The Stovebolt Site Forums The Shop Area Engine Shop Dual Carburetors?

I've been considering going to a two carburetor setup. I'm currently running a single one barrel Carter YF 2100S on a 235. I am curious if there is a significant performance enhancement with the 2 carb setup (Carter 1 barrels) if done properly with two exactly the same carburetors and settings. I'm also wondering if there is a major drain on gas mileage with the addition of the second carburetor. Just interested in what people think about this. Is it worth the cost of changing manifolds etc. Not to mention any potential tuning headaches.

Thanks.
Mark

The engine was designed to run on one carburetor and millions of them worked just fine that way. Why mess with success? Unless you do a whole bunch of other modifications, a dual carb setup is about as useful as a windshield wiper on a duck's butt. Eye candy for car show gawkers who don't know any better!
Jerry

Well I just the love the as useful as a windshield wiper on a ducks butt comment.

For me I love the look of the dual carb and I will set on the fence on this one. I will let the more experienced guys debate it since I do not have a baseline or a before and after test, so for me silence is golden.

Carb Article

Mark - this issue has been debated many times, and will likely be debated many more times.

Since I do this for a living, I will offer the opinion I have offered many times in the past.

One adds additional carburetion for functionality because:

(A) one modified the internals of the engine
(B) the original carburetor is too small
(C) to increase the volumetric efficiency of the engine

So:

(A) you didn't
(B) it isn't (and your Carter 2100s is an excellent replacement)
(C) with a perfectly stock stovebolt 6 you MIGHT gain 6~8 percent efficiency improvement because of better cylinder fill IF YOU DO EVERYTHING PERFECTLY! (proper carbs, proper tuning, proper synchronization).

Your term was "significant". If you consider 6~8 percent significant, then go for it.

Be advised if you do a less than perfect job it is possible to LOSE performance, perhaps as much as 20~25 percent.

Or as Jerry so tactfully stated, basically eye candy.

Jon.

I will forever quote Jerry's windshield wiper line.
Agreed, one carb as is on that darn nice truck of yours.

If you have to ask, then you may not be ready for two carbs. I run them because I like the look, the smoother running, and fuel mileage. They are expensive if you buy rebuilds, intake, and linkage. I bought cores and rebuilt them myself, made my own intake, linkage, fuel lines, and air filter. It was still expensive but worth it to me.

Tuning is not that hard, you may have to pull them off a couple times for metering changes, and there is some work to get the linkage set right. But daily tuning is no ore then a single carb requires.

The advantage I see is how the fuel is delivered equally to the cylinders, each carb is now feeding three cylinders instead of six. My engine is much smoother idling and smoother running down the road. I can tune for much higher air to fuel ratio's then with a single carb, I have a gauge just for this.
Best of all, after driving, I can shut off the truck and not smell any fuel in the garage.

Joe

I think multiple carbs and bigg azz tires are anything but practical. They are both more expensive and a giant PIA. The trade off is for the WOW Factor and the attention you get. It can really make you stand out in a crowd. So if you want a little more attention and are willing to put up with a little more work and maintenance...I say go for it !

http://1drv.ms/1vsbv3F

Purely hypothetical question on the "best" way to improve carburetion when desired....

I've read a lot of threads on this topic and trust those who say the dual carb thing is largely cosmetic under most circumstances. That said, I also acknowledge that very little about this truck hobby is logical...and that "eye candy" might be a good enough reason to do it, for some. Dual carbs on an inline 6 is a cool look.

Setting both perspectives aside for a second, let's say that you meet carbking's 3 criteria for additional carburetion. And you want a blend of driveability and performance....a little more "go" when you step into it.

Which would be a better approach to achieve that purpose?

1) Dual carbs properly sized and tuned with a progressive setup.
2) The small 4 barrel carb/ intake. (Clifford with a small Holley, I think).
3) Something else?


My uneducated guess is the 4 barrel. A little less monkeying with tuning and such. Although I think Clifford even says it won't work with a stock exhaust manifold or Fentons without "modification."

I know this isn't a completely fair question. Too many other variables (cam specs, head, gearing, etc, etc). Just looking for perspective.

There is a wide range of opinions on this one. I'm going to have to remember the windshield wipers on a duck's butt analogy. A lot of people like the dual carb setup. It does look cool. There are pros and cons and to each his own.

