Based on what you described, here’s the breakdown:
What the Vacuum Readings Mean
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Manifold vacuum (19–20 inHg): That’s healthy for a stock engine at idle. This tells us the engine and intake are fine, no major vacuum leaks below the carb.
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Carburetor base port (3 inHg): That’s abnormally low. It indicates the carburetor itself isn’t pulling proper signal through its venturi and idle passages.
So the problem is inside the carburetor, not the intake or general vacuum system.
Likely Causes
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Internal vacuum leak in the carburetor
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Worn throttle shaft bushings or a cracked carb body can let air bypass and kill the signal at the base.
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A warped throttle body or main body (common on old Rochesters) may not seal even with a new gasket.
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Idle circuit passages blocked
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If the idle or transfer slots are still plugged despite two kits, the carb will not develop proper vacuum at its base port. Ultrasonic cleaning or passage rod-out may be necessary.
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Incorrect or missing gasket placement
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Some Rochester B carbs use a thin fiber or steel gasket under the cluster. If it’s missing or misaligned, internal vacuum leaks occur.
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Check that the base-to-body gasket has the correct cutouts—many kits include “one size fits all” gaskets that don’t match every casting.
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Metering rod / power piston issues
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If the power piston is stuck open or the wrong metering rod installed, it can bleed air/fuel constantly, wrecking the vacuum signal.
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Warped or damaged air horn or main body
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Over-tightening or years of heat cycles can warp the castings, leaving tiny gaps between sections where vacuum bleeds off internally.
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Steps to Correct
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Verify gasket correctness and orientation – compare old and new carefully, make sure all vacuum channels line up.
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Check throttle shaft play – wiggle the shaft side to side; if sloppy, it needs bushing work.
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Inspect for warpage – lay a straight edge across the throttle body and main body surfaces.
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Confirm power piston action – it should move freely and seat firmly when at idle.
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Re-clean idle passages – use compressed air, carb spray, or even fine wire probes to confirm passages are truly clear.
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Vacuum test the carb off-engine – block all ports and apply vacuum to see if it holds, to confirm no internal leaks.
Bottom line: The engine is fine, but the carburetor is bleeding vacuum internally—most likely due to a warped base/body, wrong gasket, or blocked idle circuits.
