The Stovebolt Site Forums The Shop Area Engine Shop Change carburetor question

My question is about what carburetor to use on my Chevy Inline 6 250 engine. Currently I have 2 Carter Weber two-barrel carbs mounted on an Offenhauser intake manifold. They are new and the engine has not been started yet. I am wondering if one Edelbrock 500 cfm carb on an Offenhauser intake would be better. What I am trying to achieve is a dependable 1956 Chevy that I can drive long distances and not have any problems. The dual Carter Webers and the complex progressive linkage seems difficult to keep tuned and correctly adjusted. Would the single 4 barrel carb be a better choice? Appreciate any advice.

One carburetor is always better than two. 500 CFM is far too big to run properly on a 250 engine, however. At 6000 RPM with 100% volumetric efficiency (impossible) the 250 would only be moving 435 CFM. At 90% efficiency, the CFM at 6K would be 398. I'd suggest using a Holley 7448 2 barrel with a 350 CFM rating, adapted to an Offenhauser 4 BBL intake.

We ran the 7448 carb on a bunch of 350 Chevy claimer engines on round tracks, and they would allow the 350 to run 6K RPM, just not at the rated manifold vacuum for the CFM rating of the carb. There are plenty of tuning parts for Holley carbs- - - -main metering jets and power valves, to custom tune the carb for your specific engine and driving habits.
Jerry

A MUCH SMALLER single carb than you mentioned would be better than either of your posted options. But the imitation AFB you mention would give you points with the younger folks at a car show.

Would suggest either the factory Rochester 2G (2-barrel) or the aftermarket Carter WGD (2-barrel) for the Chevrolet 265 would be an excellent choice.

I ran a 400 CFM genuine Carter AFB with the Offy intake on a street 300 CID 6-cylinder, and it was more than necessary.

Jon.

Jerry and Jon are the members to listen to on this.

More food for thought, how far is "a long distance"?
If I was going to do a coast to coast for example, I would set-up the Belair to run today's Ethanol gas.
That's a tall order and a hit on the ole wallet.
Start with the gas tank and go forward to the carburetor.
You want all parts, including the tank, to be composed of materials that do not react with Ethanol.
I can send you pictures of my 55 pickup parts that explains better than my words.

Why not go with a stock 1bbl manifold and a Carter YF? There were a few of those made for the 250. You don't want to overdo things. There is positively no sense in running too rich and it is dead easy to do. Jerry and Jon have told you (very correctly) you need smaller...a YF 1bbl would do that for you and is an excellent design.

Now on the other hand if you want a brand spanking new 250 with spanking new manifolds and HEI, you can buy one and you don't have to leave Texas to do so:

https://www.donhardyengines.com/specials.htm (go to the bottom of this page)

You'll have to make peace with the EPA, but a brand 250 made by GM can be yours for $2300 or $2500. In my opinion that and a Carter YF would be as trouble-free as you can get and you will spend more than $2500 rebuilding one plus you'll have to watch the machinist like a hawk 'cause it ain't easy to find good ones today.

Thanks to all for the advice. I'm going with a low cfm 2 barrel. Other than the Holley 7448 are there any others that will work well on my Chevy L6 250 engine?

Jon (Carbking) has made a very good suggestion with the 2G series Rochester. It's one of the best designed, most reliable carburetors ever built. You'll need to go with an intake designed for a Mercruiser inline six, I believe it's the "165" series since Mercruiser rates their engines by horsepower rather than cubic inches, or use an adapter with the Offy 4 barrel manifold. Just DO NOT try to funnel the mixture from the 2G down into the stock 1-barrel manifold. Creating that kind of choke point cancels out any possible power gain- - - -you'd be better off with the Carter 1 barrel Jon G recommended.
Jerry

High CFM carburetors run too lean, not rich. They flow plenty of air, and not enough fuel, due to the low velocity of air through the venturis not generating enough vacuum to siphon the fuel out of the float chamber and through the main jets. That's why engines with "too big" carburetors have a tendency to stall, stumble, pop, backfire, and leave big blistered spots in the paint on the hood when a backfire starts a fire in the engine compartment!
Jerry

Jerry has nailed the bit about too large running lean because of low air velocity in the venturi.