Joe mentioned fuel delivery from each carb to 3 cylinders instead of one carb feeding 6 cylinders. But Carb King's response that all you would gain would be a 6% to 8% efficiency improvement which hinges on tuning and synchronization done to perfection kind of shoots it down for me from a cost verses benefit standpoint. I think I'm happy with what I have now. Once I get into 4th gear I can cruise fairly well and keep up with traffic. I'll probably just leave it alone.

Mark - the key to needing increased carburetion is in the modification of the engine.

For a moment, completely forget about carburetion (and ignition). Now what you have is an air pump; pure and simple, an air pump. Now paraphrasing the old schoolchild joke of how much wood can a woodchuck chuck into how much air can an airpump pump?

Well, this is a question with a answer.

CFM (at 100 percent volumetric efficiency) = (RPM times CID) / 3456.

Understand the above, and you won't have to remember.

You are dealing with cubic FEET per minute on the left; and cubic INCHES on the right. Thus to convert the cubic inches to cubic feet, you divide by (12 times 12 times 12) or 1728. BUT WAIT, as the TV commercial screams; with a 4 stroke engine, you only draw air on every other crankshaft rotation. So divide by 2. Multiplying the 1728 and the 2 on the right gives you 3456.

Note the equation is listed at 100 percent volumetric efficiency.

A good AVERAGE FOR STREET ENGINES:

Modern (1960 and newer) USA 6 cylinders maybe 80 percent
Older (stovebolt) 6's maybe 70~75 percent
Modern V-8 (Chevy 350) maybe 80~85 percent
Hi-performance V-8 (Pontiac Ram Air IV) maybe 90 percent

Now consider why one NEEDS more carburetion:

(A) the engine is modified (allowing a higher RPM)
(B) the original carb is too small (think taxi, very few others)
(C) eye candy

JW51, you asked about different methods of increasing carburetion if one needs it. Your suggestions:

(A) dual carbs with progressive linkage NO, NO, NO!!!
(B) small 4-barrel (most would choose this option)
(C) something else - OK suggestions:
(C-1) dual single barrel carbs with solid linkage
(C-2) triple single barrel carbs with solid linkage

As Joe H. suggested, multiple carbs on an inline 6 (properly done) even the fill ratio/cylinder making for smoother performance. This is the area of gain on a stock engine; but also applies on a modified engine. The multiple carbs somewhat improve the volumetric efficiency.

Jerry - which breed of duck???

Jon.

I must have confused you with the Langdon article. Can you explain the NO, NO, NO?

Progressive linkage on dual carbs on an inline engine hurts volumetric efficiency (except at WOT) rather than improving V.E.

Think about it. How many Jaguar and Austin Healey 6's with dual (or triple) carbs have you seen with progressive linkage? And the Chevrolet Corvette used solid linkage on the triple Carters in 1953 and 1954.

And while there can be some argument in favor of progressive linkage on a triple 1-barrel set-up on a six; we have had MUCH more positive results with solid linkage even on the triples.

Jon.

Gotcha. Still trying to figure all this out.

And I may have misread the Langdon thing, too. The progressivity (if that's a word) he's talking about is in the 2 barrel carbs, not the linkage.

I grew up around a shop in the 1950's where we specialized in somewhat off the wall projects. My father was the first man in Nashville Tennessee to work on automatic transmissions, he converted V-12 Lincoln Zephyr distributors to run on his own flathead Ford V8 race car, and built those distributors for his competitors, and he was the best independent shop carburetor specialist in the southeastern US. A GM field engineer made Dad's shop an unofficial research facility when they were trying to make the 1953 Rochester 4 barrel work, and some of the modifications he and dad figured out were incorporated into the 1954 production carbs. It was an interesting place for a curious kid with a burning interest in everything automotive to hang out.

Some pretty notable factory progressive linkage attempts were the Pontiac Tri-Power, the Oldsmobile J-2, the Buick straight 8 with two carbs, the Hudson Twin-H, and the Corvair Spyder with 4 carbs, one on each side that opened quickly near full throttle. All of these setups had one thing in common- - - - -they sucked like a Hoover upright vacuum cleaner!

