Carb or ignition issue

[Gray035]

Recently my 390 has been shutting off for no reason at random times. It can happen on the freeway at cruising speed or at a stop light at idle. When this motor had the same symptoms several years ago it turned out to be a bad Unilite ignition module. I installed a pertronix Ignitor in the Unilite distributor. This solved my problem for several years.

I thought the re-emergence of this shutoff issue was the result of leaving the ignition on overnight a couple weeks ago (I was doing some wiring troubleshooting on my tail lights). Apparently Pertronix units are susceptible to leaving the ignition on. So I installed a new Pertronix Ignitor I unit. It seemed like everything was fine for a couple drives, but the random shutoff continues. As an added bonus, the last time it randomly shutoff, when I restarted it the RPMs shot up to 3000 or so at idle. Then after a few minutes something clicked and it bounced back down to a normal idle.

I'm at somewhat of a loss. I cannot determine whether I have an ignition problem or a carburetor problem, or maybe a fuel pump problem. The shut-offs occur intermittently, so it's difficult to troubleshoot. Sometimes the before a shutoff occurs, the motor will skip a few times.

The carburetor is a Holley 650 vacuum secondary. I've rebuilt it in the past. Maybe it is ready for another one?

Have any of you had similar issues? What am I not seeing?

[cep62]

Random shut offs are usually ignition . Fuel starvation usually has a little more drama.
You're not using the original resister wire yet ? those will give problems.
Maybe the ignition switch ? Coil ?
Next time it happens hop out and check for spark and fuel.

[Gray035]

No resistor wire anymore. But I'm leaning toward ignition too. I haven't looked at the ignition switch. Maybe that is the culprit.

The problem is that it will start right back up after a shutoff.

[bluef250]

Any luck in chasing down issue.

Have you checked for fuel at carb after it shuts down?

[1972hiboy]

I would have looked at the Pertronix unit first like you did, once I felt satisfied that wasn't the problem I would go to the coil next. Wondering if the random shutdowns are happening only after running up to operating temp? if so could signify coil heating up and the field breaking down. Either way I would throw a new coil at it and re-test.

EDIT, if the pertronix unit is several years old throw a new one at it. I misunderstood, I thought you just put a new one in. I have had pertronix unit go bad before.

[Rocky390]

If it's a fuel issue, it could be vapor lock.

Have similar symptoms with my impala a while back... simply wrap the fuel lines with reflective aluminum heat tape - problem solved