[56ls408]

Have you tried checking at the tailpipe with your hand to feel the amount of exhaust coming out both when it's running correctly then when it is acting up that might indicate if there is a restriction in the exhaust. Years ago the factory used a double wall exhaust pipe & I have seen the inner wall of the pipe collapse restricting the exhaust same as happens today when a converter is plugged.I seriously doubt that is your situation as It's likely yours wouldn't have that type exhaust plus it wouldn't come & go with engine temperature but might be worth checking the exhaust output based on your observation .

 

[Cheapskate]

I doubt it's the exhaust because it's all new pipe (except for the headers).
Haven't found a solution yet and yardwork took some time off the evening. But I happened to compare the carb that's hooked up the one I have now in spare. I was looking how the primaries and secondaries move and close. And I noticed that the one I have on has some "extra flaps".
Here's photos of them both with wide open throttle:

As you can see the spare one doesn't have those upper ones. And the one that's on has them closed. They do move freely if I gently push them.
What's this?

EDIT: Also noticed that engine temp has no effect on this problem..or at least today it hadn't.

 

[55wagoncrazy]

Those are the secondaries and they should be wide open on full throttle. If not that will cause he engine so stumble. You may need to
make adjustments to when they open. They should open mid throttle. Otherwise the carb is running more like a 2 bbl carb.

 

[Cheapskate]

I don't know how to call those upper butterflies but the secondary butterflies at the base opened normally. I'll have to look into it why these upper ones don't open at all.

 

[Cheapskate]

OK, so I found those valves from the manual and they're auxiliary throttle valves that open only on air flow. So no wonder they didn't open with engine shut off. I've had the unit in my hand when assembling/disassembling but guess I forgot it in the middle of the night.. 🤦‍♂️ And someone must've took them totally off from the spare unit and that's why I got puzzled. Sorry.

 

[Shadetree55]

OK, so I found those valves from the manual and they're auxiliary throttle valves that open only on air flow. So no wonder they didn't open with engine shut off. I've had the unit in my hand when assembling/disassembling but guess I forgot it in the middle of the night.. 🤦‍♂️ And someone must've took them totally off from the spare unit and that's why I got puzzled. Sorry.

when secondaries open on throttle, vacuum should open upper flap and if like a quadrajet that opens the metering rods for secondairies

 

[stoveboltgeek]

when secondaries open on throttle, vacuum should open upper flap and if like a quadrajet that opens the metering rods for secondairies

Not on a 4GC. On a 4GC, airflow due to the secondary throttle plates opening pulls the auxiliary plates open - not manifold vacuum.

 

[Shadetree55]

Not on a 4GC. On a 4GC, airflow due to the secondary throttle plates opening pulls the auxiliary plates open - not manifold vacuum.

Okay, I'm a quadrajet man LOL

 

[stoveboltgeek]

One thing that's unlikely but worth checking is the action of the power piston and power valve. I've noticed both tend to not move smoothly if not cleaned well with carburetor cleaner. (No , 4GCs do not use metering rods for the power circuit. )

 

[Cheapskate]

No luck with new coil and condenser. Plugs were also gapped just right at 0.35-0.36. Checked also timing and it was on the spot.
So I've been back at the carb again. I haven't found any vacuum leaks so far. I tried to change the mixture again; put the mixture screws at "basic setting" 1½ turns out,took idle rpm screw out to the point the car barely kept running and began turning the mixture richer. It seems to be running "best" at the screws 2½ turns out but the problem still exists.
It idles fine and takes rpm (fast movements) quite well now but but at any steady rpm it stumbles and wanders around. Closing choke by hand a bit makes it run worse so it doesn't need to adjusted any richer at least I guess.
As suggested, I've looked how the choke behaves and the flap seems to stay open all the time as should at this temp. Just out of curiosity I took off the choke heat tube while it's running to see how things change and there seems to be a constant heavy vacuum on the choke tube connector. Should it be so 🤔? Didn't find an answer from the manual.
I'll check the power piston too, thanks!

EDIT: Could a too high float level cause this kind of situation? I've measured them several times though...

