i have 88 jeep wrangler i rebuilt striaght 6 cylender motor carb too. what is happing is drive up to an hour or say 30 miles then the engine dies wont start.
Sounds like good old fashioned vapor lock of the gas line. Try insulating the fuel line or moving it away from the heat source.
Probably vapour lock indeed, best to check the routing of the fuel lines and to feel if the carb ans see whether either may get too hot.
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