Hi, I've got a stock 383 which was rebuilt when I bought the car. It has what I would decribe as "lean surge" symptoms when crusing and at idle it has a miss. When I nail it the car run very good. I have changed 2 carbs which have had the metering rods changed. What other conditions would make the same symptons?, I've been told perhaps intake manifold gasket? I appreciate your comments.

cap the vacuum advance & retry it (timing/rotor phasing). Remove the metering rod springs & retry it. What's the initial and total timing? do the idle mixture screws have any effect?. What idle RPM?

I agree. Too much vacuum advance will make it stumble at cruise. Do you have one of those adjustable vacuum canisters on the distributor? Somebody may have been fiddling with it and dialed in too much too soon.

What are the carbs used and what is your vac at idle ?

MP ignition w/vac capped, Both carbs are rebuilt and one is carter 750 avs and other is carter 625 avs, vacumn is a flutting 15 at idle, to hear the car run, it sounds like running on 6 cylinders, but again, when going throught the gears, the car screams?

What do the plugs look like?

vacuum leak (valley pan)