An email asked if I could resurrect the info on the Rochester B and BC power circuit. The answer is probably. First off, please understand fuel enters the vertical mast by one of two ways: the main jet or the power valve. Please see the first image below. Unless you are accelerating or under load, you want the power valve to remain closed as much as possible. Next, please understand once fuel enters the vertical mast via the power valve, you immediately have too much fuel unless vacuum is low enough to let the spring push your power piston down (opening the valve which is just a spring loaded ball). When fuel gets in there, it is regulated by what is known as a power restrictor. This is shown in image 2 below. As I recall the diameter was about 1mm or .0394" in all the carburetors I studied. Your main jet might be anything from 56 to 59. Let's for the sake of this post say you have a 58 jet (common). Add .058 to .039 and you get .098. So, if your power piston is stuck in the "down" position, you now have a 98 size jet. Please note: if you are driving around getting 8 mpg, this is a good clue. A 98 size jet is (as you can see) almost twice too big. So you're running way too rich. Third, please note in the third image the power piston in the "UP" position (where you want it to be). This means vacuum is keeping it off the power valve (the ball) and your carburetor is only being fed by the main jet (which honestly is plenty in most cases). Fourth, please note in the fourth image the power piston is in the "DOWN" position (where you only want it to be until your engine has returned to normal operation...meaning you're at the top of the hill, you aren't accelerating, etc). Fifth, please note in the fifth image the power piston itself and the spring controlling it. Please also note in 5 carburetors I found a verified 3 different strengths of springs used. This means the vacuum needed to keep the piston up could have been as low as about 7 or as high as 11. That's a wide range and as you can see a strong spring will cause the power piston to open the valve pretty easily in some cases. 5 images is all I can post, so we'll look at more in a moment...
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
