The Stovebolt Site Forums The Shop Area Engine Shop Rochester B series study, overlook, etc

I mentioned a couple of months ago I was going to spend time studying these. Prior to that for a few weeks it seemed like a lot of mud had been slung at them around here, some pretty viciously. Right or wrong, I'm going to share what I've gathered.

I think it may be interesting and hopefully instructive to review the Rochester B series before trying to solve any problems. Thousands of these are still on stovebolt engines after all, and things which might be considered basic or even unimportant can have notable effects. I got 5 of these to study so as to have what I figured would be a decent field. Before we begin I’ll tell you the “model” for the images was a Rochester B which probably should be considered a parts carb. Along the years this poor thing suffered a great many atrocities. When I got it, it was covered in grease and mud and the air horn was held to the bowl at 1 of the 4 screw holes by what looked like a paper clip straightened and twisted with pliers. Another screw hole was occupied by a nail which had been bent (possibly with a rock) into an L shape after it was inserted. There is an o-ring used to seal the power valve where a prior artisan managed to strip the threads. But it is fine for visuals and I was surprised at how nice it looks after cleaning. If I had a good throttle body and float bowl mast with a good power valve and power piston, it could be saved. First a view of some parts of the air horn:

http://i1154.photobucket.com/albums/p535/JonGoodman/Air%20horn%20parts.jpg

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:

http://i1154.photobucket.com/albums/p535/JonGoodman/Float%20bowl%20parts.jpg

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:

http://i1154.photobucket.com/albums/p535/JonGoodman/Air%20bleed%20holes.jpg

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 fuel level = richer. Lower level = leaner.

2. In the B series, the idle scheme is simple and given the right circumstances will work well. 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 jet, the level of fuel in the bowl and the idle adj. needle. All the air bleeds and idle tubes and everything else 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:

http://i1154.photobucket.com/albums/p535/JonGoodman/Throttle%20body%20ports.jpg

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:

http://i1154.photobucket.com/albums/p535/JonGoodman/6%20no%20vacuum%20wide%20open%202.jpg

And by contrast it would look like this in the "up" position (away from the power valve):

http://i1154.photobucket.com/albums/p535/JonGoodman/power%20piston%20up.jpg

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 solely for illustration purposes and also because when I got it, it looked like somebody had been using it as a toothpick. Possibly the bent nail person. Here are the two types:

http://i1154.photobucket.com/albums/p535/JonGoodman/Power%20piston%20tip%20types.jpg

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:

http://i1154.photobucket.com/albums/p535/JonGoodman/power%20restrictor.jpg

We’ll talk more about that "restriction" in the next post and we'll discover something that might be interesting to you.

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.

more to follow...

Jon,

I’m sure I speak for all of us here on Stovebolt.com when I say a huge thank you for taking an enormous amount of time to share with us your observations and thoughts on the Roachester B carburetor. After 70 years that carburetor is still very popular, and found on many of our old trucks. Based on the number of questions I see posted in the forums it’s obvious that many of us have problems with them, and need help understanding them so we can make them perform better. You are a living encyclopedia of information about carburetors and a valued member of this site. Thanks again for always being willing to answer our questions and for sharing what you know!

Chuck

Lots of very good information. Probably won’t sway many minds but at least the Rochester will be better understood. Thanks for doing the leg work. Hope you are mentally prepared for the naysayers! 🛠

Hi Martin, Chuck,
Thanks! But please understand I'm standing on several other shoulders. Jon the CarbKing, Jerry the HotRod, others not known to this forum. Those folks have to be credited for the help they've given (and I've asked several questions). Later today I'll be sharing what I've learned about straightening these. I suspect that's what most people are going to be interested in most.

This is great useful info for me. I have managed to keep my truck running with a "B" for almost 40 years.
I have recently put a carter yf on my 292, i like the way it runs, but plan to return to the rochester.

these write ups will be very helpful. thanks for doing them.

Yes, Carbking is “King” when it comes to carbs, so I’m curious what his remarks will be regarding your posts. Hotrod Lincoln Jerry is a master at these old engines and I eagerly read his posts. Just was thanking you for taking time to share your findings, observations, and expertise with the rest of us. It’s appreciated.

