Buying a new carb, replacing old Carter AFB (LONG)
03-17-2010, 05:41 AM
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Buying a new carb, replacing old Carter AFB (LONG)
Hello gang, one project I'd like to wrap up before the snow disappears is to update my carb...possibly my manifold along with it.
Here's where I'm at now... I have a mystery motor, meaning that I have no idea whats been done to it internally.. she's a bit lopey so I think the cam has been upgraded.. but she's anything but an asphalt burner. It's a 350 block, iron heads, offenhauser 360 intake with an old carter AFB... not an edelbrock version. Off idle and low RPM performance is dismal.. couldn't roast the tires on dry pavement if I had to.
Right now I'm leaning towards a Holley SA 670 or the 4175 so I read Lars' paper on tuning Holley's which brought up another question regarding the intake.
His comments on single plane (360 degree) intakes fits with the performance issues I'm experiencing:
I pretty much accept Lars' advice as gospel seeing how he's probably forgotten more than I'll ever know about carbs 
So I'm thinking the intake should be replaced as well. Does the Holley SA 670 make sense with a edelbrock performer intake? I'm thinking the 670 because it seems to have more options in terms of tuning which may come in handy working with a mystery motor.
Also curious what the consensus is on single vs dual plane intakes for street use.. other than Lars' paper I didn't really come across any other information on the subject.
I should also say for the record that I am not interested in a Q-Jet, not because I think Holley's or Demon's are any better, but because I have limited places to source parts from up here and even more limited wrenching buddies that know them as well as they do Holleys. Being the dumb guy, I need all the help I can get
So there you have it, what would you do if you were in my shoes?
Here's where I'm at now... I have a mystery motor, meaning that I have no idea whats been done to it internally.. she's a bit lopey so I think the cam has been upgraded.. but she's anything but an asphalt burner. It's a 350 block, iron heads, offenhauser 360 intake with an old carter AFB... not an edelbrock version. Off idle and low RPM performance is dismal.. couldn't roast the tires on dry pavement if I had to.
Right now I'm leaning towards a Holley SA 670 or the 4175 so I read Lars' paper on tuning Holley's which brought up another question regarding the intake.
His comments on single plane (360 degree) intakes fits with the performance issues I'm experiencing:
Virtually all musclecars (with a only a few notable exceptions) came from the factory with a dual plane intake manifold, also known as a “180-degree” manifold. A dual plane intake is characterized by having two separate sides (right and left) which feed two different “levels” inside the manifold. Half of the cylinders are “fed” from the one level, and the other half are “fed” from the second level. A dual plane manifold promotes higher intake velocity at lower rpm, improving low-end torque and low-end throttle response. A dual plane manifold also promotes very good vacuum at idle and at low speed.
At very high rpm, a dual plane manifold will loose some efficiency. For this reason, there are single plane manifolds available. Known also as “360-degree” manifolds, the single plane manifolds typically have large intake runners and a large, open plenum under the carb. Very good for high-rpm use, these manifolds can be a real challenge to tune for low-end, street rpm use. A single plane manifold will typically significantly reduce vacuum, and this causes several problems.
My recommendation: don’t use a single plane intake on a street driven car unless you have a lot of cubic inches and you’re willing to put in a lot of time tuning the idle transition. If you’re experiencing an off-idle stumble on a car with a single plane manifold, your problem may be in the manifold and not with the carb. Consider spending the $$ for a dual plane intake and saving yourself some frustration.
At very high rpm, a dual plane manifold will loose some efficiency. For this reason, there are single plane manifolds available. Known also as “360-degree” manifolds, the single plane manifolds typically have large intake runners and a large, open plenum under the carb. Very good for high-rpm use, these manifolds can be a real challenge to tune for low-end, street rpm use. A single plane manifold will typically significantly reduce vacuum, and this causes several problems.
My recommendation: don’t use a single plane intake on a street driven car unless you have a lot of cubic inches and you’re willing to put in a lot of time tuning the idle transition. If you’re experiencing an off-idle stumble on a car with a single plane manifold, your problem may be in the manifold and not with the carb. Consider spending the $$ for a dual plane intake and saving yourself some frustration.
So I'm thinking the intake should be replaced as well. Does the Holley SA 670 make sense with a edelbrock performer intake? I'm thinking the 670 because it seems to have more options in terms of tuning which may come in handy working with a mystery motor.
Also curious what the consensus is on single vs dual plane intakes for street use.. other than Lars' paper I didn't really come across any other information on the subject.
I should also say for the record that I am not interested in a Q-Jet, not because I think Holley's or Demon's are any better, but because I have limited places to source parts from up here and even more limited wrenching buddies that know them as well as they do Holleys. Being the dumb guy, I need all the help I can get
So there you have it, what would you do if you were in my shoes?