[img]https://www.thecarburetorshop.com/Carter_multiple_venturi.jpg[/img]

The link is a drawing of an experiment done in the Carter Carburetor School which demonstrates how fuel will flow based on different venturi take-off points. Note there are four OPEN glasses of water, with tubes leading to various points of a multiple venturi, and the relative heights of water that will flow due to the negative pressure from air velocity through the venturi. But the height of the columns will vary, proportionately, if the air velocity through the venturi changes.

Jon.

Jon, that 3-stack venturi bears a strong resemblance to the ones in the primary barrels of a Quadrajet. The combination of small throttle bores and multi-stack venturis makes the QJ one of the most versatile carburetors on the market.
Jerry

Jon and Jerry -- you guys are giving me wealth of information. I plan on using an Offy 4 barrel intake manifold with an adaptor plate if I use the Holley 7448 2 barrel carb. Or would the Rochester Quadrajet be a better choice?

Jon or Jerry, can you give me model number for a Rochester 2G carb that I can purchase that would be a good choice? Thanks

Tex - there are many different. I would suggest one from either a 265 or 283.

Some of the truck versions would be manual choke.

The passenger, and some of the truck versions would be auto choke.

Look on my website for listings.

Jon.

If you are going to spend over a grand on it you might consider a FiTech or Holley Sniper self tuning 2bbl TBI fuel injection. Instant start hot or cold, dead smooth from idle to WOT, and almost always an increase in fuel mileage. You said some long distance traveling and the FI runs the same on a mountain top as at sea level. There is more work to install with the main things being exhaust O2 sensor bung and high pressure pump---preferably in gas tank.

Would a manual choke or an electric choke be better for my Inline 6 250 motor?

Okay, here is another question. I read today that a 4 barrel carb at 600 cfm is just as good as a 2 barrel carb with 350 cfm. The reasoning is that at low RPM around town normal driving the 4 bbl will only be using the primary and thus only 300 cfm. My 56 Chevy is set up with an Offenhauser 4 barrel intake so I can use either. I don't plan to hot rod and mostly cruse driving. Which should I choose? Once again I appreciate the advice.

The theoretical CFM for any specific RPM is easy to calculate. Multiply 1/2 the cubic inch displacement times the RPM, which gives "Cubic Inches Per Minute", then divide that figure by 1728, the number of cubic inches in a cubic foot.

For instance: 350 cubic inches divided by 2 = 162.5
162.5 X 4000 = 650,000
650,000 / 1728 = 376.1 CFM at 4K RPM

To get real-world volume instead of theory, use 90% of the theoretical CFM as a ballpark figure. This accounts for various flow characteristics that prevent all the mixture and exhaust gas from flowing completely through the engine.

376 X .9 = 338.5 CFM

Most people don't understand that the maximum CFM of a carburetor is seldom, if ever used. It makes a good talking point to brag about a carburetor's maximum capacity, but in real world terms, that figure is virtually meaningless!
Jerry

Alright, so when I run that calculation I get 260 CFM for my L6 250 ci motor. So either a 300 cfm 2 barrel or a 600 cfm 4 barrel should work assuming you accept that the 4 barrel will only use the primary during normal driving. Which ends up at the same question -- is the 2 barrel a better choice or is the 4 barrel the better chouice?

Is cheesecake better than blueberry cobler??? How about apple pie???

It is a question that each individual will answer themselves; there is no correct answer.

A bit more information: Carter published CFM rating on ONE, and only one WCFB 4-barrel; the 2493s "race only" carburetor for 1956 Chevrolet 283. It was rated at 380 CFM.

Repeating a sentence from my first post in this thread: "I ran a 400 CFM genuine Carter AFB with the Offy intake on a street 300 CID 6-cylinder, and it was more than necessary."

Good luck with your build.

Jon.

It seems to me you are trying to get us to pick for you. If we pick right we’re heroes and if we pick wrong we’re bums. Pick the one you really want and if it doesn’t work sell it and get something else. You can repeat that cycle until you find just the right one that puts a big smile on your face! Plus just look at all the fun you will have.

