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Re: Change carburetor question
TexApache #1417819 Fri Jul 23 2021 02:42 AM
Joined: Feb 2004
Posts: 22,981
H
'Bolter
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Change carburetor question
TexApache #1417826 Fri Jul 23 2021 03:33 AM
Joined: Nov 2010
Posts: 48
T
'Bolter
Thanks to all. Your advice is right on. Modifications are trial and error and fun.

Re: Change carburetor question
TexApache #1417848 Fri Jul 23 2021 12:40 PM
Joined: Mar 2014
Posts: 2,465
J
'Bolter
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 websearch. 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.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Change carburetor question
TexApache #1417872 Fri Jul 23 2021 03:56 PM
Joined: Feb 2004
Posts: 22,981
H
'Bolter
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Change carburetor question
TexApache #1417899 Fri Jul 23 2021 08:00 PM
Joined: Dec 2017
Posts: 969
D
'Bolter
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.

Last edited by Dragsix; Fri Jul 23 2021 08:01 PM.

Mike
Re: Change carburetor question
TexApache #1417909 Fri Jul 23 2021 09:06 PM
Joined: Feb 2004
Posts: 22,981
H
'Bolter
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Change carburetor question
TexApache #1417933 Sat Jul 24 2021 02:30 AM
Joined: Nov 2010
Posts: 48
T
'Bolter
Does anyone know where I can get Carter YF ?

Re: Change carburetor question
TexApache #1417935 Sat Jul 24 2021 02:36 AM
Joined: Apr 2005
Posts: 10,022
Grease Monkey, Moderator General Truck Talk & Greasy Spoon
Check the Truck Parts for Sale Forum, there’s one listed there by Jon G


Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne 4dr 230 I-6 one owner (I’m #2) “Emily”
‘39 Dodge Businessmans Coupe “Clarence”



"I fought the law and the law won" now I are a retired one!
Support those brave men/women who stand the "Thin Blue Line"! Hug a cop!
USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)

Re: Change carburetor question
TexApache #1417938 Sat Jul 24 2021 03:02 AM
Joined: Mar 2010
Posts: 9,759
5
'Bolter
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... smile

Re: Change carburetor question
TexApache #1417964 Sat Jul 24 2021 12:50 PM
Joined: Nov 2002
Posts: 2,224
C
Carburetion specialist
Carl - I personally love rhubarb pie; but the recipe has been modified in central Missouri to mix rhubarb and strawberries. frown

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! wink

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 wink ). 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 wink ): 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.


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
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