Last edited by AKruizer; 03-17-2010 at 06:35 AM.
03-17-2010, 06:30 AM
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Doing a little more reading it appears I was right to put the comment in there about helping the dumb guy. I think my understanding of square/spread bore and single/dual plane intakes is all out of whack...
To revise a bit... I'm understanding the Offenhauser 360 to be a single plane intake and contributing to lower vacuum and low rpm performance.
I'm shooting for having a matched carb/intake geared towards the street not the strip.
Any information or education you can offer to help me understand this better would be appreciated.
Thanks!
- The Dumb Guy
To revise a bit... I'm understanding the Offenhauser 360 to be a single plane intake and contributing to lower vacuum and low rpm performance.
I'm shooting for having a matched carb/intake geared towards the street not the strip.
Any information or education you can offer to help me understand this better would be appreciated.
Thanks!
- The Dumb Guy
03-17-2010, 06:51 AM
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Right now I'm leaning towards a Holley SA 670 or the 4175 so I read Lars' paper on tuning Holley's which brought up another question regarding the intake.
His comments on square bore (360 degree) intakes fits with the performance issues I'm experiencing:
...
So I'm thinking the intake should be replaced as well. Does the Holley SA 670 make sense with a edelbrock performer spread bore intake? Most of the treads I found it seems like guys were putting 670's on square bore intakes... I realize I'd need an adapter, reason I'm still thinking the 670 is it seems to have more options in terms of tuning which may come in handy working with a mystery motor.
Also curious what the consensus is on spread vs square bore intakes for street use.. other than Lars' paper I didn't really come across any other information on the subject. ...
His comments on square bore (360 degree) intakes fits with the performance issues I'm experiencing:
...
So I'm thinking the intake should be replaced as well. Does the Holley SA 670 make sense with a edelbrock performer spread bore intake? Most of the treads I found it seems like guys were putting 670's on square bore intakes... I realize I'd need an adapter, reason I'm still thinking the 670 is it seems to have more options in terms of tuning which may come in handy working with a mystery motor.
Also curious what the consensus is on spread vs square bore intakes for street use.. other than Lars' paper I didn't really come across any other information on the subject. ...
Does the Holley SA 670 make sense with a edelbrock performer spread bore intake?
Most of the treads I found it seems like guys were putting 670's on square bore intakes.
The best 2101 is the previous one & the EPS is only about 1/4" taller.
03-17-2010, 12:30 PM
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Get an Edelbrock Performer or Performer EPS and one of their 1406 carburetors and you should be good to go. The advantage to using the Edelbrock carb is everything...fuel inlet, linkages, etc., should hook right up the way they were. I'm not knocking Holley, but the Edelbrock carb will hook up simpler and they are simple and hold adjustments well. I'm simply advising simplicity since it's a street car.
The EPS is optimized specifically for square bore carbs and shows somewhat better horsepower and torque numbers than the Performer, though you do lose the flexibility of using square- and spread-bore carbs.
With either, you might have to use a drop base air cleaner, but maybe not.
The EPS is optimized specifically for square bore carbs and shows somewhat better horsepower and torque numbers than the Performer, though you do lose the flexibility of using square- and spread-bore carbs.
With either, you might have to use a drop base air cleaner, but maybe not.
03-17-2010, 01:11 PM
#5
Sorry to be the one to bring this up, but you may not get where you want to go with a carb and manifold.
You mentioned "lopey" cam. Lots of novice engine gurus put in a big cam with lots of overlap (lopey) without realizing it hurts low end while increasing top end. To make it worse, they do it on relatively low compression engines.
It sounds like your cam and intake were made for top end. Changing the carb, if it's tuned correctly now, may not make much difference.
Changing the intake may build more velocity at low rpm, but if the cam has a lot of overlap, you'll just bleed it off.
I went through the same issues, combined with a 3.08 rear end, on my car.
An economical approach might be to go to a dual plane manifold, rebuild your carb, recurve your distributor for more initial timing. All that can be done for less than the cost of a new holley.
If that doesn't cure it, then change the cam to one that's better at low rpm and would work better with a dual plane manifold.
My $0.02
You mentioned "lopey" cam. Lots of novice engine gurus put in a big cam with lots of overlap (lopey) without realizing it hurts low end while increasing top end. To make it worse, they do it on relatively low compression engines.
It sounds like your cam and intake were made for top end. Changing the carb, if it's tuned correctly now, may not make much difference.
Changing the intake may build more velocity at low rpm, but if the cam has a lot of overlap, you'll just bleed it off.