Why would I want to put a 4 barrel carburetor on an engine and only use half of it? That's probably why practically ALL inline sixes used in light and medium-duty trucks have ONE BARREL carburetors. Maybe those designers with millions of dollars of research and development funds and shops with equipment hotrodders can only have wet dreams about weren't such mouth-breathing dummies after all!
Jerry

Thanks to all. Your advice is right on. Modifications are trial and error and fun.

Tex, Jerry has given you the right answer above. If I were you, I'd get an original intake/exhaust manifold and the Carter YF one barrel carburetor engineered specifically for the 250. I think it would be the YF 4339S, although there may be others. When you stop to think about it, the 250 is 15 cubic inches larger than the 235. 15 cubic inches is approximately the volume of a cup of water, and that isn't much. With the 250 your net torque maxes at 185 at 1600 rpm and your net horsepower maxes at 110 at 3800 rpm. That's pretty nice, but it isn't a high performance engine by anyone's measure. If you want to see your torque/horsepower curve, go to this page and follow down to page 31:

https://www.gmheritagecenter.com/do...hevrolet-Trucks/1972-Chevrolet-Truck.pdf

Jerry may disagree, but what I've seen is too much carburetor will make an engine run rich and wonky at idle and off-idle. You can adjust the idle mix with the idle mixture screw, but you can't adjust the off-idle performance. It is built in with holes drilled in the throttle body or through a vertical slot cut in the throttle body and this off-idle zone is meant to offer a smoother transition from idle to driving speed so you don't fall off into a dead zone pulling away from a stop sign. So, when you pass the off-idle rpm spot, your engine begins running leaner (as more air passes through the venturi) and it continues to get leaner the more you open the throttle. When it starts running leaner, it starts stumbling, stalling, popping, even backfiring in some cases. Bottom line...it doesn't run well at all (with good reason). And if you want to read about off-idle stumbling, go to Google. Type in off-idle stumbling and see what you find there. I did that early this morning and got over 6,000,000 hits. You want your carburetor balanced from curb (some call it driveway) idle all the way to full out open. If you don't have that, you basically have a mismatch and you won't be happy with it. If I could offer any advice at all, it would be two things: any engine will ALWAYS run better with too small of a carburetor than with too large of a carburetor and any engine will run better with a carburetor designed specifically for that engine. Good luck.

While Jon's idea of rich running during the "off-idle" transition might be theoretically correct, I've never been able to measure that situation, even when using some pretty sophisticated test equipment that cost the bean counters in the public education system a truckload of money. The transition area between idle and high speed mixture is less than 1/8 inch of movement of the throttle plates past the off-idle ports, and the transition happens so fast that there's little, if any time for that richness to happen. If it does, it's exactly what's needed to prevent a lean stumble, along with a properly adjusted accelerator pump. I'd really like to see some documentation of the idea that off-idle richness can be measured when using a carburetor that then instantly goes lean as soon as the venturi starts supplying the main air/fuel metering.
Jerry

Pontiac installed a 4 bbl carb on their over head cam 6's way back in the 60s and those motors ran really really well. What some manufactures use to call called compound carburation was in fact multi carbs on an inline motor and those designs ran really well (Hudson comes to mind). Some designs were better then others. Some set ups ran better then others. I have run quite a few multi carb set ups through the years on some of my inline motors. The wicked and unreliable Rochesters, the better then good Carter YFs, the pretty decent but hard to find Holley glass bowl 1908 and 1920s, Holley webers, webers, holley 2 bbls, holley 4bbls. some worked better then others.

I like multi carbs and they have always run better then my stock single bbl carb. How do I know? I have the benefit of still owning my first car and know how it ran (at a drag strip) before I began modifying the 235 that was in it, and after over a period of 44 years. I have run 235, 261 250 motors in cars and trucks. All benefited performance wise from additional carbs. If I was just building a stock motor I would likely leave the single 1 bbl. But I like to tinker. Recently, I was running Holley/Weber Ford Pinto 5200s and they ran good. I most recently ran a pair of real Weber 38 DGEV carbs, they ran pretty good but I always got the feeling there was more.

