It is very likely there isn't enough vacuum to lift the piston off of the valve. There is a spring which pushes the piston down if there isn't enough vacuum. I'm talking about direct manifold vacuum here and that vacuum goes through a passage in the carburetor to the top of the power piston. When there's not sufficient vacuum and the piston is pushed down by that spring, the rod at the bottom of the power piston pushes the spring loaded ball at the base of the mast open and this floods that area with fuel. Fuel rushes in there and is available for the carburetor to suck up through the power restriction hole. When the power circuit is open, the carburetor runs much richer, and it can even do this at idle. More on that in a moment. The whole idea is that when the engine is under load, the manifold vacuum will drop. Naturally the engine will need more fuel when it is under load, however lacking manifold vacuum, it isn't easy to get enough fuel without help. That's what the power system is supposed to do. But...when your Rochester gets warped, then vacuum passing from the throttle body and up through the float bowl is lost where the float bowl connects to the air horn. How much vacuum needed to keep the power piston off of the power valve depends entirely on the strength of the spring in there. I've seen some which only needed about 7 or 8", more of them which needed roughly 9" and others which needed about 11". Honestly 11" is pretty doggone extreme. You may only have 15" or so at idle, so any bit of vacuum lost anywhere in your carburetor will let the power circuit be open...at idle. This is what causes the Rochester B to run rich all the time. The power piston isn't doing anything for you.
Please take a look below at the first image. In it 1 is the float/float pin, 2 is the main metering jet, 3 is the power valve cap, 4 is the bowl mast, 5 is the power piston assembly, 6 is the vacuum passage for your power piston (with a brass tube I added), 7 is the main nozzle, 8 is the vacuum passage lip for the idle circuit, 9 is an internal vent with an air bleed hole above it. And please take a look at the second image. That vacuum passage as you will see has no tube coming up. When I was doing all the study work on the Rochester B I decided it might help to add the brass tube, but unless the air horn and float bowl seal well, I'm not sure it matters. If there is any warpage in your air horn, it is very likely you're losing vacuum right where that passage is. I should also mention there is a vacuum break in the bore of the air horn. It is there to relieve any vacuum that might build up at the piston. If the vacuum break passage wasn't there, fuel vapor from the float bowl could be sucked around the piston and through the vacuum passage and into the manifold...and this would result in your carburetor running richer. But (a large "but" in the case of the Rochester B), you have to have a perfectly sealed air horn/float bowl for this to happen. And in my observation that's pretty rare unless the carburetor has been straightened.
And in the third image below you can see how fuel enters. Yellow arrow is your main jet. Red arrow is the power valve. If it is open you're getting much more fuel than you need. And the fourth image shows the power restrictor...if you look hard you can see it. It is a hole which allows more fuel to run up the mast along with what the main jet has allowed in there. So you can appreciate the need to keep that power valve closed as much as possible.
Hope this is helpful. Good luck!
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
