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Unread 06-24-2013, 04:59 AM   #1
jdhaas3
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287 cfm vs 300cfm carb help

I am getting a new motocraft and am wondering what CFM to go with. I have the 258 with offenhauser intake, getting the exhaust done right now and going from dual to single exhaust with a bigger muffler and 2 1/4" pipe. I am running 32x11.50 tires and dont plan on going any bigger, just trying to make this thing run a little better. Also have an HEI. The guy with the carbs said he can do a 300cfm or a 287cfm model, just wondering what you guys think I should go with. Here is a link to the 287CFM carb:

http://www.ebay.com/itm/321148560132...ht_2328wt_1399

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Unread 06-24-2013, 05:49 AM   #2
gmakra
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I think you will be fine with either since they are both close in CFM
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Unread 06-24-2013, 08:23 AM   #3
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You'd better stop and do some more research and then verify what you have before you order.

That carburetor is a 2 barrel and I'm pretty sure the Offenhauser intake is set up for a 4 barrel. You may want to consider a Holley 390CFM four barrel or the Holley 470CFM Truck Avenger Offroad.

Also, you may need to verify the minimum CFM requirement for that intake. I have a Clifford Performance intake and I know it requires AT LEAST a 390CFM carburetor. I know my Clifford is adaptable to a 2 barrel carburetor, but I don't think (and I DON"T know or sure) that the Offy can be changed.

Looky Here: http://www.exeterautosupply.com/Docs...%20catalog.pdf Go to page 11.
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Unread 06-24-2013, 08:56 AM   #4
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By the way...

That intake is most likely why you have a dual exhaust. More air in needs more air out. Most likely you have headers as well.

Take the time to fill out your Jeep's profile and be as detailed as possible. Little things like that intake and headers make a BIG difference in what the CORRECT answer might be. It was only because you casually mentioned the Offy intake that my answer is different.

What carburetor do you have now? You might not need a new carburetor at all, but rather a rebuild with the right jets, etc. Or it could simply need to be taken to a mechanic who actually knows how to tune a carbureted vehicle. (unfortunately these people are becoming more difficult to find. Try looking in hotrod or speed shops.)
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Unread 06-24-2013, 09:24 AM   #5
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Quote:
Originally Posted by WindKnot View Post
By the way...

That intake is most likely why you have a dual exhaust. More air in needs more air out. Most likely you have headers as well.

Take the time to fill out your Jeep's profile and be as detailed as possible. Little things like that intake and headers make a BIG difference in what the CORRECT answer might be. It was only because you casually mentioned the Offy intake that my answer is different.

What carburetor do you have now? You might not need a new carburetor at all, but rather a rebuild with the right jets, etc. Or it could simply need to be taken to a mechanic who actually knows how to tune a carbureted vehicle. (unfortunately these people are becoming more difficult to find. Try looking in hotrod or speed shops.)
The offy intake is their class C series, it has an adapter plate that allows for a 2bbl carb. I do not know the type of headers that are in there. The headers and intake were on there when I bought the jeep. The carburetor I have now is a Holley 2bbl, I have looked and I cant find anything stamped on there to figure out what model/cfm it is. I know it could definitely use a good cleaning, I dont really know how to find out what kit I should order to clean/rebuild it.

Even though I am going to a single exhaust instead of the dual, I am going from 2 inch to 2 1/4 for the size.
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Unread 06-24-2013, 09:32 AM   #6
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Quote:
Originally Posted by WindKnot View Post
You'd better stop and do some more research and then verify what you have before you order.

That carburetor is a 2 barrel and I'm pretty sure the Offenhauser intake is set up for a 4 barrel. You may want to consider a Holley 390CFM four barrel or the Holley 470CFM Truck Avenger Offroad.

Also, you may need to verify the minimum CFM requirement for that intake. I have a Clifford Performance intake and I know it requires AT LEAST a 390CFM carburetor. I know my Clifford is adaptable to a 2 barrel carburetor, but I don't think (and I DON"T know or sure) that the Offy can be changed.

Looky Here: http://www.exeterautosupply.com/Docs...%20catalog.pdf Go to page 11.
I was looking at the link you sent me. On page 9 it has all the information about the C series and which carbs work for it. When I get to the house I will see if I can figure out which model of Holley is on there. The other thing I saw is that they only really recommended the 2bbl with adapter for the 4 cylinder. I have that but on the 258.
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Unread 06-24-2013, 09:50 AM   #7
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Page 9 is not for you. Read deeper. There's nothing there for your engine. Page 11 is specifically for the 258. Now there may be a 2 barrel adapter on your intake, but THERE SHOULDN'T BE unless you can find a 2 barrel carb delivering at LEAST 390CFM. This could be why your Jeep doesn't run so well.
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Unread 06-24-2013, 10:10 AM   #8
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I agree with WindKnot. Even if someone does make an adapter, it's just the wrong configuration for your intake. Maybe someone can help you with a positive ID on your existing carb and help you sort it out. I think you'll be further ahead going that route.


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Unread 06-24-2013, 10:40 AM   #9
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This is what Holley says...

Look at the right side of your carburetor's choke air horn. You should see two numbers there, the top one is your List number and the bottom one is the date code location. The list number will begin with an R and consist of 6 to 8 digits. This is the number you need.

