The Stovebolt Site Forums The Shop Area Engine Shop Rochester B vs Rochester B

What are the upsides/downsides of using a 261 Rochester B (240 cfm) vs using a 235 Rochester B (200 cfm) on my '57 235 bored .030? I have both. 54 3100 that likes 40/45 mph on our back roads. [img]https://www.stovebolt.com/ubbthread...umber/52120/filename/20221230_083548.jpg[/img]

My question: why did Chevrolet use 2 different carburetors?

A 235 ci engine requires 139 cfm at 2400 rpm to run at top efficiency ( carb wide open ). The smaller carb might give better throttle response due to higher air velocity, but at higher rpm could restrict some cfm. I would use the one that doesn't leak, but if you can make both run with little to no money, try them both and report back with your finding.
There are a lot of charts and calculators for finding cfm of engines, yours will have an efficiency in low 70% range if that due to the terrible cylinder head port design. The cfm result is maximum amount of volume for a given rpm, normal driving requires much less.

Never mind

My take away from your post is that you need to shoot for stretching the legs out on that '54 3100.
40-45 MPH isn't going to keep the carbon burnt out of combustion chambers.
Live a little, man! Your engine will thank you.
BTW, I would use the carburetor designed for the engine. Save that 261 carburetor for a 261 replacement for the 235 that you are going to blow up by following my advice above.

Here's the tale... the 54 3100 has been on the farm since my dad bought it new. The 57 6400 same thing. I tried both carbs on the 54 and the 261 carb needed a freshening up. Soooo... I took them both apart, got my box of B parts and put all the best parts in the 54 carb. Then I installed all the crappy parts in the 261 carb and sent it to Florida for some R&R. After its vacation it came home good as new with all new parts. It was then when I had both side by side that I noticed one was larger than the other. I had sent the 261 to be rebuilt.
Going up the steep hill from the river bottom on SR 40 is a good 70 ft rise in elevation in 1/4 mile. At 55 the 54 carb stays at 55mph. The 261 carb will have me going 60 at the top.
Like my old buddy Vance said... "you ain't never got too much power."
By the way, Carl... I'm 71 and anyone who knows me and sees me doing 55/60 on these back roads would think something's wrong and I'm headed to the hospital! Oh yeah, don't worry, I still have the 261 from the junk 57 6400. May need it one day!

Did your carburetor vacation in New Smyrna Beach, by any chance?

Don't know... my NAPA guy sends carbs to him for overhaul. He is a one man show that sends them back better than new.
The only Smyrna I know of is in Down East Carteret Co. NC just inside the Outer Banks below Pamlico Sound.

Jethro. So you feel that the 261 carb gives you more power than the stock 235 carb? That's of interest to me as I have a 261 carb on my 235. It did seem to be running lean - if I partially closed the choke it ran smoother. There was a 57 jet in the carb and the specs said it should be a 64. I reamed the jet out a bit and it does seem better. Do you know what jet is in your carb?


(don't know how the 57 jet was installed. In the old days, some carb kits came with new a jet. Maybe a carb rebuilder in the '60s installed a 57 from a kit)

It was built to factory specs, so he probably put a 64 in it. I dared not open it. Didn't even have to readjust it. Should have more power... factory rated 40 more cfm than the 235.

Factory rated CFM? Never heard of it.

200 vs 240 rated. All carburetors have a CFM rating. The larger the number, the sooner you stop at the gas pump.

I would like to see the source for the CFM rating for a Rochester B.

There is a CFM chart in Doug Roe's book which does include the Monojet, but there are no Rochester B references in the chart.

As far as the feel of more power, I would sooner believe this is true because of "Soooo... I took them both apart, got my box of B parts and put all the best parts in the 54 carb." than the different carburetor.

Even seemingly "better" parts that are improperly matched will generally run worse than standard parts.

Jon

I think Jethro was asking the question because he bored over his engine and thought the greater displacement may require a greater amount of fuel.

