So in light of whatI can see in th drawings and your description of the response of the engine I would:
1) make sure the fit of the main well tube to the upper body is smooth and sealed well.
2) locate the restriction in the passage from the power valve to the main well tube and assure absolute cleanliness
3) remove from the upper body and clean the power valve piston and its bore to assure free movement of the piston when the vacuum drops
4) locate and clean the passages both above and below the power valve piston and look at the sealing of them through the gasket.
5) the spring above the power piston is what pushes it down to open the power valve when throttle is opened and vacuum drops.if it is missing/brokenor not free to operate your lean condition when calling for power will exist
6)the piston rod must hit the power valve check ball when the spring extends it or no enrichening
7) clean the power valve itself and try to prove it allows flow when open and seals off when not actuated
Study this power circuit and think about it's function--in my opinion the trouble is to be found there.
Good luck and keep your eyes open for other possibilities.