I just got done installing a pair of 2300 350 cfm Holley's on a 261 chevy. Runs terrific, absolutely terrific. No lean off idle stumbles, it is not running rich, no coughing or stuttering at idle, no loading up, gets to 5000 rpm in a heartbeat.

I just don't get this constant theoretical discussion poo pooing anything other then a single bbl carb and that a single bbl carb is the ONLY carb that should ever ever be used on an inline stovebolt 6. Hogwash. If that is your choice then fine but the fact remains that the inline 6 responds like any other motor to more cam, more carb, better ignition, better performance. It may not be gobs and gobs of improvement but a good thoughtful selection of parts, including carb, will give performance improvements no matter how modest. To simply write off a good multi carb set up as worthless or a good four bbl carb set up as ridiculous (notwithstanding that Pontiac did it for proven performance reasons) is just being closed minded.

All right, lol, I am off my soap box.

Try buying one of those Pontiac OHC six Quadrajet carburetors sometime. Your checkbook will need some CPR. A quarter mile needs four turns and a dirt surface before it can be called a "race track"- - - -A reasonably clever primate can stab a throttle and steer in a straight line- - - -it takes almost as much intelligence as grunting a few numbers and throwing a football.
Jerry

Does anyone know where I can get Carter YF ?

Check the Truck Parts for Sale Forum, there’s one listed there by Jon G

Jon,
Your comparison, "Is cheesecake better than blueberry cobbler??? How about apple pie???", falls flat.
Blueberry cobbler cannot be beaten, so much so that I can't remain focused enough to grasp your otherwise pertinent point which I assume was intended to be, "The choice is up to the individual."
Maybe you could edit your post to swap rhubarb pie in place of blueberry cobbler...

Carl - I personally love rhubarb pie; but the recipe has been modified in central Missouri to mix rhubarb and strawberries.

The mixture may be great for some, but I cannot stand it.

Oh, and I also love strawberry pie.

Too bad you live so far away, we have our own blueberry bushes, and LOTS of blueberries!

The post, as you surmised, was a lighthearted attempt to suggest that, DEPENDING ON THE SKILL OF THE TUNER, various carburetion combinations are POSSIBLE.

When I post on these forums, UNLESS THE ORIGINAL POSTER DESCRIBES OTHERWISE, I assume that the desired end result is a well-mannered street vehicle.

I have, in the past, done a few "unconventional" carburetor swaps. One was a "race only" Ford Pinto 2000cc engine. The rules stated "any carburetion". The owner found an intake (don't remember who made it) to put four (4) Mikuni motercycle carbs on the engine. After a LOT of time fabricating parts, it ran great for the first 3/4 of the season; and by then we were far enough ahead in points that we won the championship. In the off-season, took the Mikunis off for inspection, and they were absolutely completely worn out! Starting looking for more. Sanctioning authority made our search easier. New rules on carburetion were changed to "any single carburetor". Owner found a four-barrel intake, and I had a Carter WCFB from a 283 Chevy which we installed. Again, lots (but not as much as the Mikunis) tuning (really apples and oranges, as my inventory of Carter parts and documentation is hard to beat). When finished, this thing ran like a dream on the track. Absolutely NO "off-idle hesitation" (it idled at 2300 RPM ). Again, we won the championship.

So carburetion anomalies are POSSIBLE. But unless there is a compelling reason to do so, ignoring the laws of science should not generally be ignored.

And Dragsix, I would love to see a MAJOR article from you on some of your engine modifications. Obviously, you have spent a lifetime in testing (compression, carburetion, ignition timing, camshaft profiles, etc.) on the stovebolt. While few here are building race trucks (Dale Jarrett - where are you???), it could provide an insight into possible modifications for these engines.

And Carl, back to the rhubarb pie (opinions ): if there is a "perfect" six-cylinder, it would be the Jaguar 3.8 litre with three (3) Weber DCOE 2-barrel carbs. And IF I had been born independently wealthy, I would have at least two of them in my garage (currently, there is only a picture! ).

