Here are two pictures that show how I measure my vacuum secondaries opening. I put a zip tie around the secondary throttle shaft on the driver's side of the carb so that it is "handshake tight" and so that the shaft can rotate within the zip tie but tight enough so that the zip tie stays in the new position after the shaft has rotated. Here is the zip tie when the secondaries are closed and then the next pic shows the zip tie after a spirited run with the secondaries opening nicely.
