Well, here's a possibility, had it happen to me. Same deal car would start fine and idle fine up to around 1500 rpm. Past that point it would cough, stumble and seem to miss. After tons of trouble shooting and finally replacing the HEI with a friend's HEI it ran fine. I changed everything I thought I could in the HEI but overlooked the tiny little unit fastened to the base plate that I assume is the equivalent of a condenser on a standard points distributor. that solved my problem!

   

DanT

That's a thought. I hadn't considered that, thanks.

I did double check the wires today. They're all in the right place on the cap and headed to the correct cylinder. So at least that's eliminated.

I had decided today that I'd try swapping back the other QJet (with the choke problem) to see if the rough running goes away. But I'll try swapping a different capacitor on there and see if it makes a difference first. Easier than swapping the carb :-).

Steve

Ignition Control module.

https://www.rockauto.com/en/catalog/chevrolet,1977,corvette,5.7l+350cid+v8,1041062,ignition,ignition+control+module+(icm),7172

I have a spare ignition control module. I didn’t think they failed slowly- I thought either it works or it’s dead. Have you seen them cause this?

I had a chance to swap the capacitor Sunday and test drive. No change , so I’ll try my spare ignition module next. 

To be clear, all the ignition parts I’m trying have run fine on the car previously. I checked them out and put them away specifically so I’d know they were good in case I needed to replace a component. 

I think there is a special grease you have to put under that module.

Any possibility that a plug got bumped and closed the gap?

You recurved the timing.  Was that at the exact time of the cap/rotor swap or a completely different procedure after the fact? 

Yes, I suppose it’s possible. I thought I was being careful reinstalling but I want to pull them anyway to see if one or more are fouled so I’ll regap them at that time. 

I set the timing curve separately last fall before the carb swap. I had it up to 3000 rpm to check all-in timing and it seemed to run fine then, but it wasn’t under load. 

I do use Arctic Silver on the heat sink of the ignition module so that ought to be fine. 

in regard to the “crossfire cap” from the first reply… is that maybe referring to inductive crossfire? I hadn’t thought of that, but I’ll check plug wires 5 & 7 and make sure they’re separated. I’m using Taylor Spiro Pro wires. I’d think they would not be prone to induction but I’ll make sure those two are separated. 

Yes, The Taylor wires are worth the extra money and work well..

Click on image for full view

I’m resurrecting my old thread because I finally found the culprit. Yes, it’s been six months… lots of home improvement projects had to happen before I got back to working on the car. The wife is appreciative.

Turns out it was a bad plug wire all along. Plug #8 was oil fouled but I couldn’t see why at first. Seems the wire was close but not touching the header. That baked the silicone outer cover until it was hard and crumbly. I think the compromised wire was arcing against the header or block, but I never saw that because the view of that plug is blocked by the AC housing. I also never watched it run in the dark. I was effectively driving on  just 7 cylinders all this time.

I think I’m going to try the Accel ceramic boot plug wires. I can’t see any silicone boot surviving long near the headers. 

Steve

MOST EXCELLENT...!!!  Good sleuthing and congrats on figuring out the problem.  Now go drive it like you stole it...!!!

Jim Olson 

"The Toys"...!!!  Save the Wave!

Where I've been in a Corvette...!!!