Here’s the blunt truth: your symptom screams “lean at idle,” almost certainly from an internal vacuum leak or an idle fuel delivery problem. Since choking it with your hand smooths it out, that confirms it needs extra fuel (or reduced air) to idle.
Here’s where to look, in priority order:
1. Leaking throttle-shaft bushings
On older carbs, worn throttle shafts act like a hidden vacuum leak.
Cold = tolerable. Hot = metal expands, leak gets worse ? unstable idle ? stalling.
Test:
Spray carb cleaner directly at each end of the throttle shaft while idling.
If RPM changes, that’s your leak.
This one fools a ton of people because the carb works fine off-idle or at cruise.
2. Idle circuit not actually delivering fuel
Even if the bowl is full, the idle feed restrictors or idle mixture passages can be clogged.
Symptoms:
-
Idles fine with hand-over-carb
-
Runs fine at speed
-
Dies at stops
-
Mixture screws do almost nothing
Fix:
Pull the idle mixture screws out, spray carb cleaner through the holes.
Verify you can see cleaner in the bore.
If not, the carb needs to come apart again.
3. Vacuum leak under the carb
A leaking base gasket, cracked insulator, or warped carb base will match your symptoms exactly.
Check:
-
Tighten carb mounting nuts
-
Look for warped base (very common on 2-Jet, Quadrajet, Holley 2/1 bbl, Carter YF)
-
Spray around base with carb cleaner ? RPM change = leak
4. Manifold vacuum port or plug leaking
You said you pinched off the hoses, but make sure:
-
PCV valve isn’t stuck wide open
-
PCV hose isn’t cracked
-
Any unused ports are tightly capped
-
Brake booster check valve isn’t leaking (pull booster hose and plug the manifold—see if idle changes)
5. Float level too low
Low float = insufficient fuel at idle once warm.
Hot fuel expands and can lean out a marginal float setting.
If it runs right only when you “choke” it with your hand, float too low is always a possibility.
