A genuine HEI unit will usually work well in older higher compression engines, but the larger plug gap usage isn't necessary in older engines (due to the richer mixture fueling and the higher compression forcing a greater percentage of the mixture into the plug gap) and isn't preferable for long term durability of the module.

The basic physics is the same for your DUI setup, but I don't know the electrical specifics of that system. Unless you're running low compression and lean mixtures I'd stay at .035" gaps. Plugs will last forever at the setting, and the module power transistor will thank you.

 

Please, I blush easily.

 

You should be fine during a normal drive home. It's the WOT/High cylinder pressure/High plug voltage conditions that stress ignition modules.

 

Thank you for all of that information. While the majority is way over my head, I love having the info to learn more about how the ignition works. Hard to troubleshoot something without all the information.

I'll get the gaps back down to .35 as soon as I can get it safely back home. In the meantime, it looks like an Uber ride is in my future today.

Keith

 

The old breaker point IGN systems preferred a .35 gap for the plugs.
The newer HEI systems prefer a .45 gap.

Not sure where you came up with super wide 55?

The goop that goes under the modge is not really an insulator for heat.
It actually withdraws (sinks) heat away from the modge and disperses it to the distributor body.
That's why it's called Heat-Sink-Compound.
Never use dielectric grease or other substituits.

Under the coils dust cover, note a black GRD wire on one corner of the coils frame.
Make sure you have a bare metal connection there without any paint.

Under the coil is a Spring Bushing and rubber washer. (That is also seen protruding dead center under the dist cap)
DUI may have installed a High Resistance Bushing. Other dizzy have those as well.
The high resistance bushing adds heat in the cap. You may want to purchase a Low Resistance Bushing, around $19.