First, make sure that diapragm under the cover with the four screws is nice and pliable. If not, you will never get it tuned right. They often dry out and get stiff, and have to be replaced.

That diaphragm has a spot in the middle of it that presses down on a very small lever that raises a needle valve. That lever is normally adjusted to be about level with the surrounding metal surface. Very carefully bend it so that it is slightly above the surface of the surrounding metal, then reassemble and give it another try.

AV8TOR