I went through the same issues, combined with a 3.08 rear end, on my car.
An economical approach might be to go to a dual plane manifold, rebuild your carb, recurve your distributor for more initial timing. All that can be done for less than the cost of a new holley.
If that doesn't cure it, then change the cam to one that's better at low rpm and would work better with a dual plane manifold.
My $0.02
03-17-2010, 01:32 PM
#6
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Gary 68 nailed it, honestly....unless that AFB has a worn throttle blade shaft, then maybe do the Edel replacement....
the last carb you want for a street machine is a Holley....
the last carb you want for a street machine is a Holley....
03-17-2010, 04:06 PM
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If you're leaning towards a Holley for street use, consider spending a few extra bucks and upgrading to the new "Ultra" series carb. Holley has refined their metering blocks to correct some old problems, making the Ultra carbs very well tuned for modern pump gas blends - this has been a problem with other Holley carbs.
With a correctly set up carb, you can run the single plane intake on the street. Although a good dual plane will give you a little better off-idle and low rpm performance, you won't notice significant differences in idle quality between the two: If' you're trying to smooth out a lumpy cam idle, you'll gain better results from timing curve tuning than from a manifold swap. Shorten your advance curve so you can run about 18 degrees initial with the total limited to 36.
If you decide on the SA carb, be sure to read my SA problems paper - you need to do a little tuning on those carbs for good performance.
Lars
With a correctly set up carb, you can run the single plane intake on the street. Although a good dual plane will give you a little better off-idle and low rpm performance, you won't notice significant differences in idle quality between the two: If' you're trying to smooth out a lumpy cam idle, you'll gain better results from timing curve tuning than from a manifold swap. Shorten your advance curve so you can run about 18 degrees initial with the total limited to 36.
If you decide on the SA carb, be sure to read my SA problems paper - you need to do a little tuning on those carbs for good performance.
Lars
03-17-2010, 11:55 PM
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Lots of good information here thanks guys 
A little more background info:
Someone familiar with the car before I owned it thinks a better carb/intake may have been in use before it was sold and that what I have was more or less a product of what lying around in the garage when they decided to keep the 'good stuff'. I'm fine with that if it's true, it's not like it changes the fact that I have a big grin on my face comfortably cruising down the highway.
So one element where the mystery motor is definitely working against me is in regards to the cam. I agree with Gary 100% that the carb/intake may not get me where I want to be if I have some novice engine work at play. But I wouldn't complain if I could get a little more bite out of the 3.73 gears though
One reason I have the carb/intake in the crosshairs now is that replacing them could eliminate some of the unknowns...there's plenty of potential issues with a 20+ year old carb and while a rebuild would be more economical, if/when that carter comes off of there it's not going back on. The fuel line and throttle cabling need work as well so this will be a good chance to eliminate a little more of bubba's handiwork. Besides Lars has another great write up on fuel lines that I intend to put to use.
For me another strike against the AFB is that there is a lot less information on this forum for tuning them compared to the qjets and holley's. I feel a lot more confident fine tuning a carb knowing that I have some really good write ups by Lars and other members here!
Gary's approach makes sense...try tackling the carb and intake and if the results aren't there then it's time to look at the cam.
Lars, thanks for the info on the utra...summit may have just earned a few more bucks As you stated getting the timing fine tuned is definitely in order.. does it make sense to eliminate an unknown AFB from the equation before I sink my teeth into dialing it in? Or am I tackling things out of order here!?!?
Hell, I'm probably just publicly rationalizing spending some money on the car and dragging you all along with me
A little more background info:
Someone familiar with the car before I owned it thinks a better carb/intake may have been in use before it was sold and that what I have was more or less a product of what lying around in the garage when they decided to keep the 'good stuff'. I'm fine with that if it's true, it's not like it changes the fact that I have a big grin on my face comfortably cruising down the highway.
So one element where the mystery motor is definitely working against me is in regards to the cam. I agree with Gary 100% that the carb/intake may not get me where I want to be if I have some novice engine work at play. But I wouldn't complain if I could get a little more bite out of the 3.73 gears though
One reason I have the carb/intake in the crosshairs now is that replacing them could eliminate some of the unknowns...there's plenty of potential issues with a 20+ year old carb and while a rebuild would be more economical, if/when that carter comes off of there it's not going back on. The fuel line and throttle cabling need work as well so this will be a good chance to eliminate a little more of bubba's handiwork. Besides Lars has another great write up on fuel lines that I intend to put to use.
For me another strike against the AFB is that there is a lot less information on this forum for tuning them compared to the qjets and holley's. I feel a lot more confident fine tuning a carb knowing that I have some really good write ups by Lars and other members here!
