The Stovebolt Site Forums The Shop Area Engine Shop The Rochester Power Circuit...

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...

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.

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.

Before I go any further, it has been correctly and politely pointed out by a observant gentleman who was more attentive in math class than I that the area and not the diameter should be the determinant and following that line of thought a stuck power valve would produce an overall jet equivalent to size 70 (or about 21% greater than 58). So you're not quite as obnoxiously overcarbureted but instead you'll be running something like 20% too rich...not that 20% too rich is in any way an acceptable level. And also I was asked about the two tiny holes at the top of the crossbar (not associated with the power circuit, but still a good question). Please see image below. These and the diagonally leaning tube are also air bleeds.

Did I try other attempts at solving the power piston problem? Yes, there was one other effort involving a thin aluminum piece (mounted under the mast) which held in place a small o-ring which would hopefully seal the piston and stop fuel from traveling up into the power piston area. You can see two images of it. Why didn't any of these ideas work? Well, actually this scheme made things worse. Here's why: the power pistons and their bores by now are worn out. Vacuum seeping around them and the constant sloshing of gas means the piston will be always bathed in fuel and it doesn't take long for that gas to gum things and stop free movement. In this particular case, the o-ring helped retain the fuel which got sucked up into the piston area and obviously that wasn't good. Sorry, but I couldn't figure out a suitable solution. The old image links are dead. I'm certain you've heard about Photobucket and the problems there. Hopefully this helps with questions about the power circuit. It certainly has been kicked around here enough. If anyone is interested in the difference between the Carter YF models and why many people believe they work better, we can go into that also. Seems to me things here are kind of loping along, so it may not be a bad idea.

Awesome info, thanks for posting!
Nice vaccum port extension. Also, I wish I'd have polished my power piston now! Fortunately, it wasn't stuck to begin with.

Here's a picture of my viton gasket experiment. I'm hoping this gasket will have enough compliance and rebound to seal my carb without having to use your straightening method.
It starts out about .060" thick and compresses to about .040-.045 in tight parts of the joint. I'd guess my air horn has about .010" of warp to it. Most noticeable on the gas inlet side.

We'll see how long this lasts! Or if it even works... i haven't started the engine yet.

I was going to laser cut it, but found out that viton releases HF (extremely toxic gas) when you burn it! So i spent an hour with an X-acto knife instead.

Good luck with the gasket. I hope it works for you. Jon (CarbKing) and I discussed gaskets of that nature as a possibility and decided there probably was some merit to the idea. We also discussed pvc and other materials.

Thanks john, good stuff! Learned sumthin to!

Nice job on gasket! I make most of my own gaskets as well

Here's a tip on making gaskets: the holes can be easily and quickly cut using either a common hole punch (like kids use in school...you can get all different sizes in both SAE and Metric sizes) or a hole punching tool (tubular type steel which is hit with a hammer). The second variety isn't my favorite as you can't line the hole up as well, but it will work. Also, when making a gasket like this, don't worry about the outside edges (and occasionally the part in the inner bore). Cut those larger and leave them sloppy. After the gasket is installed, you can trim around using an X-Acto knife or razor blade and usually you can do the same thing inside the bore...may have to reach a bit in there but if your knife is sharp it will work.

You know, I love these old buggers, I have rebuilt and worked on them for 56 years. Jon G has probably forgot more than I will ever know. Doc

Additional tips on making gaskets.

If you need a gasket for which you have no sample:

(1) Clean the casting to which the gasket will be applied.
(2) Place the casting on your scanner, and scan it to your computer CAUTION: different scanning programs may not scan 1:1, I use Adobe Acrobat, which gives an exact dimensional scan on my system.
(3) Acquire a sheet (8 1/2 x 11) of gasket material from your FLAPS
(4) Print the scanned file on the gasket material. Now all you have to do is cut it out.

One additional tip: your significant other's fingernail scissors have curved tips, and are extremely useful for cutting curved gaskets.

Jon

Thanks for the tips!
I'll definitely have to use punches next time, scanner sounds very handy too!

Here's a rumor you may be able to confirm or deny:

Rochester B is more reliable than many other carburetors offroad and for driving on steep inclines because of the circumferential float bowl

Sounds like a questionable claim.
Seems like jet position within the bowl is more relevant?

Several decades ago, I used to watch the Perry Mason show.

I remember well one of the most-used lines in the show, when attorney Mason was in court.

"Objection Your Honor, calls for a conclusion by the witness" :p

You would probably get different answers from different folks.

Having said that, Carter pioneered the use of the spring-loaded fuel valve for use in carburetors used for marine and off-road applications in the 1930's.

The primary civilian off-road vehicle during the run of the Rochester type B carburetor was Jeep. Jeep used Carter.

As for military only vehicles, I have no record our military EVER used a Rochester type B.

Again quoting Mr. Mason - "Your Honor, the defense rests"!

Jon