Most likely, you didn't get a venturi insert, but the sleeve that SuperTigre used on the original mufflers. This reduced the size of the hole in the case so the smaller carb could be used.

Here's the insert than goes into the venturi:

SUPG2420 22623366 CARB RESTRICTOR 3000K $4.99

It has a 10mm O.D.

These parts can be ordered through any hobby retailer who sells SuperTigre engines and accessories.