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Do you have a return line? Having one in the first place would suggest that there is an internal pressure regulator that routes gas back to the tank when not consumed by the carb/engine. Or it just runs a higher volume to provide gas to the carb and return line. So if you don't have a return line that may be the problem.
Well, hypothetically, let's say the return line is capped off, ... or not. How might either result in a high idle on a carbureted engine?