If an engine has more than one carb, they need to be synchronized and completely matched for airflow and metering.
Jerry

Jerry, I had a '47 Buick 8 with the factory dual carbs - loved it for the highway, but it sure did Hoover up the gas!

Bill

Back to the progressive linkage, running on just one carb would be fine if it was centered on the manifold, but since dual carbs are at each end, can you picture the far cylinders being feed equal amounts of fuel as the ones right near the open carburetor? Thats the first draw back, the second is carb size. If its big enough to run on one, then adding a second will be to much even if it is at full throttle only.

A progressive carb is totally different then progressive linkage, don't confuse the two. A two or four barrel progressive carb is any with the second half opening at a later time using linkage or vacuum.

The little Pinto carbs being sold for duel setups work so good because of there size. One is not nearly big enough but two are just right, add the secondary at high rpm and you are slightly over sized but still with in the range of the engine.

Joe

Red, those Buicks did a good job of guzzling gas with one carb- - - -I'm sure another one would just add to the problem. Straight-8 engines in general were gas hogs, Pontiac, Buick, Hudson, Oldsmobile, Packard, etc. I worked on a mid-1930's Buick 8 several years ago that was almost identical to the engines they used into the early 1950's.
Jerry

yes, the 'fireball 8' was what was stock in the Flxible bus I have, used from the late 30s up to the 50's, when they started with diesels, fair HP and torque for back then, not sure if the dual carbs really added much tho

Bill

Im running the dual Weber / Carter setup on my 235. I had big issue with the first 2 but now that they have been replaced the truck runs 1000 times better. I still do feel they are drowning the engine when you mash it. The secondaries possibly could be slightly toned down i think to help with that. Now that i have a bad set of carbs from the first go around i might try to take the secondary progressive linkage and play around and mod it to see if i can change the opening ratio. I will however only do that after early next season as i will first install a new camshaft with a different grind next spring. Possibly the too much fuel issue now might not be an issue with a more aggressive cam installed.

Nope- - - -you're going lean, not rich. Opening up too much venturi area slows down the air velocity so much the vacuum in the venturi drops, and it can't develop enough vacuum to draw sufficient fuel into the airstream. Instead of messing around with the rate of opening, restrict the opening of the secondary barrels to about 1/2-2/3 of their total travel. That will moderate the flat spot somewhat by preventing the throttle plates from opening fully. If you could rig a HEAVY return spring to come into play at that point then it would make you think about giving it any more gas until the RPM comes up. It's not a substitute for over-carbureting the engine, but it sort of keeps you from giving the engine more throttle opening than it needs. The spring-loaded air valve on a Quadrajet is a good example of an idiot-proof carburetor. No matter how far the throttle plates are opened, the air valve only allows the right amount of air to enter the secondary barrels.
Jerry

How much carburetor area is too much (viz., no more improvements)?
When the WOT vacuum is just below the rated vacuum: 1.5" of Hg for 4 bbl. carbs and 3.0" for 1 & 2 bbl. carbs.
If the carb is too big (or engine too small) the carburetor will not deliver its full rated CFM.
The reverse is also true: if the carb is too small (or engine too big) the carburetor will deliver more than its rated CFM.
What else to watch for: the fuel curve (air bleed, emulsion tube etc.) for almost all carbs is based on it operating in its designed WOT vacuum range. If you add too much 1 or 2 bbl. carbs so that the WOT vacuum falls below their rated 3.0" the air correction will be out of range and run lean at high RPM. The reverse (Holley 390 on a 454) will run rich.

[quote=Hotrod Lincoln
The spring-loaded air valve on a Quadrajet is a good example of an idiot-proof carburetor. No matter how far the throttle plates are opened, the air valve only allows the right amount of air to enter the secondary barrels.
Jerry
[/quote]

Jerry - the Q-Jet is one of my favorite carbs, but it isn't idiot-proof; the idiots will adjust the airvalve to open sooner than it should!

The Carter AFB with its weighted airvalve is more idiot-proof; but the determined idiots will grind mass (weight) from the airvalve so it opens too soon!

Jon.

