On my offy 3x1 setup. Im getting closer to needing to setup the choke system. I decided to scrap the original heat-operated choke and go to a manual pull-choke.
ALSO, I have (due to the extreme difficulty and near impossibilty in my steup) scrapped the idea of progressive throttle linkage and gone with direct linkage.
Having gone to direct linkage, do I have to choke all three carbs? Or can I just choke the center one? If just the center, do I need to wire the outers open?closed?

PS ~ Please no comments about how I may or may not be a moron for not using progressive linkage. I dont care about the loss of MPG.

Thanks, you guys have always been a big help.
I'm especially looking forward to Twisted6's and RevOD's responses.

I've runnig this setup in alcohol with direct linkage and with no choke at all. Nothing, None, Zero.
Even in the coldest winter days, what i used to do is inject some gasoline on the center carb and run. I can't forget to mension my intake manifold is water heated. But anyway if you want to use a choke, i suggest you to use only on the center carb. I think it'll works perfect.
What makes me believe this is the reason when the car is idle if you cover the center carb it turns off while if you cover the others doesn't

Good luck

I had this setup on a 57 235 some 40 years ago. I was told to run with direct linkage as it would lean out the outer cylinders. I think with 3 accelerator pumps, starting wouldn't be a problem.
I assume you have jetted down all the carbs. I didn't and could flood the engine while driving by tapping the pedal.
Oh well, live and learn.

will6er

All the carbs are stock. Rebuilt. I think they have the standard jets in them.

Give it a try.

Get a wide band O2 reader for jetting.

I would not touch the jetting,(for now) remember,you do have more fuel going to the engine,,,,but you also have three carbs of more air to the engine.

MBHD

sorry folks but i tought 250Nova was asking about choke sources, not for jetting.

I think i missundestood the topic.

Sorry

I would think if you have direct linkage you may want to choke all of them the same. With a pull choke it shouldnt be hard to wire them all up to operate off the same pull I would think.

Direct linkage is okay and you won't need any chokes. Just block all three 'open' is all.

No one ever ran "progressive" in the 50s and it always works, where the other needs adjustment constantly. Gas was a quarter then.

"one pump" should be plenty for starting cold.

Good luck.

When setting with a Choke and it has a High speed idle control Like most carbs do Only one choke should? be needed to Hold for the High Idle.
Two things a choke does
1) it sets restricts air flow But increases fuel
mix.
2) it sets and Holds high idle till the motor is
warm enough to run on it's own. Then the choke is now wide open.

So my 2cents is That if you choke all 3 carbs You will then be dumping in Way to fuel to air and could very well end up floading the motor.

What i have done in the past for a choke less set-up is richen the fuel mix up for cold weather. This really depends on just how cold it gets on how well it works. But a choke is the better way to go.

I appreciate the responses so far, very helpful. Would still like to hear more ppl's suggestions.

Thanks

I run three small Corvair carbs on a 250 with only the center choke working. All three are tied together and act as one carb. It starts just fine with only center choke. I even use the original exhaust manifold and choke linkage. Joe

I am running a 2 carburetor manifold on my GMC 248 and using Rochester carbs. I have choke cables to both carbs and find it necessary to use both to get it started. When you only choke 1 of multiple carbs the vacuum transfers to the other (not choked) carbs and there is no benefit to choking. Only my experience, but I had a GMC 270 with a Howard 5 carb. manifold years ago and had no chokes, just one pump with 5 accelerator pumps gave plenty gas for a cold start.

UPDATE:

This thread can be deleted if necessary. I put a single manual choke on the center carb. But as chevman32 described, I havent found it necessary even on cold starts.

Hi I run a mildly built 235 with 3 rochesters on a Offy manifold I only use the center choke, it works great just pull it for for a moment until the bowls are full then side it in and it idles great.Ive never seen a progresive linkage on a triple setup. I followed the advice in the teck tips for setting up the carbs and they idle like a stocker for 2 years now. hope I helped? Bri...

