That leather plunger you're seeing is on the accelerator pump. The power valve is a long rod that rests against a ball bearing and pushes it off its seat under low-vacuum conditions. Under higher vacuum, the rod is lifted, and the valve closes, forcing all the high-speed gasoline to go through the main jet. The power valve assembly is attached to the top of the carburetor, and hangs down close to the bottom of the float chamber. The vacuum piston is lifted against a light-tension spring to the lean position, and drops to unseat the power valve when extra gas is needed for pulling.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!