The Stovebolt Site Forums The Shop Area Engine Shop The Rochester B and BC work...revisited

It has been about 4 years since I did the Rochester B study and since it never became a tech tip nor even a sticky all that work has become harder to locate now. I understand that, so I'm going to resurrect that work and correct a few things in the process. Besides that the images were hosted by photobucket and that service has become virtually extinct it seems...at the very least you will not be able to see images on the prior work. So here’s a repeat:

I think it may be interesting and instructive to review the Rochester B series before trying to solve any problems. Thousands of these are still in use after all and things which might be considered basic or even unimportant can have notable effects. When this study was done I purchased 5 of these in varying condition. The carburetor that served as the “model” for the images was just a parts carburetor. When I got it, it was covered in grease and mud and the air horn was held on to the float bowl at one of the 4 screw holes by a nail which had been bent (possibly with a rock) into a crude L shape. It did clean up ok, however. There is an O-ring used to seal the power valve where a prior genius had stripped the threads. But it is fine for visual use.

So, first a look at some parts of the air horn: (please see image one below)

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 something we’ll discuss later, 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 next a view of some parts of the fuel bowl: (please see image two below)

1 is the accelerator pump hole, 2 is the accelerator check ball assembly, 3 is the main venturi area, 4 is the boost venturi area, 5 is the power piston vacuum passage.

And a look down inside the bore to see the cross shaft: (please see image three below)

1 is two of the air bleed holes, 2 is an internal vent tube.

1. Because of the design of the B series, the fuel level is very important in determining whether you run rich or lean. As you’d expect, atmospheric pressure plays a part here and the level naturally affects the distance the fuel has to travel for the idle circuit and also the main circuit. Higher lever = richer, lower level=leaner.

2. In the B series, the idle scheme is simple and given the right circumstances will work well enough. At idle there isn’t enough air flow to make the venturi do anything so the carb relies on the main jet, a vacuum passage, an idle tube in the float bowl, four air bleeds, discharge ports and a mixture needle. Vacuum pulls fuel up through the float bowl via the idle tube and into a cross shaft a couple of inches below the top of the carburetor in which air is mixed with fuel via 3 air bleeds and also an internal air bleed that is sort of hidden. Then the mixture is drawn back down through a passage in the fuel bowl and fed into the engine below the throttle valve. There are only 3 things you can change or adjust in a correctly working idle circuit: the main jet, the level of fuel in the bowl and the idle adj. needle. All the air bleeds and idle tubes and everything related to those are fixed.

Located above the idle discharge hole are other discharge holes known collectively as the "off-idle" port. They're activated as the throttle is opened, drawing air across them and increasing the same fuel/air stream already in play at the idle discharge hole. As the throttle is opened further and air flow increases, air pressure in the boost venturi begins to drop and fuel starts being sucked out of the main nozzle (which interestingly also works as one of the air bleeds in the idle circuit). This fuel travels through the main venturi down into the engine and this almost completes the idle/off-idle scheme. I say almost because the idle port will still be feeding fuel into the engine at this time. Rochester engineers believed this would create a smooth transition from idle to driving speed, and it wasn’t a bad thought. Here is a view of the throttle body showing those plus the distributor vacuum holes: (please see image 4 below)

3. When air flow increases and causes more fuel to go through the main nozzle, the main metering system begins to take over. Ideally fuel will be metered predominantly by the jet and that fuel will travel through the bowl mast (and still through the idle tube, naturally but with lesser importance now). The fuel once again mixes with air through the same air bleeds that fed the idle circuit and gets discharged into the engine through the boost and main venturi. And if the Rochester engineers had stopped at this point, things might have been Jake. But they didn’t. They took one strange step further and invented...



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: (please see image 5 below)

Okay, must move to another post...have used our 5 image limitation here.

I was thinking about something the other day. How much of the B’s poor reputation is due to design in original condition vs. condition issues? (Set aside for a moment that the design caused the warping).

Or to ask it a different way, has anyone compared a pristinely-restored B to a pristinely YF on the same engine? And if so, are they running characteristics noticeable?

(I can delete this post if it detracts from your intent, Jon)

Now for just a moment let's look at some more images regarding the Rochester Power System since that will be the source of many problems for those who continue to use these carburetors.
Image one below shows how gas enters the mast. Yellow arrow = through the main metering jet. Red arrow = through the power valve.
Image two below shows the mast and setting inside the mast is the idle pick-up tube (normally it will be connected to the air horn securely but many are missing thanks to the fine and careful work of "professional" rebuilders).
Image three below shows the idle pick-up tube and to the left of that (retained by my finger) is the power piston. You need to have a good relationship with your power piston.
Image four below shows the parts of the power piston. The piston and extending portion and the spring. At this point I'll mention the strength of that spring can be important. It regulates the piston movement. Weaker spring = less vacuum needed to pull it up. Stronger spring = more vacuum needed to pull it up. Many different tension values were used. I don't believe anyone really knows for sure what the answer is and these were all mixed up by "professional rebuilders."
Image five below shows the parts of the power valve itself. As you see this is simply a threaded plug holding in place a weak spring and a ball bearing. That is your power valve.

