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Old 06-19-2010, 08:47 AM
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Can't get spark from HEI dist without tapping cap

I've got a 350 small block in my '72 GMC pickup.I've replaced the points ignition system with an aftermarket (napa) HEI ignition, and it's run fine for 5 years. Recently, i can't get a spark out of the thing unless I tap the distributor cap, at which point, it will fire right up. I've replaced all the bits, cap, module, coil, etc, to no avail; I've even run a new hot wire from the starter.

Any thoughts?

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Old 06-19-2010, 08:59 AM
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I suppose the couple things I would check are the ground strap under the coil and the wires going to the module from the pick up coil. Not the coil in the cap, the one that is next to the reluctor.

Good luck.
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Old 06-19-2010, 11:24 AM
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or possibly broken wire inside insulation for pickup coil etc

it happens....

can be real tricky

you may opt to just swap in a stock HEI from any 75 to 79 car or truck or '80 up heavy 3/4 or 1 ton truck(these were not EST or ESC equipped)

good luck
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Old 06-19-2010, 12:55 PM
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Quote:
Originally Posted by fast68
or possibly broken wire inside insulation for pickup coil
Yes- those wires.

Every time the vacuum advance moves, the wires move, too. Over all the years and miles, they will sometimes break.

I've seen them cause intermittent problems when the vacuum advance would cause the advance plate portion of the distributor to turn- it would miss, then catch, etc.
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Old 06-19-2010, 02:10 PM
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I would also check the pin and spring that connect the coil to the rotor. \Once it starts is it ok?.
Another possibility is the physical distributor ground to the block, not likely but it is possible.
You may also want to look at the rotor, they are known to burn through under the contact.

Where exactly do you tap on the cap to get it started.
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Old 06-19-2010, 02:30 PM
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yes, once it starts, it runs just fine. I tap it right on top, doesn't seem to matter where. I thought it would have something to do with groundstrap or coil, but both seem ok, new coil, it tests fine. I'll tear in to it this afternoon to have a look at those wires.
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Old 06-19-2010, 02:33 PM
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Quote:
Originally Posted by roadbase
yes, once it starts, it runs just fine. I tap it right on top, doesn't seem to matter where. I thought it would have something to do with groundstrap or coil, but both seem ok, new coil, it tests fine. I'll tear in to it this afternoon to have a look at those wires.
T-bucket brings up a good point about the carbon center post- the spring that contacts the bottom of the coil is made of tiny wire, it wouldn't take much for rust/corrosion to affect the contact.

You can easily check it when you take the cap off- it's under the coil (4 screws holds it and you're there).

Be sure to use dielectric grease on the rubber washer if it's dry.

Last edited by cobalt327; 06-19-2010 at 02:39 PM.
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Old 06-19-2010, 02:48 PM
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Quote:
Originally Posted by roadbase
yes, once it starts, it runs just fine. I tap it right on top, doesn't seem to matter where. I thought it would have something to do with groundstrap or coil, but both seem ok, new coil, it tests fine. I'll tear in to it this afternoon to have a look at those wires.
I would think that rules out a bad connection as most likely any bump would give you problems. One other thought. How is the crank and start feed to the HEI connected. If they are spliced together at the lug at the cap I would be suspicious of a bad connection in the circuit that feeds during cranking. Next time it wont start, try checking for voltage while cranking at the cap.
One other thing to consider is maybe when you are taping on the cap you are also moving something else in the wiring.

I am real suspicious of the crank feed to the HEI due to the fact that once it is running it is fine
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