In this test you will see if the "air cut off" or "enrichment valave" is leaking.

Disconnect the vacuum home to it. This hose is from the right carburetor. Between the butterfly and the intake tube. Plug the nipple on the vacuum port on the left cyclinder. Can not leave it open!!

Then Disconnect the larger hose in the top of the valave. If you plug it with your finger and the idle does not hang, then the valave is leaking. If no change in idle, the valave is working properly.