How long is the fuel line to the carb assembly? Are there any "connectors" in the fuel line for refueling? As I had your mid-throttle symptoms I was using a lengthy piece of fuel line to the tank that had a white plastic connector inserted in the line and exposed outside the cowl that was disconnected for refueling. The connector was very restrictive (inside diameter maybe 1/2 of the fuel line). After re-plumbing the fuel lines I replaced the plastic connector with a short piece of brass tubing whose ID was much nearer that of the fuel line. With the tank cork tightened and the plastic connector replaced my mid-throttle problem has all but disappeared. Seriously considering a 3 line approach with a shorter carb line with no inserts.