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4300A four barrel carburetor re-build advice?

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1972Bird View Drop Down
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    Posted: 23-September-2021 at 4:06PM
Hi All,

I have a 72 Tbird with the 429, stock and I'm looking for advice on setting up/trouble shooting the factory 4300A carb.

I rebuilt the carb with new gaskets, accel pump cup, needle/seat and installed a new float and an electric choke from Mike's Carb parts in WA. 

There is a little play on the throttle shaft but not a lot and I never had any lean mix/vacuum leak symptoms before the rebuild, so it can't be that bad.  Even if I wanted to address that, I don't believe there is enough material in the the plate to allow for a drill/ream/bushing operation like they can do with other carbs, so I left it alone. 

I set the choke by the book and I've got it to the point where it seems to start up reasonably well and runs OK, for a while...  After it warms up a bit and I take it off fast idle I've driven it around town at slow speeds.  Steady state driving at 20-30 MPH around town seems fine at first but after a short period of time it often stalls out and/or floods intermittently and starts running rough. 

I've had it apart several times now and the last time I set the float to 28/64" which is a bit higher than spec, hoping to solve the flooding.  I even built a facsimile of the factory float height tool and I was careful to bend the metal tabs and not abuse the pontoons. 

I carefully checked the float alignment (not much room between the pontoons and the carb body that's for sure) and even trimmed the bowl to airhorn gasket in the spots where the float might catch.  It seems better but it still floods.

I've read that these carbs are considered "tricky" (or a POS...) and they generally seem to have have a poor reputation but if anyone has successfully tinkered with the 4300 and can offer some tips, I would be grateful.

Before you say it; no I don't want to replace it with an Edelbrock or Holley because I would like the keep the car original if at all possible.  Also, I have a hard time believing the 4300 can't be made to work when you consider the sheer number of vehicles running these carbs in the 70's.  Sure there were issues with some or even many of them, but the entire fleet???

Anyway, I don't want to give up on it just yet so please let me know if you have any 4300A secrets to share.

Thanks,
James

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 25-September-2021 at 6:23AM
I have never messed with 4300 carbs.  I can understand that want to keep it original, but I have been there and sometimes it makes life more difficult when it comes to odd ball parts.  That said, there are a few members on the forum who have rebuild 4300 carbs through a company called Allstate:


There is some info on this thread about the carb rebuild. 

Vince

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 25-September-2021 at 4:25PM
I would say if the carb is intact as it was running before, not yet taken apart at all, carefully measure the adjustable items which the rebuild kit will specify.

I rebuilt my original 2-barrel carb of my 351C-2V, and it didn't work as well as the original assembly. The choke adjustment the kit described for the adjustment, was off from the OEM Ford specs. That was ages ago, maybe 1979, and I forgot what adjustments there were.

But I wished I could have measured those little detail items before taking the carb apart. I would want to be able to duplicate the original adjustment specs, not what the rebuild kit tells you to.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aquartlow Quote  Post ReplyReply Direct Link To This Post Posted: 26-September-2021 at 4:41AM
I understand the want to keep it original, I tried to do somewhat the same thing years ago on a '76 460 I rebuilt and installed into my '82 F250 4x4, had the '76 Motorcraft 4350 laying around and meticulously rebuilt it to re-use it. It was just a waste of time and money for me. I can tell you once you install a Holley or even an Edelbrock and feel the extra power of what seems like 2 extra cylinders, easier starting and better overall drivability(once tuned to your engine), I believe the idea of re-using the Motorcraft 4300 will be a thing of the past. Keep the 4300 for originality if you ever want to sell, but use a 600 or 750 VS for everyday use, you'll be much happier. I realize this may not or is not the answer that was originally asked, but just being honest with my 2 cents and past experiences.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 26-September-2021 at 5:12AM
are you running E10  / 10% ethanol fuel?

it runs a little 'leaner' than the old mix from 1972, and cooler. by 1972 the EPA mileage / emissions battle was already on the horizon, the 4300's were already Lean On Delivery. i'd try stepping up the main jet a couple numbers, possibly opening up the idle feed restriction orifices just a skosh

what thermostat do you have in it?

sounds to me like she might like a little more temp in the carb, atomization is King. today's fuel blend is engineered to be delivered from a pintle injector at 40-50 psi, not for being sucked from a venturi main booster at atmospheric pressure
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1972Bird Quote  Post ReplyReply Direct Link To This Post Posted: 27-September-2021 at 6:25AM
Thanks All,

I spent a good amount of time on the Internet researching and the vast majority of posts say to replace it with an Edel/Holley/etc.  I get that and if money were no object then I would buy something else just to solve the problem and move on the other things.   I really don't want to (won't) spend $400-600 for a new carb right now, especially since it's the end of the driving season for this car.

