The actual "power valve" is the spring-loaded check ball that piece contacts when there's low manifold vacuum and the spring above the piston pushes the ball off its seat. The V shape allows the plunger to center up on the ball. When the ball is pushed off its seat the main jet is bypassed and the mixture goes full rich. Sometimes the ball and spring gets gummed up and can't seal. Also check the carburetor base gasket to be sure there's a slot that aligns with the vacuum passage that connects to the power piston and overcomes the spring pressure.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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