First, what size prop are you using? OS tends to suggest props a bit too heavy which makes the high speed needle overly critical. Also you are better off with the needle opened more than 2 1/4 turns when starting a new engine. Set the initial position 3 turns from when you can first hear air going through the needle seat with the carb wide open. You will find that position is different from where the needle quits turning. Just blow through the fuel line and listen carefully.

Finally, when you do the pinch test (high speed test at wide open throttle), you pinch and release the fuel line as fast as you can. All you are looking for is a momentary increase in speed.