I worry about the calibration of any replacement carb although you are right in that all the gas in my area is 10% ethanol so maybe the stock calibration is wrong anyway.   I also could not figure out what carb is a direct bolt on to the stock Ford intake (Summit had no listing for my car....), which has spreadbore openings of a unique diameter, but apparently a square bore bolt pattern???  Thinking about strange mixture distribution problems sent me back to the 4300 but if anyone knows for sure what carb model and part number is a direct bolt on then please shout out.

All I need right now is for the car to start and run, even briefly, so I can move it in and out of the shop when I work on other projects.  I am coming around to the idea that I'm trying to "polish the turd" in that the original design was compromised by Ford from the start in order to meet emission regs and fuel economy goals.  I will persevere for now but if I can't get it the way I want by next Spring, clearly I will need to replace it.

I pulled it off again yesterday and took it apart thinking I would return the float height to stock and really check it over closely.  Setting the float took forever because I was very careful to get both pontoons exactly the same and also positioned between the float bowl sides as centered as possible.  I had a feeling that perhaps they were intermittently hanging up on the gasket between the bowl and air horn castings so I further trimmed the gasket in a few spots with an Exacto knife and even "deburred" the float a bit as there is a fairly thick molding line exactly at the worst possible place.  I measured the floats and they are slightly wider than the original float, so that makes it even worse. 

I also tweaked the float hinge pin area to remove as much lateral play as possible between the two posts in the bowl casting.  This is really tricky to do with making the float bind but I did the best I could and I think it's less sloppy now.  I did check the brass pin and it looks OK.  It's straight, there are no grooves worn in it and it slides in and out easy enough.

Before disassembly I measured the accel pump height and noted it is a bit lower than stock, so I will bend the arm to achieve 7/16" before re-installation.

At this point I decided to check the castings for flatness.  Should have done that as step one several months ago......  What a maroon!!!!  Sometimes I really wonder what I'm (not) thinking...  The air horn is flat, the throttle plate is flat, but the bowl casting is high in the center by as bet as I can tell, about 1/16".  I used a 12" metal ruler  form a carpenters level and depending on which angle across the body you lay the ruler, the far end leavs a gap from 1/32" to 1/16".

There is a pronounced "bulge" in the center where the air filter housing bolt goes in so I presume that over decades of heat cycling the tension of that bolt will actually raise the casting up.  What's really strange to me is how only the bowl casting is affected.  You would think the air horn casting would bend too but I can't detect much if any similar distortion.

Anyway, not to say that warpage is the root cause of my issues.  In fact I have a feeling that the flooding/stalling symptom is a result of many things adding up together AKA "tolerance stackup" and that may be why the 4300 carbs have a bad rep.

So now I feel compelled to try and fix the warped bowl casting before giving up on the carb.  If it is truly "junk" then nothing I do will make my situation worse other than leaving me without any carb at all. 

Apparently the Q-jet people bolt theirs to a heavy steel surface plate jig then heat them to 350 F for a few hours but I'm not sure that will work here because the 4300's fasteners are quite small and I'd have to make a suitable steel jig.  Milling is out because I'm not a machinist and don't own a mill.  Also, the casting has some rather thin wall sections in places and I worry any sort of milling will simply crack or break some of those areas.

Don't laugh, but I'm seriously considering my bench-mounted belt sander....  Will ponder this for a bit and update you all later.

James




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Post Options Post Options   Thanks (0) Thanks(0)   Quote mkshelton Quote  Post ReplyReply Direct Link To This Post Posted: 27-September-2021 at 7:17AM
Originally posted by 1972Bird 1972Bird wrote:



At this point I decided to check the castings for flatness.  Should have done that as step one several months ago......  What a maroon!!!! 

 Sometimes I really wonder what I'm (not) thinking...  The air horn is flat, the throttle plate is flat, but the bowl casting is high in the center by as bet as I can tell, about 1/16".  I used a 12" metal ruler  form a carpenters level and depending on which angle across the body you lay the ruler, the far end leavs a gap from 1/32" to 1/16".

There is a pronounced "bulge" in the center where the air filter housing bolt goes in so I presume that over decades of heat cycling the tension of that bolt will actually raise the casting up.  What's really strange to me is how only the bowl casting is affected.  You would think the air horn casting would bend too but I can't detect much if any similar distortion.

Anyway, not to say that warpage is the root cause of my issues.  In fact I have a feeling that the flooding/stalling symptom is a result of many things adding up together AKA "tolerance stackup" and that may be why the 4300 carbs have a bad rep.

