72 390, c6, 2100 2br
Truck has been running good, was putting down a gravel road and it died. Now it idles rough. It will start ok but idle is at at low rpm. So something when from good to bad. Other than idle, the truck runs fine including WOT

I have checked the following:
-Timing
-all vacuum lines/consumers
-alternator/battery voltage
-carb
--float valve
--choke
--idle circuit
--power valve

Pulling a plug wire or a vacuum line will kill the motor. Carb float level looks good and bowl is full. The trans modulator holds vacuum, brake booster is good, plugging PCV makes no difference. unplugging vacuum advance line on dizzy makes no difference. Truck has a few tanks of fuel through it after new plugs, wires, coil, carb rebuild, fuel tank/sender/filter/lines/hoses, air filter, ect. Good compression.
It will only run when (stovepipe) choke is still engaged, or if idle speed adjustment screw is cranked up enough.

If I cover the carb horn partially (aka choke it) with my hand it runs smooth and rpms come up signifigantly. This makes me think it is a carb issue. At idle it appears that there is fuel being delivered by the main circuit's booster venturi. I don't think this should happen at idle (but I'm not a 2100 guru). I replaced power valve, no change, existing one was still good. I have disassembled and cleaned carb 3 times. The idle circuit is clean and clear. Idle mixture screw have no effect

What to do next?

 

2100 carbs are cheap enough to have a known good spare on hand for testing.

 

All the above points to the idle circuit is plugged. Too lean at idle until you do something to get the main circuit to start fuel out of the venturis. Did you soak the carb or just spray can the circuits? Afterwards did you blow everything out with compressed air? While doing this did you make sure you heard or felt the circuits to be open? Find any ka ka at the bottom of the bowl?

 

Thanks for the info carb Dr. JEFFFAFA.
Didn't soak. Used carb cleaner and compress air to clean passages. Inspected with flashlight, all 'looks' clean and clear. Spray comes out where it should. Needles are in good shape. Bowl had a few little bits in it but otherwise clean from recent rebuild.
I think I know how the idle circuit works, it has only a few parts;
-fuel feed tube in the venturi assembly
-idle circuit tube w/ restrictor and one rectangle air vent
-mixture screw in it's cross bore

 

I don't think this is what's happening in your carb but, the mixture screws won't make a difference if you have the idle set high. Whether it be by hand or by the choke. Note the transfer slot in the pic below. That is normally used for off idle fuel flow. But I think the idle circuit is plugged somewhere from the idle air bleed on down since we know the venturis are getting fuel. You had mentioned it runs good everywhere but idle. Even WOT. If you take it apart again try blowing compressed air "up" the idle circuit with the venture cluster removed. Maybe a piece of rubber hose in between you compressed air nozzle and the idle discharge port. Mixture screws still in there but just barely. Then maybe mixture screws all the way in and hose to the transfer slot. Also make sure that idle air bleed is open.


Attachment 195206

 

 

Took another look inside the carb. Air bleeds are clear. The passages are pretty big compared to some of the other carbs I have worked on (motorcycles and such). I can clearly see the idle circuit is all clean. The only part I can't shine light through are the feed tubes in the booster, but I can shoot carb cleaner though them clear across the room.


Still runs like a turd must be a vacuum leak that has eluded me to this point.

 

I see the upper part of the idle circuit is clear. Are you sure the circuit is open and clear from there down to the idle discharge holes?
The vacuum leak idea peaked my curiousity. When you R&R'd the carb I'm sure you used a new gasket under the carb. How about the gasket under the spacer plate?

 

Methinks vacuum leak.
Tighten down intake manifold. If problem persists, spray carb cleaner or starting fluid around intake ports and anywhere that you might be sucking air.
Also, pull out idler screws, back off idle speed screw so throttle plate seats, spray the throttle plates heavily and let it soak. Liquid will be held by the seated throttle plate, cleaning varnish. Stick the schnozzle of carb cleaner into idle mixture holes and spray, spray.
If mixture screws do not affect idle, you are pulling off the main jet, as Jeff has said above.
Let us know what you find.

 

2x Vac leak

 

I checked all vacuum lines again. manifold bolts are tight. I went ahead and let it warm up on the choke and adjusted the idle speed screw and the idle mixture screws. I stopped at 3-3/4 turns out indicating a vacuum leak somewhere... The carb itself has a loose throttle shaft so I know it leaks somewhat there. It runs and drives OK but the idle is still a little rougher than it was before, as indicted by the rattling window glass in the door. It struggles mid throttle at highway speeds again now, so another indication of a vacuum leak. I have several other project going so it will have to do for now.