Ok, on to the next part...

Before I took the time to show the 5 images above I showed the open power valve. And by contrast it would look like this in the “up” position (away from the power valve: (please see image one 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. The one in this carb has been crudely soldered closed just for illustration purposes and also because when I got it, it looked like somebody had been using it as a toothpick or cleaning something off the bottom of their shoe with it. (please see image two below)

The extra fuel provided by the power system is available to the idle tube and the main metering system. It is metered by what Rochester calls a “Power Restriction,” and in the carb I’ll be using for illustration, this restriction hole was 1mm or .0394” in diameter. Here...you can see it in image three below:

I’m going to overlook the choke system, accelerator pump and throttle body. Those don’t seem to me to be classic problem areas. However if the throttle shaft bore is worn to the point you notice wobble it goes without saying it should be bored and sleeved.



The fuel level. You can adjust this up/down with the float and experimentation is easy. A lower level will also lessen the tendency to weep at the seam and my experience has been it is common for these to naturally run rich. However there is a wild card here. The Carter engineers learned the size of the orifice in the seat (needle/seat) was important. Too large of an orifice caused flooding. When the float drops to allow fuel in, the fuel flows through a large orifice faster than the float can compensate for. That is to say there will be a delay involved in a float operated mechanism and the size of the orifice of the seat will be important. An orifice too large will cause flooding. You can see in image four below:

Vacuum loss. This was a problem for 4 of the 5 Rochester B and BC carbs I observed (remember one could not be operated), and I would imagine the same problem exists universally. Every one I had was warped to some degree and in one the warpage was well over 15 thousands. How and why? Only 4 screws to hold them together certainly doesn't help, but maybe those 4 screws have a partner in crime. Please see image five below:

And on to the next post as we’ve used our 5 image limitation...

And for an air horn with more visible “lines” please see image one below.

The casting lines were designed to bite into the gaskets forming a nice seal. And they do that...old gaskets show it, but if you'll check old gaskets with a mike or dial gauge, you'll find the screw hole locations of those gaskets will be thinner than the area between the 4 screw holes. This shows the gaskets are compressing at the screw hole locations and not compressing as much in the middle. Heat and this stress is causing the castings to warp. Only having 4 screws to hold these together adds to the problem. When those gaskets age, dry out and shrink, the areas in the center are going to be the areas that promote vacuum loss and gas seeping first. And later we’ll talk about how to straighten the air horns and float bowls.

How about using 2 gaskets to fix this? I’m not a fan of that technique. Please look at image two below to see why. These 2 gaskets came out of the carburetor that was most significantly warped. They had been stacked on top of each other and this carburetor worked just as poorly as you would expect. One of them either slid out r was sucked out into the interior bore and you can also see there was a nice gap where air was being drawn out of the float bowl every time this vehicle got past the idle/off-idle point.

These old and shrunken gaskets and the warpage all were working together to cause both a loss of vacuum and fuel leakage. Did new gaskets help? Only minimally. So let's consider vacuum loss is going to affect 2 things:
1--The idle circuit. It will make it run leaner.
2--The power piston/power system. Loss of vacuum there is much more sinister as it can cause the power piston to operate erratically or to fail to work at all. If the power piston disables the power valve, the whole thing fails and you have more fuel available to both the idle and main metering circuit all the time. Consider: The main jet in this carburetor is .058" and the restrictor is .0394”. If we calculate how much extra fuel will come in around the power piston and the hole, we come up with an area of the power valve of .0123”. If we subtract the area of the rod from that (.0058”) we get an extra amount of .0165.” That added to the main jet of .058” equals .0745” and that will be the effective size of your main metering jet now. So you’ve increased the fuel to your engine by almost 30%. You’re running really rich now, so you should celebrate. Buy stock in Chevron.



Sadly all but one of the carburetors I had to study suffered sincerely stuck power pistons. When the power piston gets stuck, it always sticks in the down position. How does this happen? When you shut off your engine, the power piston loses vacuum and the spring immediately pushes it down. Gas has been sloshing around in there and will have wicked up into the gap. As the gas in your bowl evaporates (and it always will), it leaves a sticky crud on the piston which makes it hang in its bore. Once this starts happening, I can't see anything to reverse it aside from taking it apart and cleaning it. My suggestion is when you rebuild one of these work very carefully and try every single thing you know to get that piston out of there with no damage to it or the bore and then polish both it and its bore with MAAS or Flitz or something like SemiChrome polish and then grease it with cornhead grease mixed with graphite. That might give you more time before the thing sticks again, but it will stick again. It is a wonky design and I can think of a couple of different ways to achieve the same thing externally that would (a) not put a power piston down inside a float bowl and (b) not get stuck so predictably. But in fairness they didn't have the junky gas we have today and that's an issue for these.

Okay, back to some other problems...

Your idle pick-up tube may be missing. It was missing in one of the Rochesters I got. Please look at image number three below.

You can fix this with a piece of brass tubing and if you want your motor to idle you will need this. Please see image number four below.

