Maybe the carb was opened up which makes it more sensitive. 1/8" is about the max that I found to work. Maybe the tank is back too far. Maybe the clunk line is leaky, detached or pushed forward from a hard landing. Air leaks are magnified a lot when using pressure. Maybe it is just too lean to start with as a normal test is to raise the nose to set the needle. It should work well on suction if the carb is not drilled out. Try it without the backplate pressure, and plug up the nipple.That is about all I can think of that has not been mentioned.