So now I feel compelled to try and fix the warped bowl casting before giving up on the carb.  If it is truly "junk" then nothing I do will make my situation worse other than leaving me without any carb at all. 

Apparently the Q-jet people bolt theirs to a heavy steel surface plate jig then heat them to 350 F for a few hours but I'm not sure that will work here because the 4300's fasteners are quite small and I'd have to make a suitable steel jig.  Milling is out because I'm not a machinist and don't own a mill.  Also, the casting has some rather thin wall sections in places and I worry any sort of milling will simply crack or break some of those areas.

Don't laugh, but I'm seriously considering my bench-mounted belt sander....  Will ponder this for a bit and update you all later.

James





Couldn't help but hear your " what a Maroon" as Bugs Bunny in my head!
Another thing you could try is to ask around any local speed shops for someone who sells/rebuilds carbs. There is a guy in my local area who rebuilds and sells carbs of all types as a side biz and he does very good work,  he advertises on on craigslist, best of all, he is open with advice for anyone calling if they have problems and offers pretty good ideas for solutions. He typically sells used rebuilt hollers for under $300 and helps make sure the fit on the car is good.
Also, whenever you do a carb R and R, it is pretty easy to miss hooking up a vac hose that you may have disconnected either on purpose or accident. If that is the case, you will be chasing your tail forever.  Take a look at everything under the hood and also the vac lines going to the climate controller and trans modulator. Sorry if these are "duh" obvious and that you have taken all of that into account, but I can honestly say that I missed a vac line before and thought my carb was ruined.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1972Bird Quote  Post ReplyReply Direct Link To This Post Posted: 27-September-2021 at 12:41PM
Appreciate the suggestion and yes I am a huge fan of Bugs Bunny!  That Foghorn Leghorn and Yosemite Sam were a pretty badass dudes also.

I love all the original Looney Tunes and despair that if you happen to see any these days they are heavily edited to remove all the non-PC content, which is to say a good deal of it.....  I get that a lot of the original content is not acceptable these days, but as an adult who can think for himself and recognize the biases for what they were, I still think those cartoons are great. 

I owe everything I know about physics and engineering to Wile E Coyote and the wonderful products he purchased from the ACME Corporation LOL.

I noticed the bowl casting has a brass tube sticking up and I expect that is pressed in during manufacture, so probably very difficult to remove without damage and that will make the belt sander idea a bit more challenging.  Hmmm.  Will think on this further.

James
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1972Bird Quote  Post ReplyReply Direct Link To This Post Posted: 11-October-2021 at 4:16PM
Hi All,

Sorry for the long time between updates.  I had a lot going on in the last few weeks and just did not have any time for the car.  I was dreading the battle with the carb and it did put up a good fight, but I think I have it fixed now.

I really sympathize with all the people online who have messed with the 4300 and given up.  I don't know that much about carbs in general and this is the only 4300 I've ever worked on but from my recent experiences and what I've read online it seems the biggest problem is flooding.

I think the root causes of most flooding problems have to do with the float sticking and warped castings.  Calibration/performance is a whole other subject I am not qualified to comment on so the following is what I did to get the damn thing to just work.

As I mentioned earlier my bowl casting was about 1/16" proud in the center area where the air cleaner hold-down stud is located.  I ended up using an air powered mini angle grinder with a small sanding wheel to sand down the center of the float bowl casting.  This was delicate work as I did not want to remove too much material and junk the carb so I was trying to feather the material removal out from the center.  I ended up taking material off about 2" out from the center stud, in all directions.  The perimeter of the casting was untouched and I stopped frequently to check "flatness" in multiple directions using a steel ruler.

To prep for this I put set screws into all the threaded holes and plugged up everything else including the bowl itself with bits of packing foam, rolled up paper etc.  I also used little bits of masking tape to cover some very small brass orifices in the venturis.  The carb now appears dry on the outside whereas before it always had a kind of "wet look" to it, so I think there was a lot of seepage internally and externally because of the warped bowl casting.

The second issue is the float itself.  I put a new float in that I got from Mikes Carbs in WA but I had a heck of time trying to get the pontoons level and at the correct height setting.  I'm currently at 25/32" whereas the rebuild kit instructions say to use 49/64" (24.5/32").  I spent hours on it and it's the best I could do.

There are three other issues related to the float that I think are the cause of so much bad feeling about this carb:

1. The float has molding lines along the pontoons (so does the original Ford float) and the fuel bowl is quite narrow in a few spots, so I think the pontoons can easily catch on the gasket between the float bowl and air horn.  Also, the new float is a few thou wider than the original.  It's so close to casting walls that I believe this really matters, so I used an Exacto blade to trim the molding flash a little bit.