Jon.

Jon, I did some race tuning on a Datsun 240Z with three DCOE 40 Webers on it, back in 1970 when the 240 was a brand new design. The owner of the car was an IBM contractor on Okinawa, working for the department of defense, and he had more money than sense. The only street mileage the car ever saw was the trip from the dealer to his home shop, where the race modifications began. The next time the thing ran was at the Grand Prix of Macau. Somehow, the car managed to hitch a ride from Okinawa to Macau on a USAF C-130! A couple of years later, I built a Ford 289 engine for a Sunbeam Tiger that also competed at Macau. I had already gone back to the USA by then, but my engine got installed in the car after I left, and I heard that it did pretty well.
Jerry

Jon (carbking) said; "if there is a "perfect" six-cylinder, it would be the Jaguar 3.8 litre with three (3) Weber DCOE 2-barrel carbs. And IF I had been born independently wealthy, I would have at least two of them in my garage (currently, there is only a picture! wink )."

Jon, back in about '75 I had the good fortune to (while reading gas meters in a affluent part of town) spot a '62 Series 1 E-Type in a open sided car port. It was covered with about a 1/4 inch of dust. It took several attempts but I finally caught the owner home. Seems it was his daughters college car and it had quit running on the way home a couple years prior and hadn't been touched since the wrecker dropped it off.

Well, $2,000 later and after installing a new battery it was at my place. The generator had failed, a trip to the old guy electrical shop and it was good as new.

The wife and I put many thousands of miles on the little E-Type, luckley we had zero serious issues with it. It was a Joy to drive. I loved tilting the bonnet open and just looking at that dual overhead cam jewel of an engine. I used to point out to the the Vett snobs that my old car had disk brakes all the way around and would run 150 mph (130ish is all I ever saw). Old English cars and Jags in particular have a certain smell, don't know if it's the old leather, the old English Walnut or just the years working on the interior. But it's memorable and I like it.

The gear box whined, 1st gear wasn't synchronized, but the engine was perfect. Smooth, with sounds that I really can't describe. And the 3 SUs were not hard to work on or synchronize, contrary to what I read these days. Of course I'd already been driving a TR3 and TR 4A, having freshened up engines in both and been through the side draft SU's on both more than once.

I so very foolishly sold the Jaguar in the mid-90s for a few times more than we'd paid 20 years earlier. Today it'd be worth 6 figures. That said had I been smart and kept, it would't be for sale at any price.....well 7 figures and I'd have to think REAL hard.

It is still possible to pick up a pretty nice driver series 1 or 2 in the mid to high sub 100k range. Don't think of it as an extravagance but an investment that you can drive and enjoy for a few years. I doubt you'll have any regret

RonR

My only gripe about a Jag, or Brit cars in general, was that electrician's nightmare Lucas wiring! Every time it rained back in the mid-1960's, all the XKE owners had the bonnet raised, trying to dry out the drowned ignition! Spit on the sidewalk three blocks away, and a Jag would quit running!

"The Brits drink warm beer because they have Lucas refrigerators!"

Jerry

Jon, you are too kind my friend. I have spent a lifetime. I can hardly believe how quickly those times came and went, lol.

So both trucks and cars, but it started with cars. 1957 chevy was my first, and the one I kept all these years. Took my drivers test in the darn thing.

Currently, 261 powered. 302 gmc rods, venolia forged pop up pistons, Schneider cam 493 lift, 282 duration. Four speed, 4:11 rear. Car sits like I built it in high school in the 70s with the exception of the motor. The head that was on the car was one I had intended for a much more radical motor so it was ported and hogged out pretty good. I actually built the head when I was in my 20s (now 60s) for a race motor when I was in my bigger must be better mode. I needed a head so I dragged it out of storage in 2014 and used it knowing it would be a high rpm head. It developed a crack last year so I redid another head which I just finished installing.

