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Discussion Starter #1
Hi everyone havent posted here in awhile but the help has always been great.
Anyhow I bought a 66 GMC for a father son project and it seemed to run ok when I got it. Now runs very bad. I replaced wires and then foud 2 dead cyl so I replaced cap and rotor . Now still have dead cly but they bounce around sometime have 3 or 4 not running. Its a 350 engine with a HEI in it. I checked other post and ran a 12g wire from ign switch added grounds all over but nothin.
I can pull a wire off cap and give it about a 1 in gap and cly fires but then oose another one. Spark is very week. How do I test the coil and modual. only things left.
Thanks, Matt.
 

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Just a thought since I ran into this once with a 67 Chevy ramp truck I bought that had a later engine with HEI installed. Did whoever put the HEI distributor in use the original cloth covered resistor wire to feed the HEI ? If so you're not getting a full 12 volts going in and it will be hard to start & may cause it to run rough. Temporarily run a hot wire directly from the battery to the distributor and see if the problem goes away.
 

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Discussion Starter #3
Yes I ran a new 12gage wire from ign switch to dist and still does the same.
 

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If you have 12v to the HEI then the coil is what is giving you fits. The module just tells it when to spark, not how much to spark.
Now show your kid how to change the coil. And don't forget the dielectric grease! :D
 

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Discussion Starter #6
Was always told that a coil either worked or didnt. But will change and see what happens. Plugs are newer and gaped at .45. thanks will let ya know. Matt.
 

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Coils can break down internally and arc between the windings. Windings can short and still give a spark although weaker. The epoxy filled HEI coils are particularly prone to overheating as the epoxy doesn't conduct heat well. Good luck on the coil change - hopefully that fixes it.
 

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Hey Matt. I just pulled a GM HEI out of my car and replaced it with an MSD mostly because the weight posts were about ruined and I couldn't find e-clips or bushings for it. It was still running perfectly though.

Anyway, the one I pulled out has a nearly new module, Accel cap/rotor, and Accel Supercoil. If you want those parts off it, you can have them for the cost of shipping. Even if it doesn't fix the problem, you can't beat the price. PM me if you want 'em.

Also, if you're missing the little ground bracket/strap inside the cap that seats under the coil and runs to the middle of the cap connector it'll run but very poorly.
 

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Also, if you're missing the little ground bracket/strap inside the cap that seats under the coil and runs to the middle of the cap connector it'll run but very poorly.
That's my best guess as to what is wrong. I've seen many cars without this strap after the DIYer's replaced the cap.

Jeff
 

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Id make sure the shaft didnt have to much slack then check clearances at the pickup coil...JM.02
 
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