Need help diagnosing a miss in my 77 Jimmy

[Jimmydriver]

My 400 CI small block has a random miss. It occurs in neutral or under load. It can be heard in the exhaust and felt at the exhaust. It happens with the engine hot or cold, The miss sometimes feels like a single cylinder missing, other times multiple cylinder misfires in a row maybe 2 or 3, and sometimes the engine shuts off briefly maybe a ½ second or less and then comes back to life. When it misses you can see the engine jump. It feels as if the ignition momentarily turns off and then back on.

It has a Summit HEI ignition, maybe 3 years old. The plugs, wires, rotor and cap are new. The 12-volt supply to the distributor is consistently there even when the random miss occurs. Bad distributor module or something else?

 

[Hogback Fabrication]

I had the same thing happen to me with an MSD HEI that isn't 1/2 as old as yours. It was the ignition module. I would think a coil might cause this as well. Try swapping a working ignition module or coil onto yours and see what happens.

 

[diesel4me]

another possibility..

I've had a few pick up coils cause intermittent misfiring..I'd guess that could be your trouble,since it seems to be random,and not always the same cylinder thats misfiring..could be the module or coil too,or a wire thats bad or corroded..I'd check and see a full 12V is at the "hot" wire too..low voltage to an HEI can cause misfires ..