This may cause a necissity for a magnifying If you look real close you will see there is a highly machined area of the venturi ahead of the tool that is slid over the e-tube. The rule says there must be a portion of the highly machined venturi ahead of the e-tube. Some carbs might need to have the gauge tipped upward or downward to get the venturi to come into view which is exceptible. The use of a magnifying lens may sometimes be called for.
Go to youtube to see video. https://youtu.be/8JmS606QzTo