4 bbl carburator basics - JeepForum.com

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post #1 of 18 Old 06-21-2010, 08:01 PM Thread Starter
BioTex
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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.
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post #2 of 18 Old 06-21-2010, 09:56 PM
JeepnBlake
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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.
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post #3 of 18 Old 06-21-2010, 09:56 PM
texasimport
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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
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post #4 of 18 Old 06-22-2010, 03:18 AM
cgeorge5150
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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.
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post #5 of 18 Old 06-22-2010, 06:16 AM
Happy Joe
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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.
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post #6 of 18 Old 06-22-2010, 06:16 AM
JeepHammer
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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.
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post #7 of 18 Old 06-22-2010, 06:56 AM Thread Starter
BioTex
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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
Attached Thumbnails
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
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post #8 of 18 Old 06-22-2010, 07:04 AM
JeepHammer
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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.
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post #9 of 18 Old 06-22-2010, 03:19 PM
turbogus
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'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
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post #10 of 18 Old 06-22-2010, 03:31 PM
JeepHammer
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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.
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post #11 of 18 Old 06-22-2010, 03:52 PM
turbogus
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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
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post #12 of 18 Old 06-22-2010, 06:11 PM
colojeepguy
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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.
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post #13 of 18 Old 06-22-2010, 07:12 PM Thread Starter
BioTex
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Here are some pictures of a Carter thermoquad carb.
Attached Thumbnails
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
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post #14 of 18 Old 06-22-2010, 07:14 PM Thread Starter
BioTex
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Here is a Carter QuadraJet mfd. for GM. I also have a Rochester quadrajet not pictured.
Attached Thumbnails
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
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post #15 of 18 Old 06-22-2010, 07:16 PM Thread Starter
BioTex
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Location: Alpine, Tx.
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Here is a strange one! An Autolite?
Attached Thumbnails
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
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