 

[Shadetree55]

No luck with new coil and condenser. Plugs were also gapped just right at 0.35-0.36. Checked also timing and it was on the spot.
So I've been back at the carb again. I haven't found any vacuum leaks so far. I tried to change the mixture again; put the mixture screws at "basic setting" 1½ turns out,took idle rpm screw out to the point the car barely kept running and began turning the mixture richer. It seems to be running "best" at the screws 2½ turns out but the problem still exists.
It idles fine and takes rpm quite well now but but at steady rpm it stumbles and wanders around. Closing choke by hand a bit makes it run worse.
As suggested, I've looked how the choke behaves and it seems to stay open all the time as should. Just out of curiosity I took off the choke heat tube while it's running to see how things change and there seems to be a constant heavy vacuum on the choke tube connector. Should it be so 🤔? Didn't find an answer from the manual.
I'll check the power piston too, thanks!

have you checked vacuum advance on distributor?? is it sticking?? not moving at all etc.

 

[Cheapskate]

have you checked vacuum advance on distributor?? is it sticking?? not moving at all etc.

At least connecting the hose changed timing on rpm when I checked with light. Also sucked the hose and it wasn't leaking. Don't know any other good tests.

 

[stoveboltgeek]

2 1/2 turns on a 4GC is fairly rich. 4GCs tend to run a bit better if not too rich an idle.

There should be vacuum on the choke heat tube - that's normal.

 

[Shadetree55]

At least connecting the hose changed timing on rpm when I checked with light. Also sucked the hose and it wasn't leaking. Don't know any other good tests.

Physical and visually check under cap. Is it advancing and not returning completely.

 

[Cheapskate]

Physical and visually check under cap. Is it advancing and not returning completely.

Will check that. The dizzy is new so it should work. Although...the old original one seemed to be overall better quality.

 

[Cheapskate]

A buddy of mine suggested that it still sounds like a spark issue and to be precise maybe points gap wrong or vacuum advance not working (also mentioned here). So I'll do a double/triple check.

 

[Cheapskate]

Holy cow! It works! And better than ever Finally after 1½ months of scratching head and mostly blaming the carb.
Today I kept messing with the distributor. I was determined but couldn't find what's wrong. As a last effort I took the old '57 model distributor from the shelf, cleaned/lubed the centrifugal unit, put new points and condenser in it and vòila. Man it was really nice to drive smoothly.
I don't know what's wrong with the new distributor. Points gap/dwell is same as now in the old one etc. But it just doesn't work. Weird. Maybe it's just overall bad quality.

EDIT: I changed the distributor in the first place this spring because timing kept wandering around. Turned out it was just because the centrifugal unit was sticky and the shaft was missing gasket (against intake manifold). But I thought...well new is new. What did I know.

 

[panelman]

Holy cow! It works! And better than ever Finally after 1½ months of scratching head and mostly blaming the carb.
Today I kept messing with the distributor. I was determined but couldn't find what's wrong. As a last effort I took the old '57 model distributor from the shelf, cleaned/lubed the centrifugal unit, put new points and condenser in it and vòila. Man it was really nice to drive smoothly.
I don't know what's wrong with the new distributor. Points gap/dwell is same as now in the old one etc. But it just doesn't work. Weird. Maybe it's just overall bad quality.

EDIT: I changed the distributor in the first place this spring because timing kept wandering around. Turned out it was just because the centrifugal unit was sticky and the shaft was missing gasket (against intake manifold). But I thought...well new is new. What did I know.

So glad to hear you found it!! I was waiting to hear you had a plug wire with the casing burned off somewhere.

Man I hate those mystery issues.

 

[TheProphet]

Probably a good idea to check out the distributor you removed, and find out exactly what WAS the problem.

Good for future knowledge. I'll bet on the condenser!

 

[Cheapskate]

Probably a good idea to check out the distributor you removed, and find out exactly what WAS the problem.

Good for future knowledge. I'll bet on the condenser!

Will do. But it wasn't the condenser. I replaced it with the one I'm using now while doing tests.