Good response by all. I'd like to comment on jetting. A stock carb is jetted for air fuel mixture. Probably about 13-14 to 1. You could put the 1 barrel on a 454 and the air fule mixture would still be correct; just not enough air. 1 or 3 carbs are still jetted correctly are going to be very close to stock. The difference will be throttle position. With 3 holes feeding you engine instead of 1 they will be open less for the same total air, which really runs your engine.

I've run 3 Holly 280 CFM carbs at Bonneville for over 30 years and the jets have remained near stock all that time. They were stock at #52 and occaisionally I change one to a #50 on each carb, that's for the air density.

Good luck! 3 carbs always looks good under a hood.

jimmy six

You bring up something interesting.

Progressive vs non progressive, is there a noticable mpg difference, lets say if both triple setups, have carbs which only open enough to meet the engines required cfm flow (combined)?

Thanks

As jimmy six points out, the air/fuel ratio (and hence, the fuel consumption and power) is pretty much set by the jet sizes in the carburetor, and the A/F ratio should be about the same if you have one, three, or five carburetors. However, at low engine speed and part throttle, as encountered while cruising at 30 mph for example, the airflow demanded by the engine to put out only about 10 HP is so low that the air/fuel ratio will probably start to change due to the very low "venturi signal" that is generated in the carburetor. The venturi signal, which is the pressure differential that "sucks' the fuel through the jets, is proportional to the square of the airflow through the venturi. It is possible for the venturi signal to become so low that the head offset between the fuel level and the fuel discharge tube cannot be overcome and the fuel flow goes to zero. Thus, for a given airflow demand, the three-carb engine will see only one-ninth the venturi signal that will be generated by the one-carb engine, and the low speed performance and fuel economy of the three-carb engine will suffer. Conversely, at high airflow demand, the three-carb engine will see only one-ninth of the pressure drop through the carburetors, and should produce significantly more top-end power.

I had not fully comprehended these effects until I talked to Tom Langdon a few months ago and told him that I was planning on using five Strombergs on my 261. I then looked at some textbooks on carburetor performance and realized what happens at very low (but still above idle) airflow through a carburetor. If there is no pressure loss through the air filters and carburetor inlet, the air/fuel ratio leans out significantly. However, is there is much of a pressure drop through the filter, the mixture can become too rich at low airflow demand. I have generated a fairly simple spreadsheet simulation of the air and fuel flow through a carburetor and would be happy to share it with anyone as crazy as I am.

I now have my 261 (278 cubic inches) set up with two Stromberg 97s for primary carbs, with three 48s as secondaries. Both of the 97s will be independently choked.

Back to the original question about whether or not to use a choke on all “active’ carburetors. If one or more of the active carbs are not choked, the manifold pressure will be so high (or the vacuum will be so low) that the choked carburetors will not provide very much additional fuel to richen the mixture when the engine is cold. However, the additional fuel provided by the extra accelerator pumps seems to be effective. Several of the previous posters have also observed these effects. When I had three non-progressive 97s on my 261, I tried just about all possible combinations of choke plate and accelerator pump arrangements. All seemed to work equally well (or poorly when it was –10 degrees F outside). One of the best combinations was all three chokes with the leathers torn off of two of the accelerator pumps.

Wouldnt the multiple carb setup become efficient, in a non progressive setup, if the combined carbs met total engine required cfm, that would require a smaller carb(s) and would increase the pressure drop or venturi effect at low engine speeds, correct?

Correct me if Im wrong, but wouldnt three carbs, non progressive that equalled 600cfm, have the same venturi signal, as a single 600cfm carb....and the whole issue of multiple non progressive carbs isnt, non progressive or multiple but simply, as they say "too much carb"?

I dont have any experience with multiple carbs or much theory of, im just speculating, looking for answers.

appreciate your input

Hey inline300 . . .

Let's do a little math and see if your intuition is true.

First let's assume two-inch 'tall' venturi at the base of our theoretical carb(s) - this will make the math easier - because it's just calculating the area of a cylinder, which is what we'll need to hold constant at 'any one moment in time' when comparing setups (single, dual, trip).

So let's look at a one barrel setup with a 1.42" venturi. The volume of air in the this setup at any one moment in time is 3.2 cubic inches:
(radius .71 - height 2)

Now let's look at the velocity of this theoretical carb flowing on top of a 216 at wide open throttle @ 140 CFM (not unlike a Carter W1):
(radius .71 - CFM 140)

The air is flowing at some ~144 miles per hour.

