Carb swap options with 4300A-equipped M-code '71 (351C)

Hello fellows,

As I note that the 4300 doesn't particularly get rave reviews - not to mention that I've heard of a few fires starting due to a bad plug in front - I've been considering swapping to a Holley or Carter AFB.

That said, I have yet to pull the carb off the intake to verify what I have. Casting number on the intake is DOAE-9425-L, but that's a standard casting number for both the four and two barrel manifolds.





Nevertheless, if my research is correct, I should have a squarebore 4300A (P/N #D10FAAA) on a matching squarebore intake, which may or may not have a hot air passage on it (some places claim 1970-only; others say it was used in '71 as well).

That said, following are my questions:

1. I've read something about the 4300's having a slightly different throttle plate size than similar carbs. I believe these comments are in reference to the spread-bore 4300D's, and does not apply to the 4300A. Am I correct in this assumption?
   
2. If this thing has a hot air passage on it, do I have to fandangle any particular blockage plate or adapter to use a Holley or Carter AFB/Edelbrock?
   
3. Which carb will require the least amount of adaptation to the factory linkages? It's a 351C with an automatic (has kickdown). I don't know yet whether I have a a C6, C4, or FMX hiding down there.
   

Advice appreciated.

Best,

-Kurt

I had a squarebore 4300 on my 429. It had a hot air passage for the choke, but the tube internal to the engine and the tube running up to the choke were separate tubes (the "external" tube running up to the choke fit into the "internal" tube). With a little effort you can pull the external tube off the mounting plate without removing the plate, and if the internal tube is intact you will not have a vacuum leak. I swapped it with a squarebore Holley and it fit just fine. Look for a Holley with the Ford kickdown linkage. I had to grind a bit off the throttle linkage to miss the intake, but that bit I ground off was only used on Chryslers. NOTE: some carbs with the Ford kickdown will not work with AOD transmissions.

I had a squarebore 4300 on my 429. It had a hot air passage for the choke, but the tube internal to the engine and the tube running up to the choke were separate tubes (the "external" tube running up to the choke fit into the "internal" tube). With a little effort you can pull the external tube off the mounting plate without removing the plate, and if the internal tube is intact you will not have a vacuum leak. I swapped it with a squarebore Holley and it fit just fine. Look for a Holley with the Ford kickdown linkage. I had to grind a bit off the throttle linkage to miss the intake, but that bit I ground off was only used on Chryslers. NOTE: some carbs with the Ford kickdown will not work with AOD transmissions.

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I'm not sure I explained it correctly - the car has an electric choke; not a hot-air choke. The "hot air passage" I'm referring to is the milled airway shown forward of the throttle bores in this picture:

(From: http://mustangtek.com/FordIntake.html)



It is also present on some of the 2V intakes.

EDIT: I think I just stumbled upon the answer - it's nothing but a heat passageway: http://www.stangnet.com/mustang-forums/threads/1970-351c-4v-intake-manifold.796922/ My mistake.

-Kurt

I think you need to plug it like the photo.

The manifold is stock 351C 4V and a Holley

with electric choke.

mike

The 4300A is not a perfect square bore. The primaries are slightly smaller than the secondaries. Not a pronounced difference like a 4300D. The intake is also not a perfect square bore. The intake can be made a true square bore easily with a die grinder if the intake is off the engine. The original choke would have been hot air heated. If the tube loop that is on the bottom of the plate shown in the picture is intact there won't be an exhaust leak (the loop sits in the exhaust crossover passage cast into the intake).

Everyone has their preferences on carburetors. For non-stock carbs I prefer Quick Fuel or Holley. The Quick Fuel carbs have more adjustability and that is helpful when tuning the carb to your specific application. Chuck

The 4300A is not a perfect square bore. The primaries are slightly smaller than the secondaries. Not a pronounced difference like a 4300D. The intake is also not a perfect square bore. The intake can be made a true square bore easily with a die grinder if the intake is off the engine. The original choke would have been hot air heated. If the tube loop that is on the bottom of the plate shown in the picture is intact there won't be an exhaust leak (the loop sits in the exhaust crossover passage cast into the intake).

Everyone has their preferences on carburetors. For non-stock carbs I prefer Quick Fuel or Holley. The Quick Fuel carbs have more adjustability and that is helpful when tuning the carb to your specific application. Chuck

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In short, putting anything but a 4300A on this intake will require that the intake come off for modification, correct?