Look for the list number on the top of the main body casting if your carburetor does not feature a choke air horn. If your list number is on the main body casting, your carburetor is probably a model 4500.

Look up the list number on Holley's carburetor parts list. You will find the model number in the column next to the list number.

Parts List: https://www.holley.com/data/TechServ...%20Listing.pdf
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Unread 06-25-2013, 05:10 PM   #10
jdhaas3
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Thanks for the help WindKnot. With the information you gave me I found the list number of 6R1919B on the carb. Then on the side I found List-7448 2980. Here is the carb I found from Holley that I have http://www.holley.com/0-7448.asp. It is 350 CFM.

I will go ahead and make sure the profile is more up to date, but here is what I have in the jeep.

4.2L 258 w/ offy class C intake, I do not know the type of headers. 350 CFM carb, no idea what the jets/venturi is set at. I just redid the exhaust to 2 into 1 with 2 1/4" pipe all the way back. I think I would like to go the route of just getting a rebuild kit for this thing after what most people have said. However, the offy says it would like 500CFM flow, wouldn't that be a little overkill for the 258?
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Unread 06-25-2013, 05:52 PM   #11
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Here is a picture of my current setup.



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Unread 06-25-2013, 08:04 PM   #12
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Offy says anywhere from 390 TO 500CFM. You MAY be okay, but it is undersized and will starve the engine some. You'll still have a two barrel carburetor with an adaptor plate.

You have choices. Holley makes a 390 CFM 4 barrel AND they make a 470CFM "Truck Avenger" carburetor that's specifically set up to work when it's far from level. That's the carb I run with my Clifford intake which has similar CFM requirements.

But there's other carburetors that run great built by others as well. The Weber 38 draws enough and is actually recommended by Clifford. But it is a two barrel carburetor. You might want to contact Offenhauser for their opinion of putting a two barrel adapter on the manifold.

Edelbrock makes a 500CFM 4 barrel. It's been around for a million years and used to be made by Carter before Edelbrock took it over. Mine was a Carter, but it was the one I pulled because I felt it was too big. If you want it, you can have it or what it costs to ship. But know this... The manual choke needs rebuilding and the carb needs to be rebuilt. I have no idea what jets are in it but I'm sure they're probably not the ones you need. I'm at 5000ft and am sure it was rejetted just for the difference in altitude. It needs work. It won't come out of the box and just bolt on with good results.

Personally I like Holleys. Others don't. I think they're easier to work on than others. Others disagree.

If I remember right you want to DD the Jeep? These aren't going to help your mileage any. They're all (including the manifold and headers) designed to boost your horsepower. They draw more air than a stock manifold, and that means they need more gas. If you want better mileage, you may want to go back to the stock manifolds and stock carburetor. that would NOT be my choice.

I leave for Montana tomorrow afternoon, so I would need to know what you want on the carburetor. Personally I'd go with either of the Holleys. The 470 for off roading.

Others will chime in I'm sure.
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Unread 06-25-2013, 08:08 PM   #13
jdhaas3
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Quote:
Originally Posted by WindKnot View Post
Offy says anywhere from 390 TO 500CFM.

You have choices. Holley makes a 390 CFM 4 barrel AND they make a 470CFM "Truck Avenger" carburetor that's specifically set up to work when it's far from level. That's the carb I run with my Clifford intake which has similar CFM requirements.

But there's other carburetors that run great built by others as well. The Weber 38 draws enough and is actually recommended by Clifford. But it is a two barrel carburetor. You might want to contact Offenhauser for their opinion of putting a two barrel adapter on the manifold.

Edelbrock makes a 500CFM 4 barrel. It's been around for a million years and used to be made by Carter before Edelbrock took it over. Mine was a Carter, but it was the one I pulled because I felt it was too big. If you want it, you can have it or what it costs to ship. But know this... The manual choke needs rebuilding and the carb needs to be rebuilt. I have no idea what jets are in it but I'm sure they're probably not the ones you need. I'm at 5000ft and am sure it was rejetted just for the difference in altitude. It needs work. It won't come out of the box and just bolt on with good results.

Personally I like Holleys. Others don't. I think they're easier to work on than others. Others disagree.

If I remember right you want to DD the Jeep? These aren't going to help your mileage any. They're all (including the manifold and headers) designed to boost your horsepower. They draw more air than a stock manifold, and that means they need more gas. If you want better mileage, you may want to go back to the stock manifolds and stock carburetor. that would NOT be my choice.

I leave for Montana tomorrow afternoon, so I would need to know what you want on the carburetor. Personally I'd go with either of the Holleys. The 470 for off roading.

Others will chime in I'm sure.
I drive this thing to work 2 or 3 times a week and to the beach on the weekends. So i would say that I put around 150-200 miles on it per week. This thing only really sees the highway and beaches, no serious off roading.

So either way you would ditch the 2 barrel Holley I currently have?
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Unread 06-25-2013, 08:24 PM   #14
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I would. I'd go with the 390 CFM Holley, which ironically is the carb that Offy recommends. I am not an expert in carburetors. I'm fortunate to have access to an old f@rt available to me who works on Holleys. I don't know for sure, but it may be that just because it IS the recommended carb, it may be more efficient than the one you have... but I don't claim to know that. A brand new one with a electric choke (which would be cool or commuting) is about $400 though.

So how much is 40CFM? about 5 1/2 55 gallon barrels of air in a minute.
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