Bending the metal tab to center the pontoons usually screws up the height adjustment, so this  process is extremely challenging because you have to be dead on accurate or the float will catch.  Bottom line is this took me hours to mess with until I was satisfied it was as good as possible. 

Ideally the float would be a little narrower or the casting a little wider.  I wonder what would happen if the float was really thinned down where the clearance is tight but I'm not brave enough to find out.

2.  The metal tab on the new float was bent in such a way as to be a little sloppy when installed between the two "posts" in the bowl casting, so it could move side to side a fair bit.  This means the pontoons are even more difficult to set so they don't catch.  I did my best to adjust the metal tab so there was the least amount of side to side play, but the float still pivoted up and down with out any friction.  I also inspected the brass pivot pin and it looked OK.

3.  The metal tab is also slightly different than on the Ford float and the right side appeared to be very close to the accelerator pump spring.  So close in fact that I'm pretty sure I had assembled the carb with the float stuck in the closed position and that's why I could not get any gas into it!  Off came the carb for the eight time..... 

I fixed that by carefully using the mini angle grinder to sand away a little bit of the metal tab where it was likely contacting the pump spring.  After that incident I made sure to check the float would open after assembling the carb by blowing through the fuel filter inlet and listening for air through the vent.  When it was stuck closed I blew so hard that I saw floaters in my eyes and could not get the float to open.  At least I know the needle and seat are good.....

The third issue is the gasket between the float bowl and air horn casting.  A very small amount of gasket material sticks out from the casting walls near the float pontoons.  Because of the tight clearances already mentioned it really, really, really needs to be trimmed back so there is NO CHANCE for it to catch on the pontoons.  I ended up trimming the gasket in the appropriate locations using an Exacto blade.  It's difficult because the gasket width is less than 1/8" and of course it's not rigid, so it moves around a bit. 

My bottom line 2 cents on the 4300 is that, after you fix the warping issue, the assembly tolerances can easily stack up beyond acceptable limits and cause the float to operate erratically.  I think this is why I had intermittent running OK and then suddenly after going down a steep hill or around a tight corner, all of a sudden the thing starts bogging/stalling etc.  There is ZERO extra room for the float pontoons, so everything has to be perfect for the carb to work.

I hope that's the end of this story and I also hope this may help someone else in the future who is trying to make a 4300 work.

Cheers,
James


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Post Options Post Options   Thanks (0) Thanks(0)   Quote rcougarman Quote  Post ReplyReply Direct Link To This Post Posted: 14-October-2021 at 9:28AM
I have used and tinkered with 4300's since 1977 (1972 GTS w/429) and on other engines and cars as well.  My 429 ran great with this all the time I owned it and it gave decent fuel economy (17 mpg highway).  
I think you hit upon the major issue (warping of the main casting).  My most recent venture of trying to get one working right (not flooding/leaking) was on my 351C 72 GTS, I finally gave up after numerous attempts to fix it and get reliability.  I probably should go back and try the manual grinding you describe.  I was able to find a good deal (just over $300 from JEGS) on an Edelbrock AVS2.  I think it is the best carburetor I have ever ran.  Put it on, adjusted the idle speed and mixture and good to go.  If you decide to move from the 4300, please consider the AVS2.  Some say it is the nearest to FI performance you can get with a carburetor.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 14-October-2021 at 12:42PM
Great info on fixing a 4300.  You will have to keep us posted on how well the car runs.  Do you have any pics of the process?

As for the Edelbrock AVS2, this post has some great info and review on one.

Vince

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1972Bird Quote  Post ReplyReply Direct Link To This Post Posted: 18-October-2021 at 12:10PM
Hi Guys,

I haven't been able to drive the car because the weather has been crappy but I do want to get it out again just to make sure it truly is fixed. 

Up until now I've been pretty good about taking photos, but unfortunately I didn't take any after sanding the casting down.  I wish I had now because it would have been easier for someone to see where I stopped taking material off.  In this type of situation I firmly believe "less is more" but of course every carb will have a different degree of warpage.

Anyone who tries to level the casting is well advised to proceed slowly and stop frequently to check their progress using a good metal straightedge and from multiple directions across the casting.  That last part is very important because the warpage may not be even from front to back or side to side. 

Also, you obviously need a very fine touch when sanding near the brass tube that sticks up but anyone with reasonable motor control should be able to manage.  I used both hands to steady the air grinder and ran it at a low speed.  Another thing I did was to buy some cheap reading glasses from the dollar store to serve as "magnified eye protection".  That allowed me to see what I was doing much better and not care if the glasses got damaged (they didn't).

Thanks for the tip on the AVS2.  That's where I will head if the 4300 doesn't last.

James




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