If necessary, I can have tubing bent to match the position of the choke on a replacement carb; that's no problem.

-Kurt

If you are OK with leaving the primary holes as they are, a short (.250-.500) spacer plate/heat isolator that matches the bores of the new carb will allow the primary throttle plates to function properly and you won't need to remove the intake manifold. Something like this. http://www.holley.com/108-18.asp Chuck

Chuck - interesting! I put the Holley on my intake and manually operated the throttle and there seemed to be no problem with the function of the carb. The primary/secondary throttle plates aren't hitting the intake. Ran pretty good, just rich (but I'm now thinking that it's related to the ignition system). I haven't forgotten about your help troubleshooting my low vacuum issue but the car is still in the body shop.

Edit: Were all 4300 variations a not-quite-square bore? Because looking at cudak888's picture, I can see how the primaries are slightly smaller but I don't recall that on my 1970 429 engine.

Chuck - interesting! I put the Holley on my intake and manually operated the throttle and there seemed to be no problem with the function of the carb. The primary/secondary throttle plates aren't hitting the intake. Ran pretty good, just rich (but I'm now thinking that it's related to the ignition system). I haven't forgotten about your help troubleshooting my low vacuum issue but the car is still in the body shop.

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Which intake do you have? Also a '71 M-code, I assume?

I'm guessing here, but it would sound as if the only logical explanation for what you describe is that the Holley plates are the same size (or smaller) than the smaller throttle bores on the stock intake.

Edit: Were all 4300 variations a not-quite-square bore? Because looking at cudak888's picture, I can see how the primaries are slightly smaller but I don't recall that on my 1970 429 engine.

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According to c9zx, that is correct - even the 4300A isn't a perfect square-bore, and the bores differ between the relative size. Here are two photos from eBay:

A. Seller doesn't ID this one except for a Delco rebuild tag. The slight difference between the secondaries and primaries can be seen in this photo. I'd say it's safe to assume this is a 600 CFM example with 1.25 venturis:



B. This next 4300 has noticeably smaller secondaries; seller claims this one is a '69 carb from a 351W, C9OF-D. According to the following .PDF article from the Mustang Times ( http://mustangtimes.net/33-11/pg48-50.pdf ), this is the 441 CFM variation, 1.00 venturies.



Just to clarify sm3570's post, the Mustang Times article indicates that a 1970 429 would have come stock with the 4300A 600 CFM model - in which case, it would conceivably match example "A" above.

-Kurt

Unless you are going for originality, I would yard that boat anchor off, stash it in a corner, and drop an Edelbrock performer in it's place, with a 600 vac secondary holley.

Unless you are going for originality, I would yard that boat anchor off, stash it in a corner, and drop an Edelbrock performer in it's place, with a 600 vac secondary holley.

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I'm looking for originality with the utmost of drivability. I'm partial to the Carter AFB myself (I'm not quite keen on the idea on having a high-polish Edelbrock peeking out from under the air cleaner), or a Holley, but I want to understand what pitfalls to expect before I prepare for the swap.

-Kurt

Okay. I'd do what chuck suggested, run a short 1/2" spacer, and 600 cfm carb or your choice. An AFB would probably be easier to hide than a holley. And the AFB doesn't have the goofy bolt on ford kickdown like the edelbrocks have.

I've been checking some specifications, and found the following:

4300A, 600 CFM:

Primary throttle bore: 1-9/16"

Secondary throttle bore: 1-11/16"

Holley, 600 CFM - #0-1850C:

Throttle bore, primary + secondary: 1-9/16"

Carter AFB, 575 CFM - #3721SB:

Primary: 1-9/16"

Secondary: Couldn't find it

Carter AFB, approx 575 CFM (for AMC):

Primary: 1-7/16"

Secondary: 1-11/16"

Obviously, I'm having a bit of trouble nailing down what's what (Case in point, I believe #3721SB is primarily a Chevy application), but it looks to me as if the throttle bore of the intake won't be too small - if anything, the intake's secondaries will be a bit too large in comparison to the carb. Provided the gasket is designed correctly, I dare say I can get away with that, no?

