Chad19

We have a 1976 Corvette that has a mis that starts around 3,000 RPM. We have gone through the electrical - new plugs, wires, coil, distributor, etc. Installed a new Edelbrock carburetor and aluminum intake manifold. Compression test shows consistent readings for all cylinders and an inspection showed no broken valve springs. There were 2 spark plugs that looked like they were not burning as clean as the other cylinders, but other than that everything checked out. Timing was adjusted as well.

Could a bad catalytic converter cause the issue? I don't know if the converter is bad but thought it might be the culprit.

Any ideas on the possible cause are greatly appreciated.

thank you!

MelWff

can you disconnect the cat and test it?
the problem existed before you replaced all those parts?
the distributor is a new HEI unit with no cracks in the cap or rotor?
you did a resistance test on the plug wires?
what plugs are you using, make and number?
what plug gap are you using?

red topless gator

Which 2 plugs not firing right? Would it be #4 and #8?

gjohnson

Did you replace the control module?

drwet

I will assume all of the electrical components that were replaced were done correctly, and that nothing changed after they were replaced. If that's the case, then you are probably correct in looking at the catalytic convertor next. If its the original convertor, its likely to be plugged. Those early design convertors were not as efficient as modern ones at the best of times, and were more susceptible to becoming plugged. And I would say your symptoms are consistent with a plugged convertor.

Chad19

MelWff - Thanks for the reply.
I will look into your suggestions when I get some time over the weekend. The issue started before the new parts were installed. We suspected electrical so we started with the new HEI distributor, Accel coil,wires, plugs, etc with no change. Pulgs are AC Delco - I will confirm the model and gap this weekend.

Chad19

Plugs 6 & 8 are the plugs that looked wet when we pulled them

Chad19

Yes - ICM was replaced at the same time as the distributor

Chad19

Correct - All electrical parts were replaced and not a single thing changed with the mis-fire. I think disconnecting the converter will be the next step as it shouldn't be much work.
The car operates just fine under 3,000 RPM, idles smooth, starts easy.

Chad19

It has been awhile but we were able to swap out the converter but we still have the mis-fire that starts around 3,000 rpm. Could this be a bad cam issue? Is there an easy way to test this?
thanks,
Chad

1Hotrodz

A worn exhaust cam lobe can cause a popping noise thru the carb. Aside from that, a fast way to check for a worn cam shaft is to remove both valve covers and check the valves for travel, by rotating the engine. A couple valves may not open as far as the rest indicating a worn camshaft lobe(s). If you want to get really technical about this then get a dial indicator and place it on the edge of the valve retainer and measure the travel by rotating engine, do this for each valve. Your not really looking for an exact number here just a variance between the rest of the valves. Mind you intake and exhaust valve lifts are not the same, so take that into consideration when you measure.

1Hotrodz

Also, about your dist., make sure you have an "AC Delco" ignition module nothing else performs like the factory modules, ask me how I know.

MelWff

I know they are new wires but do a resistance test on them, 1k ohms per foot of wire.