Nothing crazy, 1.6 chevy stainless exhaust valves, 50-52 chevy power glide 1 15/16 intake valves. Relieved the exhaust chamber to unshroud the larger exhaust valve, hand ground the little bulb on the exhaust wall near the head of the valve, Hogged out the valve bowls a bit to make use of the larger valves, three angle valve job, some hand blending, pc seals, enlarged valve spring pockets to accommodate the Schneider springs and keepers.

The photo with the head port side up was on the mill. The first pass at .001 shows how out of flat the head was. I ended up taking .010 off the head to get it flat which I did not think was terrible. I have had heads in the past that needed a lot more so .010 was pretty good in my mind. Important part of the process, at least for me.

The other thing is, I generally don’t replace the valve guides any longer. I had a head crack on me a number of years ago trying to get one out. Turned a perfectly good 848 head into a boat anchor. So I prefer to use bronze inserts. These heads are ready old now, have been through countless heat and cool cycles and to the extent the castings will move around a bit, I guess they have moved all that they are going to. I don’t want to mess with that just to install valve guides. That and I think the bronze has some lubricant quality to them, so to speak, So bronze liners for me.

Originally I ran one of my old 2x1 manifolds, a Weiand, with a pair of 5200 Holley Weber pinto progressive two bbl. that set up ran pretty good. I then sold a 56 pickup to fund some house repairs and a new Clifford intake manifold and a pair of Weber 38/38 carbs. That set up ran really well, always a street car. But I had a pair of Holley 2300 350 cfm carbs that I have had since 93 or so but never got around to using so I decided that with the change in heads, I would sort out how to adapt the intake for the holly carbs.

First I had to make up some adapter plates to fit a holley four bbl. the carbs are actually too wide to fit so they need to be spaced a little further apart. Interestingly enough, speedway makes a two bbl. adapter set up that allows you to move the carbs around a bit. Then it’s on to cobbling up linkage and getting it installed. I did not make any jet changes. I did change the power valves from 8.5 to 6.5 because my motor makes about 12 lbs of vacuum at idle so the 6.5 were better suited. Because the bowls face the firewall, I installed jet extensions and notched floats.

Linkage was tricky but you just have to be creative and stick with it. Because both carbs have idle circuits it’s best to synchronize the carbs at idle. I have always used a uni-syn model A on my multi carb set up that used single bbl. carbs. Uni-syn also made a model 4B for carter and Rochester 4 bbl carbs. I have tried to get my hands on one but no dice as they are a little bit hard to come by so I used a moroso carb hat, drilled a hole in it and glued a model A uni-sun and then synchronized the carbs.

Finished product. Runs great. And it’s fast for what it is. Gets to 5000 rpm in a heartbeat. Idles about 900. Pretty clean at idle. It’s a street car and will always be a street car.

As for trucks, just acquired a gmc, dual rear tires, flat bed. First one like it I have messed with. Not sure if it’s a 54 or 55, still has gmc motor. It’s on the back of a friends flat bed. Not sure what gmc motor is in the truck yet. It’s complete, reasonable decent shape, but it’s been sitting. Long term project, may even be retirement project if I am lucky enough to be able to hang up my spurs in a couple of years.

Mike - thanks.

Great pictures, and information about other mods engines need to use more carburetion.

Those uni-syns for 4-barrels show up on ebay occasionally.

Ron - a dream come true.

Jerry - spot on regarding Lucas, inventor of the three-position light switch (off, on, flicker).

I learned more than 50 years ago, when one purchased a TR-3 or MGA, that one went to a junk yard and bought a fuse box out of an old Chevy. Install the fuse box, and have more than two electical circuits!

Jon

You have received some of the best advice out there. When I was young and stupid (now I'm old and stupid), we always thought the larger the CFM the better. One needs to realize the balance for efficient speed and power vs. optimum engine care. All of these guys can probably remember the piles of carbs an intakes piled up in the salvage yards due to poor applications. Look for the best balance and you will get the best performance. I have a sealed box of Offenhauser intakes for Chevy L-6 engines that I may be selling. All proceeds will go to Stovebolt. I'm concentrating on selling more usable parts for the L-6's. Some engine jewelry is not worth the effort and lost dollars it takes to apply. Doc