-Kurt

I've been checking some specifications, and found the following:

4300A, 600 CFM:

Primary throttle bore: 1-9/16"

Secondary throttle bore: 1-11/16"

Holley, 600 CFM - #0-1850C:

Throttle bore, primary + secondary: 1-9/16"

Carter AFB, 575 CFM - #3721SB:

Primary: 1-9/16"

Secondary: Couldn't find it

Carter AFB, approx 575 CFM (for AMC):

Primary: 1-7/16"

Secondary: 1-11/16"

Obviously, I'm having a bit of trouble nailing down what's what (Case in point, I believe #3721SB is primarily a Chevy application), but it looks to me as if the throttle bore of the intake won't be too small - if anything, the intake's secondaries will be a bit too large in comparison to the carb. Provided the gasket is designed correctly, I dare say I can get away with that, no?

-Kurt

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My post assumed you wanted a 750 or larger carburetor (1-11/16 all four throttle bore). Any particular reason you are considering a 600 cfm carb? Do you have an AFB with a ford kick down lever? Chuck

My post assumed you wanted a 750 or larger carburetor (1-11/16 all four throttle bore). Any particular reason you are considering a 600 cfm carb? Do you have an AFB with a ford kick down lever? Chuck

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Seeing that the 4300A is 600 CFM, I figured I'd stay with the same. It's a bone-stock M-code, and I'm not too concerned about performance. Is there any particular reason that would make the 750 superior to the 600 for this application?

I don't have any AFB - yet, or a Holley. I figured I'd start looking once I know whether there is an AFB with compatible bores.

-Kurt

Just to clarify sm3570's post, the Mustang Times article indicates that a 1970 429 would have come stock with the 4300A 600 CFM model - in which case, it would conceivably match example "A" above.

-Kurt

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Good info, Kurt. I'll have to take a look at mine when I get it back. When I rebuild the engine I'll probably put another intake, a more aggressive cam and a 750 CFM carb on it but for now the 600 CFM Holley does the trick nicely.

Unless you are going for originality, I would yard that boat anchor off, stash it in a corner, and drop an Edelbrock performer in it's place, with a 600 vac secondary holley.

Click to expand...

I'm looking for originality with the utmost of drivability. I'm partial to the Carter AFB myself (I'm not quite keen on the idea on having a high-polish Edelbrock peeking out from under the air cleaner), or a Holley, but I want to understand what pitfalls to expect before I prepare for the swap.

-Kurt

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I believe Edelbrock bought Carter quite a few years back.

I got the same setup 351c-4v and put an Edel Performer 750 carb on the stock manifold with a 1" aluminum spacer about 4 years ago. It Still runs much better than the 4300 ever did, much better drivability, no more gas leaks and it definatly has more power in in the entire range. I recommend the bigger carb, it felt like it gained 20hp, especially in top end, but that is unverified on a dyno.

Others (on other forums) have said that you won't gain much power by going with the Performer manifold though. The stock 4v manifold is actually pretty good.

As 1971Mach1 stated the stock intake is pretty good especially when used with a 1" spacer (I like HVH the most). The Ed performer for 4V is no better and I feel the stock intake out performs the Ed above 5000 RPM. A 750 cfm Autolite/Motorcraft carb was used on the Boss 351/HO engines in 71/72. Chuck

I pulled the 4300 this morning and found three gaskets shoved underneath it: Two squarebore w/1-11/16" ports throughout, and a three-barrel gasket for a Holley 3160 below that. Sounds like someone was overcompensating for the lack of having the proper gasket with the steel insert.

At any rate, I've been futzing around with a 600CFM Holley 4160 (1-9/16" ports throughout) on top of the intake. The 3/16" difference in size doesn't seem to be much of an issue on the secondary ports, and the primary ports - which are identical in size - fit decent (though it's not an utterly-precise fit, I noted).

Unfortunately, the main body of the 4160 that I'm working with is a bit rough inside the primary venturis, and it doesn't appear to have a provision for installing the ball-check to prevent the power valve from blowing out. I might buy a second body to work with and mount the parts on that.

I also found the hot air crossover blocked off on each side with a gummy sort of material. Now the big question - leave it, or open it back up?

FYI: Base plate of the original 4300A carb has the same number as the top tag. D1OF-AAA.

EDIT: I posted a new thread about the alternative PCV routing and valve for use with the Holley: http://www.7173mustangs.com/thread-re-plumbing-the-factory-pcv-to-a-holley-4150-4160

-Kurt