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Discussion Starter · #1 ·
after stumbling across a carb calculator, I discovered that a 600 cfm seems like way too much carb for a 304 it says I only need a 425 cfm carb to rev it to 5k. I've got an edelbrock manifold, cam and headers that will be installed soon. I can pretty easily get what I paid back for my 600 cfm edelbrock carb. Also is a 500 cfm carb still to much? I'm still looking at the edelbrock carb about $200 new, and I'll pick up the off-road jets and the electric choke kit. I just what to avoid flooding, most of the trails I run aren't very steep, usually just muddy trails with the ocassional big rock throw in for good measure. sorry for dragging on. TIA
-MarkyMark

80 CJ-7 Laredo 304v8 TF999 3.73's BFG M/T's
 

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According to a carb chart I have in a Rochester book, a 325 ci engine that turns 5000 rpm will need a 380 cfm carb. Holley's 390 cfm carb # 8007 would fit the bill perfect. Since Holley's are a pain to set up, (my opinion), then the next best choice would be the 500 cfm Carter AFB # 9504 or it's clone the Edelbrock Performer # 1404. Whichever one you choose, be sure to get spring-loaded needle and seats to help avoid flooding off road.

 
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Discussion Starter · #3 ·
Depending on performance part, it could be that you will need the jet, neddle & spring kit, to tune the carb.
Get the springloaded floats etc.
I have the carter 500 on a Chevy 305, Edelbrock performer, headers and a 194 /204 @ 0.050 cam.
It works ok, but it quits running when i put the nose up in the air
(I don't have the spring loaded setup....)
Also, mileage is bad, but i suspect it would be gretly improved, with the needle, jet & spring kit...

I'm now switching to a Holley 4-barrel Pro-jection kit...

Regards,
PerJ

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600 CFM seems pretty big. It's meant for engines with big breathing cams that run at high RPM's. I'd go smaller. Holley has a calculator at their website. Holley dot com, if you want another opinion.

Loose nut behind the wheel
Another right-wing conservative.....
Born and raised in Jeep-Town
 

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/wwwthreads_images/icons/tongue.gif I'm with Nut on this one. In carbs, more is not necessarily better. You give up too much low end control./wwwthreads_images/icons/crazy.gif

CJDave
 

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i also think the 390cfm Holley is a good choice, a nice vacuum operated secondary, electric choke 4 barrel, (i think most modified engines are over carburated) also with a smaller CFM you will have better throttle responce.

i have a 390 on a V-6, (262 cubic inch) and it won almost every 6-cylinder mud race it entered./wwwthreads_images/icons/smile.gif

3/4tonYJ
My Jeep Page
 
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Discussion Starter · #7 ·
I'd agree with the other replys. Don't overdo it. Just in case you want to write this down. Engine displacement (in cubic inches) divided by 1728 (converts to cubic feet) multiplied by half the the RPM (half, remember 4 cycles only suck in air/fuel on every other crank rotation) equals the CFM flow at that RPM. That is a given. If your carb is to big you only slow the velocity of air/fuel. Same amount (CFM), but a bigger hole, moves slower. Response gets sluggish. Your going about it right, too many just put some big monster carb on top thinking they'll get more then can't figure out way it would run worn a %#$!
CJ Joe

1968 CJ-5 225 V6/T14/18/27frt/44rr
 
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Discussion Starter · #8 ·
The general formula for determining carb size is (CID X RPM)devide by 3456 then multiply by volumetric efficiency(usually 75-80% in stock engines),I dont recomend the Edelbrock due to the small primary throttle bores and the inability to adjust the vaccumn portion of the secondarys,
the 390 vaccumn holley would be a good choice ,the 600 vaccumn holley would also work well in either case invest in the quick change spring kit and a jet kit avalible thru summit racing at reasonable price.
keep in mind that a propperly set up vac. holley will only use the CFM needed,
and that the above formula is ONLY a guideline!

JF1
 
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Discussion Starter · #9 ·
Just my $.02 worth-- I have a 600 cfm on a stock 350. It is a Edelbrock with the manual choke.
I had to lean the jetting down but it works very well. If you where just rock crawling then the 390
CFM would be interesting.
I think that you would find that the 500CFM would work just fine .I to would not go any bigger
with a carb than I had to. Heck theres guys on this list running 390 CFM carbs on 258 I-6's .
I would call Edelbrock teck line and check with them.
Good luck with what ever you fine out.


 
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Discussion Starter · #10 ·
thanks guys I think I'm just going to bump down the 500 cfm, I've heard Holleys are a pain to setup and the 390 holley costs about $145 more than the 500 edelbrock, with the off-road jets and the elecric choke kit. So that's probablly what I'm going with, unless some wants to cut me a deal on a used 390...

80 CJ-7 Laredo 304v8 TF999 3.73's BFG M/T's
 
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Discussion Starter · #11 ·
I think the 500 cfm carter will serve your purpose well. Steve

Mopar360yj
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I've got a 390 cfm, used to run it on a 258 I-6 and it ran fine. I'd rather sell it with the manifold though. I'd
think it might be a little small for the 304 anyway. We ran 650s on 283s but gas was cheap then too.

As to figuring the carb size, I know you would take the displacement of the engine and multiply by the
RPM, multiply by the efficiency and divide by two (two revolutions to fill all cylinders). But wouldn't that
be for a single barrel or standard two barrel. A four barrel or two barrel with progressive linkage does not
normally run on the whole carb and isn't the rating with all barrel butterflies open?


 

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Check out the Jeepster/Commando BBS on this site. Rich wrote a book of really good info. Helped me alot

Scott
'72 Commando 304

 
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Discussion Starter · #14 ·
you can use a 600cfm carb with no problem as long as it has non mechanical secondaries. for most off road usage, carter or edelbrock are easy to set up and relatively inexpensive. my opinion though is unless you need big power a two bbl carb is better. you have less fuel in the carb, less venturies to pour fuel into the intake when you dont want it to. a motorcraft or holley two bbl carb are my personal picks. both can be set up for little money and give very good performance.

dan

I do not intend to offend,I offend with my intent
 
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