Chuck

I just posted the instructions on straightening them. Now we get to the fun part...

There is absolutely nothing wrong with a "good" model B. I ran one for years on my '52 216. I even let the truck sit in the driveway for 15 years without ever starting it. One day I got fired up to do something with that rust heap, so I put five gallons in the tank and as soon as the gas got to the carb, it started right up and purred like a kitten just like it did 15 years prior.
Lets see you do that with a Carter YF. The accelerator pump diaphragm will not respond. The Model B has a leather pump and just needs to be wet to work.
Having said that about the Carter, I find that they perform better than a "good" model B. No dead spots upon acceleration is one thing that I notice.
Carl

Rochesters are good; Carters are better, Zenith is probably the top of the heap if you can find the right one and your bank account doesn't have a coronary! Spending my adolescent and teenage years around a Carter authorized service center might have influenced my opinion just the tiniest bit, though!
Jerry

Chuck - I think Jon G. is offering a lot of useful information to those who like the Rochester B. I have posted for years about straightening the castings. Most of the other information Jon is presenting is available in various locations, but most of those locations are not READILY available to everyone. There is some information in the first edition of "Rochester Carburetors" by Doug Roe and Bill Fisher. More information can be found in the Rochester "9X" manuals, and still more was available in the Rochester bulletins. Up to about 1958, Rochester published specifications such as what Jon G. is providing for all carbs, but these bulletins are quite rare.

So if some one is truly in love with the Rochester B, it CAN be made to work (albeit NEVER as well as the Carter) for awhile, and then one gets to redo it, and redo it, and............

If I personally owned a vehicle originally equipped with a Rochester model B THAT WAS A TRAILERED SHOW CAR THAT I SHOWED IN NATIONAL SHOWS, then every 3 to 5 years, I would go through the Rochester B. I did build several of these some 35~40 years ago for the show car crowd, but found that in the same amount of time to PROPERLY do the B, I could restore TEN Rochester Q-Jets. So, as a professional owning a shop, how should I price the rebuild of the Rochester B??? And I could NOT offer the normal 5-year warranty that we offered with carbs we did, when we were still doing carbs.

(Opinion) For any other use than show, there are better (and much more reliable) carburetors available for the 216/235/261 and other Chevrolet six's.

Just thinking about performance, if I were to assign a number from 1 to 100 for carburetors commonly mentioned one these forums that are in factory condition, it would be as follows:

Stromberg B series - 98
Zenith 28/228 series - 97
Carter YF series - 95
Rochester B series - 85

After 5 years of normal driving and maintenance:

Stromberg B series -98
Zenith 28/228 series - 97
Carter YF series (using ethanol) - 85
Carter YF series (using non-ethanol) - 95
Rochester B series - 60

As a point of reference, the two Carter AFB's on my shop truck have been untouched for 18 years. As one ages, one finds one's most valuable asset is time. The need to periodically redo simply does not fit my agenda at this time of my life.

My definition of performance would consider drivability, power, and fuel economy. Reliability is a different consideration, and I refer to that with the Carter YF if using ethanol.

Summing - for a show car, or for someone that just enjoys redoing the mechanicals periodically (there are those that do enjoy this), or someone who enjoys doing research such as Jon G., then use the Rochester. But better units exist for everyone else.

And a HUGE thanks to Jon G. for providing the information to everyone.

Jon.

This thread started by Jon G has been turned into a Tech Tip. Today (11/17/23) we posted it as part of two on the Rochester B Carb that Jon G put together with collaboration from carbking and others. It's in a PDF format so it's easy to print out, with all the images embedded in the text. The B-series overlook and issues is the subject of Part 1 and includes some basic operation of a Rochester B series carb.

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.

HERE IS THAT LINK for both, found in the Engine Tech Tip forum.


It's one of many that Jon and others have provided for the Stovebolt Collective and we thank them for that!

We are going to lock down this thread and we will look for any feedback over in this Engine Shop thread. Thanks. (Plus, this thread is really old! )