So has anyone had a problem getting an HEI conversion to work well with a Mopar distributor? All the articles I read here and elsewhere say that it should "just work" once you find out which polarityu to use when connecting the magnetic pickup leads.

Well, mine didn't. With the leads connected one way, it won't start. With the leads connected the other way, it starts, but runs ratty and dies. The only thing I can figure is that an HEI is made to be inside the distributor with VERY short leads between the pickup and the HEI, whereas I wanted to mount mine where the Mopar electronic unit mounts on the fenderwell, resulting in a long wire run to the HEI. In fact, I mounted the whole HEI system in a gutted Mopar electronic ignition box, using the heat sink that was originally for the power transistor so that it is plug-compatible with a Mopar box. That way I can switch back and forth just by swapping boxes and bypassing the ballast when the HEI is in the circuit. This is on a big-block V8, but that shouldn't matter.

Of course the other possibility is that I just got a bad HEI from the parts store. Or that its a GM plot to get back at me for pointing out how much they suck for the past 30 years

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Remember the HEI module is a housing-ground unit, which presents no problem when the module is mounted inside the distributor but if you're mounting it externally, you have to take special care to make sure the module gets a really good ground. I'm betting that's what you're up against. I do not know the consequences of putting power to a poorly-grounded module; it may toast it, so keep that in mind to avoid chasing your tail once you verify and/or fix your ground. As long as it is properly grounded and properly connected, the module doesn't know or care whether it's mounted inside the distributor or on the firewall, fender, etc.

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

If the grounding and wiring is good, a bad module is a possibility. They can behave oddly as they die, just to make diagnostics fun.
I had one junkyard HEI module that ran beautifully for 30 seconds, Then turned into a brick. After carefully verifying my wiring, I put in a second module, which has been running for a couple of years now.
Another module (in a Vega, years ago) did a pretty good imitation of vapor lock as it died.

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"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

I bought one at an auto parts store so I would have a new one and keep a used one for a back up. The new one did not work so I just went to the JY and got a second used one for a back up.

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Maybe, but I grounded the SNOT out of that thing. Star washers under both posts to ground it to the gutted Mopar module that its mounted on (which bolts directly to star-washered posts mounted to the chassis) and an external ground wire on top of that.

You'd think, but it still acts like its getting marginal/inadequate trigger. The Mopar box works fine. I'm really not game to buy another HEI just for a marginal performance improvement. Maybe if the Mopar box dies again- an HEI does now cost about $10 less than a Mopar box.

Any thoughts on whether the HEI box might make greater demands on the *coil* and thus show up a weakness? I wouldn't think so since it actually spends less time passing current through the coil primary than a Mopar igntion does.

FWIW, here are some pictures of mounting the HEI to the gutted Mopar electronic box, plus shots of a Nippondenso alternator installation using a factory 100-amp alternator bracket on a 440:

http://lonestar.texas.net/~sglacker/HEI_and_ND_alt/

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Yes, the HEI box makes greater demands on the coil. Also, erm, the coil has to be connected the right way round.

I am not 100% certain, but I think you may still have a ground issue between the HEI module's bottom surface and that anodised aluminum heat sink on the Mopar box. I'm assuming you used heat-conductive grease between the module and the heat sink, right? (Right?). You might want to try sneaking a wire between the module and the heat sink, running the other end of the wire to ground, and seeing if things improve. Other than that, I would suggest trying a different module. You ought to be able to get a half dozen for $3 or so at a self-serve wrecking yard. Seems to me there are also 2 kinds of 4-pin module; perhaps you accidentally got the wrong type.

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

It might be worth trying some jumpers between the distributor and the module directly, as well as the power, coil and module directly. There may be a weak connection with the connector on the modified mopar box.

This is the time on Shprockets where we say "duh."
Probably, but that is NOT the desired ground connection. Even in a "normal" HEI installation, that surface is covered with heat-sink grease. The grease supplied with the HEI is non-conductive (white zinc oxide based) and can't be counted on for a good electrical connection. The ground connection is normally through the metal tabs around the mounting screw holes.

I may still try your idea, just in case the module I've got has a bad internal connection to the grounding ear on the screw hole. That is if/when I decide I want to screw around with it any more. Seems like more hassle than its worth.

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It only seems like more hassle than it's worth because it's not working for you yet!

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

I had problems with mine at first but i ran a ground wire to the box i used to house the module then from their to the firewall and it worked great after that