Well the story goes like this:
I did the GM HEI upgrade a few months ago to get my slant to idle and start better, and it worked like a charm for about a month. Then one day it died sitting in traffic. I hooked the old ecu back up and away I went.

I'm a slow learner so I put a new gm module in and things were mostly fine for a few weeks, I say mostly because I noticed that upon some hot start ups there was a slight to moderate misfire that would come and go, but not enough for me to worry about until this weekend.
I installed a 90amp alternator on Saturday to offset the drain from the electric fan and the system, and again the / spun to life with little effort and I went for a cruise (at night) bright lights, a/c, some kickin tunes..
Sunday rolls around an again it starts right up, and off to church..now here is where it sucks... start it up after church and the slight misfire is there...(no problem..it's like 95* outside) but this time the thing will barely get up to 40mph and is coughing and sputtering through the carb.....so I pull over and pop the hood..leave the engine running....and blip the throttle from the engine bay, yep it's misfiring, but with the hood is up it's starting to run better...so I close the hood and start for home. Within a mile it is back to running like crap..............once home I swap back to the mopar ecu and all is well....

So I'm having this problem with 3 different HEI modules (one is an accel) and the car idles like crap with the mopar ecu hooked up..

There is only 3 things it can be right? Coil, module, or pick-up?
What am I missing here guys..?

The gm module is grounded and mounted to the factory heat sync and another 1/8 inch alum plate about 4 inches across with heat compound, the gm coil is mounted to the inner fender about 6-8 inches behind the alt.
The heat sync does get hot (hot enough where you don't want to touch it), but it wont burn you..
I'm about at wits end with this whole thing.

Get a multimeter and see how much power the HEI box is getting from the ignition circuit. I found out a while back that if it's getting too little power, it'll die prematurely. It needs to be getting pretty close to full battery voltage to run reliably.

To solve this I just installed a 15A relay that'll provide full battery voltage to my HEI box (actually to the coil +, but the HEI box runs off of the power supplied to that anyway), and had the IGN lead trigger the coil. Works like a champ, even after sitting in my garage for eight months

Edit: It also ran far better overall with full voltage getting to the coil and the box. Before adding the relay I was seeing about a 1.5V drop at the coil +.

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

This may sound wierd but I was having a simular problem with my Mopar ECU and found the battery wasnt up to snuff. It would crank all day long but not start up. Battery was drained just enough that the ECU would get goofy. New battery now no problems.
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

Well, after some reading last night I went to troubleshoot. I swapped the coil and the module and was able to make it misfire with the heat gun on the module..so it has to be the module right?..or the power to the module.... So I added a relay that supplies power directly from the battery in start and run positions and it runs fine (all of the time, so far). I suppose I should have done that first but I didn't think that would cause it to run strange SOME of the time.
Jopapa, as of last nights test, I was also seeing about 1.5v loss to the module but still above 12v, now it's 13-14
So in hind sight, it seems the HEI modules are as sensitive to low voltage as to spikes.
Thanks

This makes sense, given that GM vehicles' line voltage has for many years been on the high side of what's considered normal. Some of their present voltage regulators have setpoints as high as 15.1v! Undervoltage to most any electronic or electrical component can cause reliability/response and burnout problems, and 1.5v is a great deal of drop. Seems we have enough collective experience here that relay and power feed info ought to be added to the big article. I'll add it. Keep an eye on it, and keep us up to date, eh?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Glad to see it worked out for you. Looks like my initial HEI failure (halfway home from our squadron picnic last summer) was something of a blessing in disguise for others doing the HEI conversion.

As part of my "pretty-up" project on my engine bay, I'm trying to design a fully integrated HEI box that'll have all the wiring, relay and a fuse in place internally and will only need + and - connections for power, IGN1 and IGN2 connections for trigger leads, distributor signal leads, and coil + and - leads. It's alot of wiring with the addition of the relay, but I think with the use of Weatherpak connectors and the Mopar distributor connector, I can make things look far cleaner and just a little more "factory" under there than they look now. It'll be a DYI project, of course, but if I'm happy with the results I'll post up plans so others can do it as well.

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

This sounds related to part of what I just posted about. My concern in part was partially the high voltage level I was showing at the module and coil. I'm seeing 13.7 to 14.6 at both.

Just don't want to fry anything.

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From Mopars to classic Minis and back to Mopars in 19 years flat!

Will work for roast chicken crisps!!

I'd say as long as you have the same voltage as what you're seeing at the battery, you should be in good shape.

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me