Engine needle valve adjustment: 

The Pinch Test

[printed in the SDF newsletter, winter 2002]
from BAMAC, via Contact Tim Mihalski, editor
13 Jacqueline Drive
Hockessin DE 19707-1008

If you pinch the fuel line, and the engine speeds up, it is on the rich side of the adjustment. How much it speeds up shows how close you are. If it speeds up a lot, you are rich. If it speeds up just a little, you are just right. If it doesn't speed up at all, you are just going lean. If R slows down, you are lean.

This test temporarily starves the engine for fuel and is reliable to test for a too?lean condition. At full throttle, quickly pinch the fuel supply line. The engine should momentarily increase rpm before starting to die. If it starts to die immediately, then it's already too lean and should be adjusted.