4 bbl carburator basics - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > Models > Jeep CJ Forum > 4 bbl carburator basics

Rampage Bowless Soft Top Oconee offroadAlloy USA Still AvailableAdvance Adapters SYE Kits

Reply
Unread 06-21-2010, 08:01 PM   #1
BioTex
Registered User
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Alpine, Tx.
Posts: 3,173
4 bbl carburator basics

I have a large pile of 4 bbl carbs (some new) at my disposal. I have at least one of every major brand. I was comparing them side by side, and got to wondering which style of primaries/secondaries would give the best gas mileage?

When I turn them upside down, and compare the butterflies, I see on one carb. where the secondaries are the same size as the primaries. Then I see where the primaries are tiny, and the secodaries are huge.

And then I see the secondaries are just a tad bit larger than the primaries on others.

What style is best for a Jeep V8 304, that sees a bit of everything? When I say best, I mean a mix of good fuel economy, and off road performance.

Would the answer be the same if the engine was a 360?

__________________
'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20.
1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231

Last edited by BioTex; 06-23-2010 at 06:13 AM..
BioTex is offline   Reply With Quote Quick reply to this message
Unread 06-21-2010, 09:56 PM   #2
JeepnBlake
Registered User
2004 TJ Wrangler 
 
Join Date: Jan 2008
Location: Lincoln Nebraska
Posts: 3,098
The one with the small primaries and massive secondaries sounds like a quadrajet, a rochester carb used on a great many chevy cars trucks etc. Supposedly pretty good carb all around, at least that's what I hear from the chevy guys.

As for aftermarket, I choose Holley. I've heard of the brand before and I hear more good out of them than Edelbrock carbs. Jeephammer is the man on carb stuff though, hopefully he sees this and puts in the good and bad about each one.
__________________
-Blake-

Build Thread: http://www.jeepforum.com/forum/f22/j...thread-985907/
Dad's Build Thread: http://www.jeepforum.com/forum/f22/d...build-1474170/
OEM Fog Light Connectors, Conversions, and Extension Info:http://www.jeepforum.com/forum/f20/o...ngler-1008928/
JeepnBlake is offline   Reply With Quote Quick reply to this message
Unread 06-21-2010, 09:56 PM   #3
texasimport
Registered User
1978 CJ5 
 
Join Date: Aug 2009
Location: Loveland, Colorado
Posts: 338
No expert like others here but I would think you need to figure out the CFMs you want to narrow down that pile first, 500/600 range I bet.
__________________
78 304-CJ5
texasimport is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 03:18 AM   #4
cgeorge5150
Registered User
1975 CJ5 
 
Join Date: Apr 2010
Location: california
Posts: 10
I have the holley 4 bbl on the I6 258. Great performance bidue to the positioning it has a problem idling down because of the tension from throttle cable. Stiffer spring has yet to fix it... Possibly going to try an additional side spring to pull it frontward... Don't know if anyone else has experienced that with a Holley or not but if you can see what will fit the best as well.
cgeorge5150 is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 06:16 AM   #5
Happy Joe
Registered User
1985 CJ7 
 
Join Date: Jun 2007
Location: Colorado
Posts: 2,365
My experiences with carbs off road are not good, pretty much any carb will work on the street though.
For best mileage go with a small primary, correct the jet size for your altitude and optimize the vacuum enrichment circuit to match your driving style. Secondaries need to open only when needed.

To size a carb (quick and dirty) take 1/2 of the engine displacement (in cubic inches), divide by 1728 (to change cubic inches to cubic feet), multiply by the maximum rpm expected (be realistic). this gives 520 CFM for a 360 at 5,000 rpm. Note other factors like volumetric efficiency come into play (for regardless of cost/maximum performance applications) but this will get you close for street applications. You will find after running the numbers that most enthusiast engines are over carbed. Note also that 2 bbl carbs are normally rated at a higher manifold vacuum level than 4 bbl carbs so the CFM numbers are not completely comparable.

Holleys (dual float bowel) are typically sensitive to bumps and whoops as well as steep climbs and leans so a lot of folks (most) have problems running them, off road.
Edlebrock (Carter design) also have less than stellar reputations off road.
The Rochester carb, 4bbl, design are one of the few that seems to give most people fair results off road (after being properly setup). However setup seems to be beyond beyond the abilities of many carb guys.

