The Stovebolt Site Forums The Shop Area Engine Shop Rochestor 2 barrel on a 235

Guys I have a rochestor 2 barrel that came off a 70' 307 that I rebuilt for it but went a different route. Would this be a good candidate for a 235? I have a single weber/holly progressive I got from Langdons on there now and it starts great, drives good until I get to about 40-45 MPH then power flattens out. I'm thinking it's to restrictive on air flow. I have the original 4 speed with a 3.73 Dana 60 rear end and 29" tall tires and I can cruise 55 at about 2400 RPM but the last 10-15 MPH tapers off on power. Up until that point she's pretty snappy.

I Had the same carb on my 261, and it ran out of power quickly. i put a mono jet 1 barrel off of a 250 cu inch chevy six and it runs great with that carb. The linkage for the original carb works with this like it was made for it. The two barrel won't bolt on without some kind of adapter , and the linkage will probably be a real interesting afternoon of how am I going to make this work?

You confirmed that same problem I'm having, she dogs out at upper end. The adaptor for the holley/weber will also fit the 2 barrel I believe. I just picked up two rochestor B's with NAPA rebuild kits off of ebay for $82. I figure between the two I should be able to work up a good carb. Somewhere I have the stock linkage for it......

If this is the common "Chevrolet" 2G it has the smaller 3¼” X 1â…ž” SAE base pattern.
The problem is that it's synchronous - both barrels open together, and the larger total venturi area exposed when you roll the throttle open will give shaky response.

A second problem is the 2->1 adapter. Fabricating a two-barrel intake would eliminate the adapter issue and give somewhat more on the top, but see Panic's post above about low RPM response.

If you are looking for more performance, a Zenith from a slightly larger (270) GMC would also eliminate the adapter and give somewhat more; but remember, you still have a 235, not a 427.

Jon.

The two most common mistakes people make is putting too much camshaft and too much carburetion on too small of an engine. Any piston engine is simply an air pump, and a carburetor works by converting air flow through the venturi into vacuum to draw the fuel from the float bowl into the air stream. Opening up a bigger hole for the air to flow through (or two of them) slows down the velocity of the air stream, and you get less, not more fuel from a bigger carburetor. If you're planning to run full throttle at high RPM all the time, a bigger carb MIGHT be a good choice.

The big bore 2 barrel Rochester used on the 307 is FAR too big to function well with a 235. It's more suitable for engines like the Chevy 350 or the Pontiac 389, and when it's used on a 305 Chevy or a 307 Olds, etc., the float level must be set very high to avoid a lean stumble. If you want to use a Rochester 2 barrel on a 235, find one of the small bore carbs that were used on the 265 Chevy V8, fabricate a modified intake manifold that allows the carb to be bolted on without an adapter, set the float level as high as possible without flooding the engine at idle, and learn to feather-foot the gas pedal at a stop sign. Have a good fire extinguisher handy in case of a backfire!
Jerry

The 32/36 carbs are just a little too small to use as a single carb. It’s a shame because the progressive feature makes them attractive to use. But the Venturi is fixed and just not enough air flow when it’s most needed. Terrific in a dual carb set up, but just not enough for use as a single stand alone. Just curious here, but why not consider getting yourself a properly rebuilt stock carb?

I like Jerry's use of "a piston engine is an air pump".
A carburetor will not add one molecule of gasoline into an engine until it is sucked in by a piston going down while both valves are closed (it is an air pump after all).
That piston going down pulls a very specific amount of vacuum. The stock carburetor is designed to allow a very specific amount of air/fuel mixture to be sucked through it from that specific vacuum, resulting in optimum combustion.
Being armed with this information, how anyone would expect that adding a carburetor with specifications different than stock (larger or smaller CFM) on a stock engine would improve overall performance is beyond me (emphasis on overall).
Bigger CFM carburetion is for significantly modified engines.

By reducing pumping loss, and improving VE.

A variable jet is probably available for that Zenith. If one is, would there be any benefit from using it?

bztguy,

Here's a discussion of your topic on the chevytalk forum:

https://www.chevytalk.org/fusionbb/showtopic.php?tid/345309/

The 307 engines used the small 2G carb. It was also used on 265 & 283 engines. I think it would work good on a 235. The problem would be modifications needed to install it. You would need to find or build an adapter to mount it on the manifold & fab the throttle linkage.
The better choice would be a carb from a 261 or 292. Or the Zenith carb previous mentioned.
I had a 55 at one time with a 235 & a 261 carb & split manifold w/dual ex. You would think you were driving a v-8. That was before Stovebolt.com. I didn't know enough then to check but it may have had a 261 engine in it.
George

If you are going to use a two-barrel, what you don't want is a 2->1 adapter. Gasoline and air mixed do NOT like extra corners (think about an older river with curves.......where is the current???). We did quite a bit of testing 2->1 adapters years ago, with negative results.

Can it be made to work? Yes, it can. Internal combustion engines are remarkable flexible. You can even get one to run by having a 15-year-old stand on the running board and, with the hood off, pour gas into the engine out of a leaky boot! How well will it work, compared to an engineered system??? I would rather have the 15-year-old!

If you absolutely have to have a two-barrel, fabricate a new intake manifold with a plenum large enough to accomodate the air/fuel flow from the two-barrel DIRECTLY INTO THE PLENUM, with space around the outside of the air flow.

And you might check the second line in my signature

Jon.