HEI module going spastic?

BillGrissom said
I watched the video, but didn't see the HEI module. Not sure where "under distributor" means. Someone made a plate to mount HEI under a slant six distributor. Do you have similar on your small block? I like your diagnostic. I do the same when checking spark. You shouldn't have missed sparks at idle since it is easy for a spark to jump the gap at low cylinder pressure. It is under WOT at low rpm where one gets misses from an excessive gap.

Did you tightly twist the wires from the distributor pickup to the HEI module? Not doing so can give cross-talk which causes spontaneous firing from positive feedback. I experienced such when testing with my HEI module using temporary jumper wires. Perhaps your #3 plug wire runs closer to the pickup wires? Why this matters - an open loop is an "antenna" which picks up high-frequency magnetic signals like that generated by the current in a spark wire. By twisting the wire, you form multiple loops in different directions, which cancels out any pickup. Another thing is to verify the HEI module is well grounded.

Yes Bill - I have the same setup as was devised for the Slant Six. I tightly twisted the wires and then put shrink wrap over it to hold the twists. I rerouted the #3 wire so it does not come anywhere near the pickup wires. I do not have a separate ground for the HEI module does - would it not be sufficiently grounded by the heat sink-to-distrbutor?

69MOPE said
Not enough info.
New install?
What type HEI?
Is that a MSD in the video?

P.S. Please get the spark plug wires away from the headers!

It is a new install - brand new rotor and cap, brand new wires, new spark plugs, new GM 4 pin HEI module. The MSD is a rev limiter but I have not connected it yet.

Ouch - the wires are at least an inch and a half away from the headers at any point. I'm still smarting from having to buy a complete second set of wires just to get two right-angle plugs to run the #5 and #7 wires under the headers.

Oklacarcollecto said
What coil are you running?

My coil is a new MSD Blaster 3.

trapster said
check your cap for cracks, try a different one if you have it.

I inspected the cap but couldn't find any cracks. It's brand new. I can't even see any carbon on the inside yet. I tried the old one but it made no difference.

DFX 340 Duster said
X2 Since it is only on #3 I would suspect something other than the module, such as the cap, wire, or reluctor.

That makes sense. But I suspect the problem is not the occasional miss on #3 but rather the fact that it may be firing too often.