There is a vacuum lip that can be knocked off in clumsy rebuilding. If this leaks (and it sits right next to a thin gasket at the air horn bore) it will make your idle mixture too lean. Take a look at image number five below.

To repair this, you can cut and insert a piece of 1/8 inch brass tube. You may need a tiny bit of JB Weld to secure it. This piece will need to extend a bit below the gasket surface...not too far, though. I did make this prettier before I finished. Please see image number one below.

And this important vacuum passage (the one for the power piston) can be improved by adding a brass tube. Please see image two below.

Rochester actually added tubes similar to this along with rubber o-rings in some of the BV and other variants to minimize problems they knew existed. I believe with a straightened air horn and float bowl you won’t need an o-ring, but I’m not against adding one. The trick is to get one slightly less thick than 1mm. No sense adding extra pressure here.

These are the problems I noted (the ones above the throttle body...which really are the ones which will cause your Rochester B or BC to not work correctly). Now please stay tuned for an explanation on how to straighten your warped castings...

Hi JW51...thanks for the thought. No post or comment detracts from this topic. Many if not most of the Rochester B's problems are the result of rebuilding work through the years. And I have poked fun at "hamfisted" or hungover workers, but if you ever have been inside one of these rebuilding shops you'd know there is absolutely no way the parts of a carburetor can be kept in any way sacrosanct. Many years ago (early 70s) I was part of a team that inspected one of the largest rebuilders in the whole N. Texas area. It kept burning down and the insurance company wanted to know why. The condensed version of their MO was: evening shift disassembled all like carburetors and tossed all like parts into 3 baskets. Tossed being the important word. One for large parts (air horns, throttle bodies, float bowls), one for medium parts (floats, masts, removeable choke parts, etc). and one for screws and small parts. In the case of a Rochester B or BC, everything that fell out of it when it was disassembled. They dropped these into solvent vats and went home. Then early morning the crew would haul the baskets out, let them drip dry and take them over to the assemblers (who are paid based on the number of carburetors they assemble and have minimum quotas per shift). From those beginnings, "rebuilt" carburetors were created...and if any parts matched where they came from it was a pure accident. The floor manager when asked about this took drag from his cigarette, spit and said "oh no...we never try to keep those parts together. No reason to do that. If a carburetor doesn't work we'll just give them another one and rebuild it again."

I tried to find a pristine Rochester B or BC when I was doing this study. I couldn't. I have found about 10 New Old Stock Carter YF models, though. It is getting harder to do, though. The Carter YF is a much better designed carburetor from nearly all angles, however. The Rochester B uses what some people have called a "snatch idle" scheme. That works ok, but only ok. If all stars are in alignment, the Rochester B can be ok. What I found were worn-out power piston bores, bores damaged by attempts to force power pistons out of their bores, knocked-off vacuum ports/damaged passages, throttle body ports rusted shut...all sort of stuff like that. And main jets that had been drilled larger by who knows who. That is something you have to check with each carburetor, I believe. But the bottom line is the Carter YF is a much better engineered carburetor. Even from a common sense standpoint it is better.

The comparison you are talking about happened back in the 50's and early 60's. It's part of history. Many of us were either kids or not born yet so we read about it today.

Turn back time to 1949. Carter W1 on a Chevrolet 216. Works well and still does today. Then in 1950 GM buys Rochester, gives Carter the boot out the door and Chevrolet begins installing the B/BC because they can save money. The Rochester works okay for a while until problems start showing up after the warranty has expired. Some people remember the old Carter reliability and want it back. Carter's new aftermarket/replacement YF has less trouble and also performs better (advanced metering technology), their mechanic recommends it so they swap out the Rochester. They like the YF. Not everybody does the swap (Chevrolet owners are typically cost conscious and new carburetors are $$$) but many do. Thanks to someone who purchased my YF many years ago, today I have an excellent carburetor on my 1955 engine. Cleaned it, checked for wear, put in a kit and installed. I also have 2 B's that I haven't been able to tame so they are sitting on the shelf.

If they're sitting on the shelf my experience has been they may not be leaking.

Jon G: Thanks ever so much for this update and getting all this information in one place. I may not use it but doubtless someone will and it will be invaluable data.

Manay thanks for recreating this information. I am going to have to print it out and save it the old fashioned way, in paper.

My pleasure. There is an experimental post at the top containing this and some other "tech tip" type files. Hopefully it will work.

Yes thank you Jon, very informative.

Thank you very much for taking the time to write this up. I just rebuilt a "B" so a lot of the information posted here is still fresh in my memory. I am sure I will need to refer back at some point...

We have a two-part Tech Tip that covers this thread and well as the How to straighten your Rochester B or BC thread. They are in a nice, neat PDF format, very suitable for printing out.

The Tech Tips cover:
Part 1 - Common Issues covers a lot of ground (12 pages - give it time to download ) with a helpful schematic on the last page.
Part 2 - Casting Fixes (five pages) looks specifically at repairing warped Air Horn and Fuel Bowl castings, with helpful details and images.

Both parts can be found HERE in the Engines Tech Tip forum. There is also a follow-up thread HERE for Q&As, comments and the like.

We are locking down this thread so the discussions can be held in the Engine Shop.