Gary's approach makes sense...try tackling the carb and intake and if the results aren't there then it's time to look at the cam.
Lars, thanks for the info on the utra...summit may have just earned a few more bucks
As you stated getting the timing fine tuned is definitely in order.. does it make sense to eliminate an unknown AFB from the equation before I sink my teeth into dialing it in? Or am I tackling things out of order here!?!? Hell, I'm probably just publicly rationalizing spending some money on the car and dragging you all along with me
03-20-2010, 05:20 PM
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I ahd a holley with a vac secondary did not like it to tune it you had to replace springs in the secondary vac chamber and you could not get that 4 barrel kick, swiched to a edelbrock mech secondary much easyer to tune and you get that kick.
03-20-2010, 05:45 PM
#10
Melting Slicks
I just have to throw this in. I see people in every carb thread saying this carb or that carb is no good and the only carb to run is X carb.
I have run Holleys, Quadrajets, the old Rochesters, Carters; both AFB and AVS, and if they are matched properly to the application and tuned correctly, (which is almost never done), they all work well!
Match the application and tune it, period!
I have run Holleys, Quadrajets, the old Rochesters, Carters; both AFB and AVS, and if they are matched properly to the application and tuned correctly, (which is almost never done), they all work well!
Match the application and tune it, period!
03-21-2010, 05:12 PM
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I agree with v2racing they are all good when proerly adjusted, I went to a edelbrock because it is easy to adjust ( I think ) and I t was the only one with a mech secondary they had in the store in the size I needed
03-21-2010, 07:33 PM
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Definitely good advice from V2racing, sometimes there may be a tendency to take the easy route and hope/expect a new carb to automagically solve all your performance issues when more often than not it is more a matter of having your system properly tuned.
In my case I don't see much benefit from rebuilding and tuning the old AFB because at the end of the day I have a properly rebuilt and functioning carb that GM abandoned over 30 years ago and I suspect they did it for a reason. Not trying to dump on the AFB or subsequent generations I know there are plenty guys out there who are absolutely pleased with theirs and honestly I'm sure if I had a properly tuned 1406 on mine it would suit the motor just fine...so it comes down to my personal preferences and plans for the car... at the end of the day as long our C3's are cruising down the road and leaving a few fords in the dust it's all good
So part of this is getting my timing/engine properly tuned and the other part is updating a few key components in the process... should be a bit easier to dial everything in with some new hardware.
Side note... haven't pulled the trigger on a purchase as of yet, seriously considering taking Lars advice on the SA Ultra. Can't seem to find any info on what all ships with the carb.. do they ship with fuel lines, gaskets, studs, throttle cable mounting hardware? If it's just the carb itself no problem, any advice on commonly overlooked items that I'll need? Trying to line up all my parts and take care of it in one order.. it's criminal what they charge for freight to this crazy foreign place called Alaska
I'll have to see if I can dig up my screen shot from Tire Rack that quoted me $ 99,999.99 to ship some TTII's... I'd hate to have to sell my new ZR1 to pay form some new treads
In my case I don't see much benefit from rebuilding and tuning the old AFB because at the end of the day I have a properly rebuilt and functioning carb that GM abandoned over 30 years ago and I suspect they did it for a reason. Not trying to dump on the AFB or subsequent generations I know there are plenty guys out there who are absolutely pleased with theirs and honestly I'm sure if I had a properly tuned 1406 on mine it would suit the motor just fine...so it comes down to my personal preferences and plans for the car... at the end of the day as long our C3's are cruising down the road and leaving a few fords in the dust it's all good
So part of this is getting my timing/engine properly tuned and the other part is updating a few key components in the process... should be a bit easier to dial everything in with some new hardware.
Side note... haven't pulled the trigger on a purchase as of yet, seriously considering taking Lars advice on the SA Ultra. Can't seem to find any info on what all ships with the carb.. do they ship with fuel lines, gaskets, studs, throttle cable mounting hardware? If it's just the carb itself no problem, any advice on commonly overlooked items that I'll need? Trying to line up all my parts and take care of it in one order.. it's criminal what they charge for freight to this crazy foreign place called Alaska
I'll have to see if I can dig up my screen shot from Tire Rack that quoted me $ 99,999.99 to ship some TTII's... I'd hate to have to sell my new ZR1 to pay form some new treads
03-22-2010, 02:52 AM
#15
GM might have abandoned the Carter AFB, but it was still being produced by Weber for marine use until Merc went to fuel injection in the mid 90s.
And Edelbrock still sure seems to sell a lot of them. The Edelbrock is the same basic carb as an AFB.
