Lets see if I can get this straight in my head here. Are you blowing in to the return line in to the fuel tank or in to the carb? Also which carb you have make and model? This because I don't know the Pope configuration but it does sound like it has is either Walbro or Zama carb.
I don't you a venting problem as that usually either causes fuel starvation during operation. Leaks from the fuel tank at the vent. The vent is design to only let air in the fuel tank and not out. Basically it is an one way duck bill valve
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