Ok, now back to your carburetor question. Here is a cut from a study I did here on Rochester B carburetors a few years ago:
THE POWER SYSTEM
4. And anyone who has ever worked on one of these should know about this. If they ignored it or just didn’t understand what it was, they seriously wasted their time. This scheme was designed to provide additional fuel as needed...meaning whenever the vacuum drops to a certain point. This could be going up a hill, when you are passing another vehicle, when you're using your windshield wipers, operating your wolf whistle, etc. And as we'll learn later, it can also provide additional fuel when you don't need it or want it. Here’s how it works: there is a very important vacuum passage going through your carb which routes manifold vacuum up through the throttle body, on through the main well and up into the air horn. When vacuum is high enough (in my observation it could be around 9” to 11” and in some cases more) that vacuum will pull the power piston up and very slightly off of the spring-loaded power valve (basically a ball pushed up by a weak spring). When vacuum falls below 8 to 11”, then a much stronger spring in the power piston vacuum chamber will force the piston straight down and unseat that ball. When this happens, you get much more fuel in the bowl mast. Oh, I should also tell you the power piston is vented back into the main air horn bore. This is so fuel won’t be sucked past it and back down that vacuum passage I mentioned. Here...you can see what it looks like when the power piston is in the “down” position:
(first image below)
And by contrast it would look like this in the "up" position (away from the power valve):
(third image below)
Rochester used at least 2 styles of power piston designs and I wasn't able to unscramble that egg. One had an open shaft and one had a closed shaft and otherwise they seemed identical.
(second image below)
I have pages of work and several images about the Rochester B (including how to straighten the castings using a 3/8" flat piece of metal and a common gas grill). Just let me know if you want me to resurrect those. They are not a tech tip, by the way. I'm not really completely certain why.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
