Ok, back with 5 more images. Let's talk about your power piston. It could have been of two styles: open tip or closed tip. And any "professional rebuilder" could have swapped your piston for what was handy, what was on the floor, what was in his parts basket, anything. The first image below shows the two types.
THE VACUUM GAME
Please see image two below. The red arrow shows your power piston. The blue arrow shows where vacuum travels through a passage in your float bowl and (hopefully) activates the power piston. The yellow arrow shows a vent hole leading back into the carburetor bore (to allow free movement of the piston). The green arrow is pointing to the crossbar, and that's where all the extra fuel (via the mast tube) will be going and down into the engine via the large hole in the center. Let's focus carefully on the blue arrow now. Without a leak-proof vacuum passage, you have no hope of making your power piston work as designed. This very honestly speaking is the Achilles Heel of the B (and other variants of it). When the castings get warped, vacuum will be lost. In math applications, that would be what we'd call a "given." That's one reason I chose to replace and lengthen the tube leading into the float bowl. I can't promise it will, but it may help with vacuum transfer. Some of these models also used a tiny O-ring around the base of that tube to hopefully help seal things as the Rochester boys knew the flaws of this design, the warping, the cockeyed idea of only using 4 screws early on in the game. Why they didn't change may indicate the designer was an otherwise unemployable son-in-law of Mr. Rochester, but anything you can do to prevent vacuum loss will work to your benefit.
The third image shows the power piston. Remove it (it may be stuck so use care), clean well and polish using 1000 grit sandpaper or something like MAAS polish. You also want the bore it rides in to be nice, clean and smooth. Today's ethanol gas will quickly leave a sticky varnish-like deposit which will make your power piston stick in the bore...and please trust me...it
never sticks in the UP position. The spring makes certain it sticks in the DOWN position. All 5 of the carburetors I studied had stuck power pistons. One almost could not be removed it was stuck so tightly. Be certain it is free. I've thought and meditated as hard as I could and tried all sorts of things to keep the piston from sticking...tiny o-rings, special grease, aluminum collars with rubber nipples that would fit under the mast (like you see in image 4 below), you name it. It looked good, but nothing worked. You can see another image below. Personally I think the only way to make this design work is to somehow move it to the outside of the carburetor where it has a chance to remain operational over time. I'll be back in a minute. Time to take a walk with my wife.
