This is a really out-of-left field question. Is the gravity feed system properly vented?
I had a fellow VCCA member who had set up a gravity feed gas supply to temporarily run the engine in a Model A. One evening our VCCA chapter was at his place for a meeting. He was describing the exact same symptom. The engine would start fine but only run and 20 seconds. Throttle movement did not affect anything.
I looked at his one gallon rectangular metal can for the fuel supply. He had soldered an outlet line into the cap. He had a make-shift hanger to hold the can upside down about the same height as the cowl fuel tank in the Model A. I asked how he vented it. No response. Just a sheepish look.
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