Due to the lack of any carburettors ability to provide well controlled mixtures off road; the use of an MSD brand ignition box (MSD-6a) is highly recommended as it will fire poor mixtures better than a stock ignition.

For best results off road a multi-point injection setup is the way to go.

If a throttle cable is too short, get a longer one (holds true for both carbs and injection).

Enjoy!
__________________
...a well prepped, well driven, vehicle should do well in any terrain, including the highway.

Carburetors became obsolete during the last century... do what ever it takes to get fuel injection...It makes bigger grins off road.

Last edited by Happy Joe; 06-22-2010 at 06:34 AM..
Happy Joe is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 06:16 AM   #6
JeepHammer
Running On Empty...
1973 CJ5 
 
Join Date: Aug 2007
Location: South West Indiana
Posts: 10,208
Quote:
Originally Posted by BioTex View Post
I have a large pile of 4 bbl carbs (mostly new) at my disposal. I have at least one of every major brand. I was comparing them side by side, and got to wondering which style of primaries/secondaries would give the best gas mileage?

When I turn them upside down, and compare the butterflies, I see on one carb. where the secondaries are the same size as the primaries. Then I see where the primaries are tiny, and the secodaries are huge.

And then I see the secondaries are just a tad bit larger than the primaries on others.

What style is best for a Jeep V8 304, that sees a bit of everything? When I say best, I mean a mix of good fuel economy, and off road performance.

Would the answer be the same if the engine was a 360?
When the bores are the same size, we call that a 'Square Bore'
When the primaries are smaller than the secondaries, we call that a 'Spread Bore'...

Smaller primaries will give better fuel control and mileage if you don't put your foot in it and open the secondaries.

You should also be working the throttle linkages,
Watching the throttle valves (Butterflies),
Some will only open the primaries, these are VACUUM secondaries.
If the linkage opens BOTH sets, then it's MECHANICAL secondaries.

Some pictures would help us to identify the carbs...

You will probably be looking at a few basic types of carbs,

Holley, Rochester, Carter, Motorcraft, ect.

Carter had two basic types of carbs, the AFB and the Thermoquad,
ThermoQuad is easy to identify, it has a 'Plastic' venturi body (actually bakealite) to keep heat away from the float bowls.
The AFB is being reproduced by Webber as the 'Edelbrock Performer' carb...
Still leaks like a sieve and is hard to tune... But it's out of under patent, so it 'Open Source' now and anyone can reproduce it.

Holley is pretty easy to identify, they usually cast 'Holley' right in the float bowl,
Once you know what you are looking for, it's VERY easy to spot the vacuum canister on the right rear of the carb to know if it's mechanical or vacuum secondary.
Every Holley comes with a list number facing forward on the choke horn that will tell you EXACTLY what you have in your hands.

Motorcraft carbs were often Holley designs, but made more simple for lower cost of production,
About every Motorcraft carb made worked pretty well, with the exception of the 'Vairable Venturi' model... Easy to identify, one large opening with long rectangle throttle valve in the venturi bore.

Rochester is GM's primary builder, and that includes the famous (or Infamous, depending on how you look at things...) QuadraJet and DualJet carbs.
QuadraJet carbs came in three sizes, Large, Huge & EXCESSIVE!.
You can still find sites that will run the numbers down for you.

Like I said, pictures might help us help you with your parts sorting chores.
JeepHammer is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 06:56 AM   #7
BioTex
Registered User
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Alpine, Tx.
Posts: 3,173
I'm not looking to identify what carbs I have, as much as I'm looking for carb theory 101. For instance, which is better for a Jeep, vacuum or mechanical secondaries, and why?

With a spreadbore, if the primaries are tiny, won't it be too hard to keep the foot out of the secondaries, and will this hurt mileage more so than a square bore setup?

I will add more pictures later, but here are a couple I have already. Sorry for the crappy pictures. I'll get better ones
4-bbl-carbs-001.jpg   4-bbl-carbs-002.jpg   4-bbl-carbs-003.jpg  
__________________
'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20.
1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231
BioTex is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 07:04 AM   #8
JeepHammer
Running On Empty...
1973 CJ5 
 
Join Date: Aug 2007
Location: South West Indiana
Posts: 10,208


Carter AFB. I think that one was called a 'BlackHawk' back in the day, but I'm not sure.
Got a 70's Scout out in the 'Jeep Pasture' with that same carb on a 345 CID engine.
Leaky, slow fuel delivery on primary,
Yours missing choke canister...

