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Old 02-03-2019, 07:39 PM   #1
Acgreen3256
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Default 4 barrel or 2 barrel carb?

Ive got a 77 scout II traveler with the 5.6L V8 2 barrel factory carb, wondering if i should get a 4 barrel carb. what are the pros/cons of more barrels, how will my engine handle a 4 barrel carb, and if i do decide to go for 4 barrels what brand/model of a 4 barrel carb should i get?
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Old 02-04-2019, 08:27 AM   #2
Scoutboy74
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Default Re: 4 barrel or 2 barrel carb?

A good 2bbl carb is all that engine needs. Aftermarket Holley 2bbls in the 350cfm rating work well on these engines right out of the box. Avoid the 500cfm rating, however. To do a proper 4bbl conversion, you also need a matching 4bbl intake manifold. Further, there are both spreadbore and squarebore versions of 4bbl intake manifolds for the 345 and 392. Same with the 4bbl carbs. Squarebore means all four holes are the same size. Spread has small primary holes and much larger secondary holes. Its not a one size fits all scenario, so definitely not something to do in willy-nilly fashion. Using an adapter to make a 4bbl carb fit a 2bbl manifold is totally pointless. You don't get the use of the secondaries with that adapted setup, which defeats the purpose of having a 4bbl carb in the first place.
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Old 02-04-2019, 01:30 PM   #3
1975IH200
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Default Re: 4 barrel or 2 barrel carb?

Holley List 0-80350 for a 350 CFM carb with electric choke.

Holley List 0-7448 for a 350 CFM carb with a manual choke (cable operated choke).

Don't bother with a 4-barrel carb for your SV-345......unless you get a good Carter ThermoQuad carb and spread-bore intake manifold and know what you are doing.
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Old 02-05-2019, 07:22 PM   #4
Acgreen3256
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Default Re: 4 barrel or 2 barrel carb?

Thank you for the input
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Old 04-15-2019, 12:46 PM   #5
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Default Re: 4 barrel or 2 barrel carb?

Quote:
Originally Posted by 1975IH200 View Post
Holley List 0-80350 for a 350 CFM carb with electric choke.

Holley List 0-7448 for a 350 CFM carb with a manual choke (cable operated choke).

Don't bother with a 4-barrel carb for your SV-345......unless you get a good Carter ThermoQuad carb and spread-bore intake manifold and know what you are doing.
My truck came with a Motorcraft 2bbl carb with a broken flange. Since I had to get a new carburetor I decided to go the 4bbl route. Easy as pie. It should take a few hours or so.Or so I thought. So I found a spread bore manifold on ebay for a reasonable price. I got the manifold and tried scrubbing off the 40 years of baked on grime to no avail. I finally gave up and took it to a machine shop and had it hot tanked. While that was the first instance of the cost and effort going up, it had a bright side, because I found a machine shop with guy with a big grey beard that said "I havent seen one of those IH SV engines in years" (So, you HAVE seen one before!) He hot tanked it for 25 bucks and I threw some Case IH Red tractor and implement paint on it (for another 10 dollars for the paint and hardener, which BTW held up perfectly and still looks great) I found a Thermoquad on a 74 1110 at the local wrecking yard a month later for about 50 bucks. So far, it appeared my luck was holding. Then I found out the previous owner had completely destroyed said Thermoquad. (which was probably why the otherwise serviceable looking truck was sitting there to begin with.) On top of that, it didnt occur to me that I could have made ANY similar Thermoquad into the one I need for that truck by using the rods and jets from the ruined one, so the box containing the dissasembled Thermoquad was thrown away when I moved) I then got an Edelbrock 1406 that was taken off a running truck from Craigs List (arent they all) and an adapter plate for the spread bore manifold and an aftermarket throttle cable bracket. After much tuning and many sets of rods and jets and a full rebuild with an Edelbrock rebuild kit,and a Edelbrock phonelic spacer to stop the fuel from boiling in the summer ($+$$+$$$), I finally got it running reasonably "not particularly bad". (I still would not call it good). I also found that Edelbrock does not sell the manual choke bracket for their carburetors, so its far more difficult to convert a automatic choke to a manual one than it is to go the other way. After many hours on the phone to them, I was told that the part was simply not available and that even though they would love to sell one to me that they just could not get one. So I changed tracks and called aftermarket tech support and told them that the bracket on my brand new carburetor had developed a crack, broken and as far as I could tell probably fallen off on the road somewhere. Two days later the bracket arrived in the mail. So here I am some years down the road, and I still don't like the way it runs so I finally broke down and picked up a "Superquad" aftermarket Thermoquad (which the previous owner, bless his soul, told me absolutely does not work and at the very least needs a rebuild kit if its even fixable at all, which as it turns out it is) and found three of the original International Harvester master rebuild kits that contain the jets and rods and plan on putting that on this spring, so finally I will have an fuel system that SHOULD work right. Remember, this was all supposed to take an afternoon to get running.

The moral of this story is, while changing things around CAN end up with a vehicle that runs correctly (for various values of "correctly") its usually a lot more effort than you expect,and a lot more money than you expect, and the farther you change it from stock the more effort it takes to get it all tuned in and running right.

Last edited by mallen; 04-15-2019 at 01:06 PM.. Reason: corrections
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Old 04-15-2019, 05:29 PM   #6
FDChappie
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Default Re: 4 barrel or 2 barrel carb?

I put a Holley 390cfm square bore carb on my warmed over 345. Got about $1k into it and can get 14mpg on a tank. It also has better throttle response at low rpm's. There is a write up on the tuning process on this forum.
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