Agreed- - - - -when we idiot-proof something, the idiots just get more creative! I remember people putting a screw and nut into the return linkage of a vacuum-secondary Holley 4-barrel to force the secondaries open too soon. That resulted in a huge lean flat spot and horrendous backfires most of the time!
Jerry

Another question for carbking, since this thread has already gone in all sorts of directions:

It is my crude recollection that you're not a big fan of the 2 barrel Carter-Weber setup Langdon sells for 2 carb setups. (not trying to put words in your mouth...just my memory here).

If that memory is correct, what is your specific criticism of that carb?

Purely academic question. I have no intent of going 2 X 2 on my truck. I know many folks here have that setup and like it, but also value your knowledge and opinion.

JW51 - I have nothing against the carburetors at all; it is the lack of manifolds.

We did extensive testing of the Holley-Weber, Aisin, and Hitachi progressive 2-barrels back in the '70's and 80's.

Two reasons (both CHEAP):

At that time used Toyota and Datsun (Aisin and Hitachi) carbs were basically scrap metal prices (how times have changed)! And when Holley blew out the various progessive two-barrels, I could have bought them by the 1000's new for less than $10. each!

These carbs were designed for high-reving small displacement engines hence the progressive design. Some of them were bolted to manifolds with split runners. All were bolted to manifolds designed for two-barrel carbs.

Air/fuel mixtures do NOT like corners!

And it doesn't matter if you are putting a progressive two-barrel on a single barrel manifold, or a square-bore Holley on a Q-Jet manifold (both requiring adapters); the efficiency of the setup is significantly compromised.

A carburetor works because of air velocity in the venturi area. The use of the adapter messes with the air velocity.

Still another issue for those wanting performance is the diameter of the opening in the manifold. For those of you that happen to be NASCAR fans, think restrictor plate. You can bolt a Holley dominator (would like to see that adapter ) on a single barrel manifold and never approach the performance of the stock carburetor (assuming the stock carburetor was the correct size for the engine).

But if one likes the Carter-Webers or the Holley-Webers, by all means use them. Personally, I define increased performance as both more power AND better economy. Tough to do using adapters.

Maybe if enough enthusiasts get together, they could convince edelbrock or Offenhauser to cast a dual manifold but using Pinto plenums and runners.

Personally, as long as there is a reasonable supply of used Carter W-1 carbs, I will be happy.

Jon.

Ah....it's the adapters.

So, given all that....do you have a favorite 2 X 1 manifold? Does one brand screw less with the air velocity?

Wallaceracing.com has awesome calcs that will tell you pretty accurately what your flow numbers are based off cfm, ci, bore, stroke etc.... You need quite a bit of info to get really accurate but it does work and can you tell you a lot. Volumetric efficiency is what you're after I think. Get some base measurements and figures of your engine and plug and play with carb cfm and manifold cfm based on what's available for your project.

Wallace Racing is an awesome resource for the hobby (and I own a GTO)

However, one thing many forget is that race parts don't necessarily work well on the street. MUCH easier to modify a carburetor for racing than for street use!

JW - I do not have a favorite 2 x 1 manifold for the stovebolt. Have helped customers with 2 x 1 setups for lots of different intakes; so far (knock on wood) no complaints.

On V-8 engines, have had a lot of issues with manifolds beginning with an e. Personally will use Offenhauser for my own engines if Offy makes a manifold for the application. But if you like a manifold made by someone else, by all means use it. If there were one perfect solution all of us would be using it; and life would be much more boring.

One other tidbid: while an adjustable main metering jet is a wonderful thing for a SINGLE carb installation; I do NOT recommend using them in a multiple carb installation (unless the customer has ready access to a dyno).

Jon.

Well not to beat a dead horse however I really want to thank you guys, that was really a comprehensive carb 101 lesion for me on the old Stovebolt engines.

I really don't like when someone tells their opinion with out explaining why. I have always heard about the "eye candy" term but never understood why. Carbking did a great job explaining why and not just say what other people like to do argue and say just because I said so. The rest of you guys with the add on and tidbits explanations it really helps. Bravo to all.

I will agree with Hotrod Lincoln with his post "The engine was designed to run on one carburetor and millions of them worked just fine that way. Why mess with success? Unless you do a whole bunch of other modifications, a dual carb setup is about as useful as a windshield wiper on a duck's butt. Eye candy for car show gawkers who don't know any better!" The eye candy part is true because the first 235 I saw is when I bought my truck the old boy had a 56 Chevy car with a 235 in and the motor was paint to match the body all chromed out and it really was a beautiful sight. Right then I knew I wanted to do the same.

