Make sure that the limiter screw on top, allows the barrel to fully close.

Check the cleanness of the little hole at front, if your carburetor has it.
Save the position of the low mix screw in your mind, by turning it all the way in.
Then unscrew it completely and clean the tip and the seat in the body.
Screw it back and try again.

Tune the thing 1/8 of a turn at a time, running the engine at full rpm for just few seconds before each pinch test.

Check this article:

http://webpages.charter.net/rcfu/Hel...s/TVAProc.html

Regards!