Honestly, unless the throttle shafts are worn/leaking I think it would be worth the effort to rebuild it. Last time I did one the kit was about $35. You can compare current jets/rods/springs to Edelbrocks setup on their website.
BTW, I've got Carters on my cruiser. It starts easier and runs better than most fuel injected boats.
And Edelbrock still sure seems to sell a lot of them. The Edelbrock is the same basic carb as an AFB.
Honestly, unless the throttle shafts are worn/leaking I think it would be worth the effort to rebuild it. Last time I did one the kit was about $35. You can compare current jets/rods/springs to Edelbrocks setup on their website.
BTW, I've got Carters on my cruiser. It starts easier and runs better than most fuel injected boats.
Last edited by Garys 68; 03-22-2010 at 03:45 AM.
03-22-2010, 03:52 AM
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Thanks for the suggestions on the AFB Gary, no doubt a rebuild would improve performance. I may end up rebuilding regardless just for the experience. My Carb searches here on the forum though suggest that although the AFB may be the easiest of the bunch to tune and is very dependable... from a performance standpoint it doesn't measure up to the QJets or Holley's. I'll let Lars sum up the AFB:
That statement carries a lot of weight in my book.
Why in the world would you want to replace a sophisticated, high performance 750 cfm carb with an old, outdated, obsolete, 45-year-old 600 cfm Carter AFB that GM deemed "obsolete" in 1966? The Q-Jet will outperform a 600 cfm AFB by 20 hp on a bad day (I've run them back-to-back on a dyno, and I have the numbers to prove it). Is your car running too well and too fast with the Q-Jet so you need to downgrade it a little...? The AFB is a huge downgrade from the stock Q-Jet.
03-22-2010, 04:20 AM
#17
Hmmm, obsolete...... Careful, one could make that statement about Qjets and fuel injection. lol
I was just pointing out the economics....
Rebuilt you Carter $35, buy an Edelbrock, you have the same basic carb for a few hundred bucks. A holley, a few bucks more.
Go with a Qjet, guessing you will buy used and have to rebuild too.
Or just throw stupid money into it and go EFI.
I was just pointing out the economics....
Rebuilt you Carter $35, buy an Edelbrock, you have the same basic carb for a few hundred bucks. A holley, a few bucks more.
Go with a Qjet, guessing you will buy used and have to rebuild too.
Or just throw stupid money into it and go EFI.
03-22-2010, 05:16 AM
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Thanks Gary, what isn't obsolete right? Dino-juice isn't exactly cutting edge is it Not really looking forward to my ion powered hover-vette, what kind of future can there be without burnouts
I gave up on economics when I rationalized owning a vette in Alaska, may have been more practical to buy a plow truck!
Not really looking forward to my ion powered hover-vette, what kind of future can there be without burnouts I gave up on economics when I rationalized owning a vette in Alaska, may have been more practical to buy a plow truck!
Last edited by AKruizer; 03-22-2010 at 05:20 AM.
03-22-2010, 09:12 AM
#19
One other consideration, If you have a big cam, you'll probably have to do some tweaking on the carb. I can buy Holley jets, powervalves, pump diaphrams, etc at the parts store. And there are entire books available on tuning them Same with a Carter/edelbrock, and their website has a lot of useful info.
Go to the parts store for Qjet tuning parts.....blank stare....
Not sure if Lars travels that far north.
And dont forget to tune up your ignition before you start on the carb.
Go to the parts store for Qjet tuning parts.....blank stare....
Not sure if Lars travels that far north.
And dont forget to tune up your ignition before you start on the carb.
03-22-2010, 10:33 AM
#20
Burning Brakes
Thanks for the suggestions on the AFB Gary, no doubt a rebuild would improve performance. I may end up rebuilding regardless just for the experience. My Carb searches here on the forum though suggest that although the AFB may be the easiest of the bunch to tune and is very dependable... from a performance standpoint it doesn't measure up to the QJets or Holley's. I'll let Lars sum up the AFB:
That statement carries a lot of weight in my book.
That statement carries a lot of weight in my book.
One thing I think you may be missing is that the newer vertion of the AFB (Super Quad, Eldbrock) are no where the same as your old Carter AFB. Someone like Lars may or may not like the newer AFB's but that was not the carb he was talking about in the above quote. Anyway from all I get from reading what you have I would suggest a cam change now, why? no matter what carb and manif. you put on will not correct the wrong cam. You will be redoing you carb and manif. instalation to in stall a cam so I would strongly suggest you do it now and then get into all that talk about which one to use. Well make it simple for your self and check out something the full cam manif. carb packages out there and then just ride and smile
my 2cents