I read Tom Langdon's article and really trusted his experience after talking to him but then I read so much negativity about the dual carbs. Now I know why.

So if I am understanding this correctly, the set up I purchased from Tom is the dual carter webers which the carbs are progressive part and the linkages are not progressive. So on both carbs the one bbl opens as your diving normally, then as you pour the coals to it your vacuum decreases and it allows the second bbl on both open or modulate as the engine demands for it or has a need for more air/fuel.

Lastly What would you guys consider "whole bunch of other modifications" vs just adding it to a stock motor.

(1) Camshaft with higher lift and increased duration will increase volumetric efficiency (when used with 2, 3, and 4)

(2) Better exhaust will increase volumetric efficiency

(3) Higher compression will increase volumetric efficiency

(4) Larger runners in manifolds/head will increase higher RPM volumetric efficiency (and will LOWER VE at lower RPM's)

Any/all of the above MAY allow higher RPM which will then require somewhat more carburetion; HOWEVER, higher RPM may require mods to the lower end (better rods, crank, etc.).

And any/all of the above may require more ignition.

Jon.

I have been driving my truck more now that fall weather has set in, and I really like the way the dual W-1s are working. Not only is the fuel milage better, the over all feel is better. The engine is much smoother in all gears. I haven't checked milage yet since the gauge just came off full after 140 mile trip. It's never gone this far on so little fuel. A couple more weekends of driving and I can top it off.

Joe

So if I am understanding this correctly, the set up I purchased from Tom is the dual carter webers which the carbs are progressive part and the linkages are not progressive. So on both carbs the one bbl opens as your diving normally, then as you pour the coals to it your vacuum decreases and it allows the second bbl on both open or modulate as the engine demands for it or has a need for more air/fuel.

No. The secondaries are opened via mechanical linkage on the side of the carb. The linkage people refer to as linear or progressive is generally the main one going from your pedal to the carbs.

Ford Model 740 Carburetor documentation seems to refer to the mechanical linkage of the carburetor as being a progressive linkage?

yup that's the progressive linkage setup on the carb side. no real way to adjusting that unless you try to make new components.

Great discussion! I am rebuilding my 1958 235 because it was losing power and had little compression on #2 and #4. Took it apart and found my pistons had holes in the ring grooves. While rebuilding I have acquired an Edmund's 2 x 1 water heated manifold with two Stromberg BXOV adjusable main jet carbs. But the discussion has me rethinking...

What are the opinions about the small 4 bbl? (A Holley 390 I think) How is performance with these single, but bigger carbs? The manifolds are available but does the design over saturate the middle cylinders? Opinions and experiences please!

Chevrolet used a 4-barrel that was approximately 385 CFM on their 265 high-performance V-8. How closely does your 235 compare to the high-performance 265? Compression? Cam? Exhaust?

I did put a Carter 400 CFM on a basically stock 300 CID six with excellent results.

Suggestion: acquire fixed metering jets instead of adjustable ones for your Strombergs (depending on which Stromberg). The Stromberg BXOV is an excellent carb, assuming what you have is correctly sized to your engine.

Or maybe (if the Stromberg is a BXOV-3) use ONE of the Strombergs with the adjustable jet on the 235.

Jon.

My plan is to install flat top pistons .30 over with a 261 cam and keep my fenton headers. Not much else on engine work. I have no idea how close that comes to the engine you speak of.

As for the Strombergs they are BXOV-2'. I'll look for the non-adjustable. Why do you suggest the fixed jetted ones over the adjustable? Ease of tuning?

What is the stamped (NOT raised) identification number on the airhorn of the carbs?

The adjustable jets are great for one carb; difficult to synchronize with multiple carbs.

Jon.

What was the quardra jet size on the 250 OHC Pontiac? From what I remember, they ran pretty good.

Joe

Joe - the Q-Jet used on the Pontiac was a 750 CFM; HOWEVER, the Q-Jet is a spread-bore design with only 150 on the primary side. And the Pontiac OHC 250 had a much higher VE than the stovebolt 235, and a higher RPM limit.

Jon.