----------------------------------------------------------------

Generally speaking,
MECHANICAL SECONDARIES are used with manual transmissions and that's about all.
The secondaries open with the throttle pedal stroke no matter if you need them or not.
NOT RECOMMENDED for vehicles that don't have high performance engine and/or automatic transmissions.

.................

Vacuum activated secondaries will usually give you better fuel mileage,
Work best with Automatic Transmissions, ect.

The vacuum has to be 'Right' before the secondaries open, so that saves you fuel, allows automatic transmissions to shift without banging on them, ect.

Last edited by JeepHammer; 06-22-2010 at 03:29 PM..
JeepHammer is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 03:19 PM   #9
turbogus
Registered User
1978 CJ5 
 
Join Date: Jun 2005
Location: Albany~town, Oregon, Oregon my home and for 200 billion slugs
Posts: 1,463
'Black Betty' came with an Edelbrock

Carter clone~NOT.

Occasional spitting and sputtering led me to a rebuild and a full tilt boogie jet kit. The problem randomly persisted until finally it went up in flames (luckily I always keep a charged fire extingusher on the roll cage). Back in my lockers from my SBC days I had the genuine article, so after de-tuning for my mild AMC 360, She's been purrin' like the proverbial kitten. Have yet to do any rowdy off roading but so far so good.
__________________
The parts shop that stocks part for Skylab II will not have parts for our year/model of Jeep
We cannot accurately judge the trajectory of a speeding critter (cat, dog, sasquatch)
Record heat waves and floods only occur when we visit that area
turbogus is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 03:31 PM   #10
JeepHammer
Running On Empty...
1973 CJ5 
 
Join Date: Aug 2007
Location: South West Indiana
Posts: 10,208
Actually, many of the AFB parts still fit on the Edelbrock version, Webber didn't change much when they started casting them for Edelbrock...

I keep the old 'BlackHawks' and other AFB based carbs around here for parts when I run onto them still in serviceable shape.

The Chrysler restorers buy up the Thermo-Quads as fast as I find them, so I know there aren't any of those or parts laying around here...

That's a 60s or early 70's version in the pictures, no emissions vents.
JeepHammer is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 03:52 PM   #11
turbogus
Registered User
1978 CJ5 
 
Join Date: Jun 2005
Location: Albany~town, Oregon, Oregon my home and for 200 billion slugs
Posts: 1,463
Quote:
Originally Posted by JeepHammer View Post
Actually, many of the AFB parts still fit on the Edelbrock version
Yeppirs the Jet Kit rods and springs fit both of my carbs
__________________
The parts shop that stocks part for Skylab II will not have parts for our year/model of Jeep
We cannot accurately judge the trajectory of a speeding critter (cat, dog, sasquatch)
Record heat waves and floods only occur when we visit that area
turbogus is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 06:11 PM   #12
colojeepguy
Registered User
1970 CJ5 
 
Join Date: Jan 2010
Location: Colorado Springs CO
Posts: 1,112
That second carb pictured is a Rochester 4jet (not to be confused with the Quadrajet). It's a good carb, but uses an oddball bolt pattern & parts are difficult to come by.
colojeepguy is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 07:12 PM   #13
BioTex
Registered User
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Alpine, Tx.
Posts: 3,173
Here are some pictures of a Carter thermoquad carb.
carter-thermoquad-001.jpg   carter-thermoquad-002.jpg  
__________________
'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20.
1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231
BioTex is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 07:14 PM   #14
BioTex
Registered User
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Alpine, Tx.
Posts: 3,173
Here is a Carter QuadraJet mfd. for GM. I also have a Rochester quadrajet not pictured.
carter-quadrajet-003.jpg   carter-quadrajet-004.jpg   carter-quadrajet-005.jpg  
__________________
'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20.
1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231
BioTex is offline   Reply With Quote Quick reply to this message
Unread 06-22-2010, 07:16 PM   #15
BioTex
Registered User
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Alpine, Tx.
Posts: 3,173
Here is a strange one! An Autolite?
autolite-006.jpg   autolite-007.jpg  
__________________
'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20.
1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231
BioTex is offline   Reply With Quote Quick reply to this message
Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.
Note: All free e-mails have been banned due to mis-use. (Yahoo, Gmail, Hotmail, etc.)
Don't have a non-free e-mail address? Click here for a solution: Manual Account Creation
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.


Thread Tools


Suggested Threads





Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.