I don't think .030 over is enough of an increase in displacement to warrant a major increase in carburetion. Just my opinion.

Just about any carburetor will flow more air than its published CFM rating- - - - -it just won't do it at the manifold vacuum where the rating was calculated. The "Street Stock" racing class I built engines for at the Nashville fairgrounds 5/8 mile track required us to run a "350 CFM" Holley 2 barrel. A 358 cubic inch small block Chevy engine would run 6100 RPM on that carb before it ran out of breath- - - - -which calculates to 630-something CFM. The manifold vacuum would rise until the engine simply couldn't suck any more air through the carb, no matter what we did to cam, valves, or porting and polishing, etc.

The trick to running faster was to put 1/16" flat washers between the carb and the intake, and draw more air into the manifold without going through the carb venturi. The engine wouldn't idle, but it ran like a scalded cat!

A 305 engine with the same carb and some Sneaky Pete tricks on a HEI distributor would turn close to 7K RPM.
Jerry

OK...I will simplify what I originally wanted to know.
I mistakenly sent the wrong core to be rebuilt. I now have X amount invested in a like-new 261 carb. Is it OK to use it on my 235. The answer seems to be "maybe, maybe not, probably not."

Someone would probably pay a hefty price for the proper 261 Rochester.

Rebuild the 235 Rochester yourself, for just a few dollars, and run it. Or better yet, use the proceeds from the 261 Rochester sale to purchase a non-Rochester carb than will run much better.

If I were you, I'd install it on the truck and go for spin. Drive around at 30-40 and see how it goes. Jack-rabbit launch at the stop light and see if it stumbles. Floorboard it on the highway and see if it stays steady.

If the drive doesn't result in any intolerable behavior, leave it on. If it does, rebuild the other carb.

Back when I was 16 years old and knew everything, I replaced the 2 barrel carburetor on my 47 Ford coupe with a 239 cubic inch flathead V8 with a carb for a 272 engine from a 55 Ford OHV V8. It was a bolt-on swap since the 3-bolt footprint of the carbs was identical. The difference was in the venturi size- - - -the 239 carb had a 15/16" venturi, and the 272 carb had a 1 1/16" venturi. The result was a disaster- - - -there was a nasty flat spot at about half throttle that simply wouldn't go away unless I eased into the gas pedal like I had an egg between my foot and the pedal. Opening the throttle up quickly would result in a total refusal to accelerate, and usually a series of bad lean backfires. Gas mileage dropped by about 3 MPG. A week of that was enough for me to go back to the right carburetor.
Jerry

In the "For What Its Worth" category:

Over the last 5 decades we have sold thousands of carburetors.

The "rule of thumb" we use when suggesting a different from stock ORIGINAL EQUIPMENT carburetor is that the "donor" engine should be within three (3) percent displacement of the "donee" engine.

So three percent of 261 is 7.83 (round it to 8.0 for ease of calculations).

Thus, by using our rule, a real carburetor calibrated for a 261 could probably be successfully used on engines with similar RPM requirements from 253 to 269 CID.

Whether a Rochester B would work on any of the above is subject to discussion

Aftermarket carburetors are deliberately calibrated too rich, so the above rule of thumb does not apply (well, it does, but the percentage increases slightly).

Jon

Actually, a 235 can be bored out to accept a 302 piston- - - - -as long as you're not too picky about having things like cylinder walls. Any good legal beagle knows to answer the question asked truthfully, without embellishment, even if the answer is patently ridiculous. Stupid questions deserve equally stupid answers. My brother spent several decades as a very accomplished criminal defense attorney!
Jerry

Well, I sold the 261 carb to a fellow Bolter today for the cost of the rebuild. Will wait for the fuel pump kit from Then and Now and see how it acts before I get a proper kit for the 235 carb. Will be seeking a 2100S core. Thanks for all the input sans the pot belly stove country store comments. Jethro.