Now let's compare this same setup with a two carb setup that consists of two theoretical carbs that are exactly half the size of the Single setup. A one inch venturi size will give us two columns of air that are 1.6 cu inches in volume each (again at any one moment in time):
(radius .5 - height 2)

Now let's look at the velocity of this twin carb set up on top of a 216 at wide open throttle @ 140 CFM (70 CFM through each venturi):
(radius .5 - CFM 70)

The air is flowing at roughly the same ~145 miles an hour as in the single carb setup.

So I agree with your analysis, most duals and trips are a less drive able setup because the venturi size of the carbs selected is perhaps two big (Roger Huntington warns us about this in his book). Look at what happens when we use two Carter W1s on a 216:
(radius .71 - CFM 70)
Velocity is cut in half at the same airflow.

But what if we assume that the stock manifold is restrictive and that the 216 engine can actually flow 20-30% better with the dual:
(radius .71 - CFM 80-90)

In spite of the added volume (160-180 CFM) the air is flowing at only ~83 to ~93 miles per hour though the two venturi.

This decreased velocity will impact drive ability because of the lower vacuum signal - which will impact throttle response and port vacuum pull on the vac advance in the dizzy. Without some tuning adjustments the dual W1s will likely be lean and the timing late at low RPM (as Hoyt points out).

regards,
stock49

So it is a matter of carb/venturi selection.


Appreciate the detailed post!

i was running 3 33mm on my setup and the car hasn't no torque at low and medium speeds. After i've changed for 29 mm veturis the ride got 100% better to drive.
The throttles are 40mm on this setup.

My three Rochesters are Model H off a Corvair. They are about 78 cfm each so all three are in the 240 cfm range give or take. This is why mine runs so good, its not much bigger then a stock Monojet and the throttle plates are very small. Mine has 20 inches of vacuum at idle. Joe

I had not considered vacuum signal when running my 3 Holleys. First they are 280 CFM each. (actually off AMC V-8's). I have watch guys use the same Clifford tubing manifold using 500's and it did not perform as well. I have 12-1 compression with a 310 adv deg cam and .615" lift. All of this works as used. The power valves are removed from the carbs. When I did drag race it the car always stumbled off the line. I learned to drive through that.

I have also used a single 2 barrel from Braswell which flow over 700 CFM. A very trick carb. With many annular discharge holes in the venturi this carb responds very well to the 50+ cubes of each cylinder. The vacuum signal is perfect but a slightly bigger 4 barrel with the 4 venturis gave me a slightly higher mph. The 2 barrel gave better ET's

If were to use a 3 carb setup on the street the center one for sure would be smaller if using a progressive linkage. All 3 would be small for straight linkage. I like a street engine with as much compression as possible to run without detonation and a cam which doesn't kill the low end. To me that's where a few degrees advancing come in.......

This a great discussion. There are many theories but hearing from practical use on what works and doesn't work gives a guy a chance not to make as many mistakes as many of us did in the past....J.D.

great thread.......
i'm in the early stages of a 3 x 1 setup using Carter YF carbs on a Ford 300 (similiar to the one in the tech section)

I'm sure i'll be experimenting with different choke/tune-up settings, this thread has really got me scratching.

i originally was planning a progressive linkage setup and started to stray away from the idea, BUT when wondering through a junk yard with a friend this weekend i found and bought a 200 ford with 3, 1 barrels that has progressive linkage........

thought i'd share the pictures......... they are the bottom 2 on this page http://www.pirate4x4.com/forum/showthread.php?p=6698269#post6698269

fyi, i just building my 300-6 for a weekend driver.....

You won't need a choke (1 or 3) in Pa. to get started in Winter. A throttle cable will let it run "fast idle" till warmed up etc.

This is with (non progessive) linkege which is what most use on Hot Rods.

Generally speaking; The owners/builders of these vehicles can afford the fuel for their 'part time' operation.

Good luck.

