The Stovebolt Site Forums The Shop Area Engine Shop Carburetor jetting on a 235 with dual Rochester carb setup

Hi,
I got a Offenhauser intake for a dual carb setup.
Now I would like to install two Rochester 1bbl carbs. Is it necessary to re jet the carbs ?
Cause I guess with the original setting of the carbs the engine would run to rich.
Please advise.

Cheers,
Mario

Mario - build the carbs perfectly stock to begin. You now have a repeatable "baseline", and tune from there.

With the Rochesters, make sure that your power circuit is operating normally. Often, with the Rochesters, the power circuit is always open due to warpage causing an internal vacuum leak. If the power circuit is always open, then the jetting will be too rich.

Normally, if everything is working normally, one has to install larger jets due to the reduced air velocity because of the two carbs rather than one.

Jon.

Generally speaking, two too-big carbs make an engine run lean, not rich. The air velocity through the venturis will be so slow that very little fuel gets mixed with lots of air. Low speed and midrange performance will be nonexistent, with lots of stumbles and backfires, so keep a fire extinguisher handy. You're beating a dead horse, trying to make the engine work with twice the carburetion it was designed to use, but it's probably going to be necessary for you to learn the hard way.
Jerry

I'm running two 2 stage Holley-Webers from Langdon's Stovebolt on a 261 ran great out of the box, but did have to increase jet size a bit. Now he stocks Webers and Carter-Webers. Love using new vs. old worn out whatever brand.
Best of luck

http://www.langdonsstovebolt.com/

What did GM do on the early Corvettes with the two Rochesters?

The Corvettes ran three side draft Carter carbs, factory-calibrated to work with the 235 engine. Each individual carburetor had way less CFM volume than the single downdraft Rochester used on the cars and trucks. There's nothing wrong with multiple carburetion as long as it's engineered to work by the factory, not an add-on by someone who thinks he's smarter than the guys who designed the engine.
Jerry

Jerry,
thanks for that information on what they did with the Vette. I haven't seen one yet but I hope to some day.

I have seen a few dual Rochester setups at shows where looking closely it appeared that one wasn't actually in use!

Jerry, actually, the Carter YH carbs each had about the same CFM as the single Rochester used on the 235 passenger, but the YH had better boosters to increase the low RPM air velocity.

And while I know you know this, for the benefit of others, the three YH carbs used on the Corvette used solid linkage, NOT progressive.

Virtually ALL factory multiple carb units used solid linkage on inline engines. The Corvette, and Hudson in this country, and a host of Jaguars, MG's, Triumphs, Volvo's, etc. used solid linkage. The carbs were sized and calibrated for the engine. The engineers didn't just throw on what they had under the workbench!

The one exception I can readily think of which was progressive linkage on an inline was the 1941-1942 Buick Compound Carburetion on a straight 8. Two two-barrels were used, with the front carb being the primary, and the rear carb being the secondary.

Jon.

Thaks for the straight scoop, Jon! I've worked on a few of those progressive-linkage Buicks- - - -they're a royal pain in the tailfeathers to synchronize properly.

If somebody wants 2 or 3 carbs for visual effect, no problem- - - -just put a solid blocker plate under the extra carbs and run on one! A fancy multi-carb system on a Stovebolt is mostly eye candy.
Jerry

Here is a picture of the Corvette manifold and side draft carbs.

https://lh3.googleusercontent.com/-...KVrQq0/w1118-h629-no/20140225_144936.jpg

Here's a similar application of the Carter side draft- - - -on a Nash!

http://bringatrailer.com/wp-content/uploads/2008/02/1953_Nash_Healey_Roadster_Engine_1.jpg

1953 Nash-Healey roadster. It was also used on some models of the Nash Ambassador inline six.
Jerry

Note the Nash used automatic chokes, and the Corvette used manual chokes.

The zinc alloy choke brackets shown in Don's picture are now made from unobtainium, or at the least, verypricium!

For those interested, there are four distinctly different generations of the Carter YH. To those not familiar with the carbs, they pretty much all look the same (other than the manual chokes on the Corvette).

For years, the only repair kit available was one from Standard Hygrade, which had only 2 of the four bowl cover gaskets (each generation had its own bowl gasket), and the incorrect fuel valve. Since the kit was listed for all, it just had to be the right kit

Because of the use of the incorrect parts, the YH got a bad reputation it certainly did not/does not deserve. How would a truck with 15 inch wheels perform with 16 inch tires?

Jon.

And here is another application, an aftermarket supercharger produced by Latham that used four of the YH carbs:

Well, that didn't work. Do a search on Latham supercharger images, and you can see the four carb unit.

Jon.

Don,
Porn is not allowed on this site! Where is the moderator?!

Thx for the many opinions and news I got from you guys. Makes me think how to progress now. Maybe I have to digg a bit deeper in that issue and have to test and evaluate whats best for a daily driver with a solid engine setup.

I built one with a pair of zeniths on a tattersfield that runs like a swiss watch, lots of power.
The carbs are solid linkage, both have idle circuits and the truck has 18" vacuum at idle.
each carb is jetted as if it were the only carb, then the fuel curve is correct as it supplies fuel to the amount of air going thru that carb.
I have the tube style zeniths with the smaller venturi, I tried the larger venturi and had the bog from hell.
The setup works so well a choke isn't needed to start, even in winter, you do need to open the throttle for high idle until the engine warms and I have water running thru the intake.
I tried progressive linkage and the engine leaned out on the front cylinders, the intake didn't share the fuel mixture at all.

The two zienith carbs is the path I was headed down...have an early offenhauser intake with the smaller carb base mounting pads... did you use carbs off a 228, 248, 270? Any special setup other than solid rebuild? any re-jetting?

I was going to post a picture but don't see how.
I don't know what the zeniths were originally used on, I made 2 good ones from several cores, the 2 carbs are slightly different and have different setups. I believe the intake required the different setups more than carburetor differences.
If you want specifics and pic shoot me an email:
ojh4@verizon.net