Likely the larger carb is reducing vacuum to the petcock and shutting fuel flow off. To help maintain vacuum to the petcock you can do a few things, completely remove the smog system and block off the exhaust PAIR vent. This also greatly simplifies hose routing. Then run the vacuum line direct from the intake manifold to the petcock, when you remove the smog system this makes it a direct run with no T's which can reduce vacuum to the petcock.
You might need to switch to a manual petcock.
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