52'F3, who ever put that 300 ford together sure didn't worry about carburators being the same! Its hard telling how it ran...... Joe

If one assumes a "perfect" carburator, eg, one which produces the proper air/fuel mix at all airflows from idle to maximum, there would be no need for a progressive linkage.

Stock49 is correct; it comes down to air velocity through the carb. This is because carbs are not perfect, and because at idle and off idle, the mixtures are acheived using different mechanisms. The transition from the idle circuit to the main circuit is difficult at best.

With a single carb, one spends little time in the transition stage; normal driving requires some minimum horsepower output, which is above the transition level.

With multiple carbs, however, that minimum horsepower output level (and consequent airflow demand) is now split between multiple carbs forcing all of the carbs to operate for extended time periods at the point where the carb doesn't know if its idling or trying to produce power. Operating within the transition range between idle and main mixture circuits makes the resultant mixture unstable and tough to drive.

Is this a vote for progressive linkage? No. A manifold designed for even air and fuel distribution under WO conditions isn't going to be able to provide balanced distribution under part-throttle conditions. Typically, on an in-line engine, the outer ports run leaner than the center, and this during the output level where one spends considerable driving time. All in addition to the maintenance hassles.

Is this then a vote for non-progressive linkage on multiples? Not neccessarily. While if a driver understands why the carbs stumble at small throttle opening and makes a point of driving in such a fashion that the throttle is either at idle or opened to above the stumble range. This kind of off-on driving will be sure to displease the mother-in-law.

Multiple carbs are a hot-rodding tradition because at the time engines were being tweaked to develop higher power outputs, single carbs capable of delivering the total airflow needed simply did not exist. Multiple carbs to get the total airflow were the only route, and the driveability problems were just accepted as part of the cost for that power.

A larger single carb on a well-balanced manifold is far more likely to provide both the total power and the driveability. If one looks closely at many of the multiple carb setups on hot rods, you will find that many of the carbs are actually dummies... Installed for the look, left disconnected to retain the functionality.

Like the nostalgic look? Consider multiport EFI. Manifold pressure-based EFI systems (Like the GM system) don't care how many throttle bodies are installed, they just care about the total pressure in the manifold. Three two barrel air valves, or six single barrel air valves would retain the look and the functionality.

If my memory serves me right some of the factory 3 x 2 setups used progressive linkage with the two outside carbs completely closed at idle position, no chokes and no idle adjusting screws ( the rochesters didn't even have idle circuits on outer carbs). This way the engine idled on center carb and responded to about 1/3 to 1/2 throttle without opening the outer carbs at all this gave good drivability, better fuel mileage, and yet when you stood on it you had three fully functional carbs making horsepower. One of the reasons tri-power was no longer offered was most people didn't understand how to properly adjust it to make it perform, so the factories built bigger and better four barrels.

i've chased the gremlins that live between "idle" and "Wide-Open" in a few carb models during years of off-roading. We experimented with different accelerator pumps/linkage adjustment, fuel bowl levels, inlet PSI, idle mixer and jet sizes, bla bla bla...... all in search of the smooth controlled off-idle transition performance.
we finially figured it out "fuel injection".

anyway, i just want all 3 carbs.

but i just can't get it through my head why running of the center chocked carb alone, and then somewhere past off-idle the other 2 carbs via "progressive linkage" kick in, would be so hard to tune?

sorry for being so off post topic, just enjoying the dicussion

52F3 They are not a problem for people with any common sense knowledge but you would not believe the dumb things people did thinking they were going to make thier car faster. With a properly designed and adjusted progressive linkage they are no problem to tune. The problem comes when the outside carbs don't close at idle or they open to soon or they don't open all three carbs at full throttle. Progressive linkage is very easy for me and I could never understand what the problem was but it existed. It is sorta like some guys can time an engine by just listening to it and others can't time it with a light.

Not only are the two carbs different he didn't even put one on the first pad. Those cylinders #1 and #2 are gonna run really lean!

What an IDIOT!!!

lol,
i have since seen the error in my ways and have corrected lean cylinder problem.....

joe, i had pictures of a 300 and a 200 in the same thread.....

I guess we need to give him credit for trying. There was some welding and thinking to do!!! Joe