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So, the 429 is topped by an Autolite 4300 carb. At sea level it ran great, up here at 7,000+ feet it runs rich and performance sucks (but the engine sounds great). I have a couple questions:

 

1) I've been told the carb needs to be rejetted for high altitude. I know I have to retune it, but my experience with carbs is minimal. I've got a bunch of books I've been reading, but they are lean on 4300 information. How tough would doing this stuff be?

 

2) Should I look at investing in a better carb? I've read very mixed reviews on the 4300 (it was rebuilt ~10 or 12 years ago and only has about 50 miles on the rebuild).

 

3) For the 429, does the 4300 flow enough or should I look at a carb that flows better?

 

Note: I'm not looking at racing, just cruising. Also, MPG's are good!

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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In my opinion re-jetting your 4300 would be a good bet. See if you can find a parts breakdown to figure out how to replace the jets. I'm not sure where to find them, maybe the Ford dealer.

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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Thanks Jeff, I'm surprised how little information I've been able to dig up on the interwebs but it doesn't look all that tough to re-jet. I just don't know what size to put in (I don't even know what size is currently in there). I posted over on 429-460.com as well, if I get any good info from over there I'll cross-post in case other newbies have questions.

Steve

 

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pull the top off, the jets are brass right under the float. You can simply get a set of small bits and drill them out. I'd go about .002 larger

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pull the top off, the jets are brass right under the float. You can simply get a set of small bits and drill them out. I'd go about .002 larger

 

Really? I thought I'd have to go smaller to get a leaner mix (granted, I know next to nothing about this stuff!). Higher altitude and all...

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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pull the top off, the jets are brass right under the float. You can simply get a set of small bits and drill them out. I'd go about .002 larger

 

Really? I thought I'd have to go smaller to get a leaner mix (granted, I know next to nothing about this stuff!). Higher altitude and all...

 

Hey sm3570,

I keep a little sticky on my tool box to remind me the 101's about jettings. I use this for my motorcycles and I'm sure it's the same for cars.

 

Smaller Jet = Higher Altitude (Leaner Condition)

Larger Jet = Lower Altitude (Rich Condition)

Air Temperature High = (less air so you have lean out the mixture)

Air Temperature Low = (more air so you have to richen the mixture)

 

Altitude High = (lean the mixture)

Altitude Low = (Richen the mixture)

 

Humidity High = (Lean out the mixture)

Humidity Low = (Richen the mixture)

 

Also, the autolite 4300 is 605cfm and the 4300D's were 715cfm. I just replaced my 750 for a 600 on my 351C and it runs like a champ straight out of the edelbrock box. I recently picked up a 71 with a 429 and it has a holley 850 on it. Never got around to firing it up yet, but I will soon.

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pull the top off, the jets are brass right under the float. You can simply get a set of small bits and drill them out. I'd go about .002 larger

 

Really? I thought I'd have to go smaller to get a leaner mix (granted, I know next to nothing about this stuff!). Higher altitude and all...

 

Hey sm3570,

I keep a little sticky on my tool box to remind me the 101's about jettings. I use this for my motorcycles and I'm sure it's the same for cars.

 

Smaller Jet = Higher Altitude (Leaner Condition)

Larger Jet = Lower Altitude (Rich Condition)

Air Temperature High = (less air so you have lean out the mixture)

Air Temperature Low = (more air so you have to richen the mixture)

 

Altitude High = (lean the mixture)

Altitude Low = (Richen the mixture)

 

Humidity High = (Lean out the mixture)

Humidity Low = (Richen the mixture)

 

Also, the autolite 4300 is 605cfm and the 4300D's were 715cfm. I just replaced my 750 for a 600 on my 351C and it runs like a champ straight out of the edelbrock box. I recently picked up a 71 with a 429 and it has a holley 850 on it. Never got around to firing it up yet, but I will soon.

 

That's kinda what I figured, but how much smaller is the question. I'll tune the carb first and see how it goes. I know the 4300 came stock on the earlier 429's and since I'm not racing it then 600cfm might suffice, but I guess I'll just have to see. Thanks for the feedback!

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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ooops !

 

Lol, not many people realize the altitude that I'm at. I feel the elevation every time I strap on my running shoes!

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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is the 429 bone stock? if so, then maybe a new 650 carb would do the trick. summit have their own carbs that are very inexpensive. if its not stock then most 460's can do absolutely fine with a 750.

 

just a fyi, having a too small of a carb can hurt just as much if not MORE, than having a too big of a carb.

 

for changing jets on holley's, the front four bolts in the front of the carb will drop the entire front bowl and the jets will be exposed.

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is the 429 bone stock? if so, then maybe a new 650 carb would do the trick. summit have their own carbs that are very inexpensive. if its not stock then most 460's can do absolutely fine with a 750.

 

just a fyi, having a too small of a carb can hurt just as much if not MORE, than having a too big of a carb.

 

for changing jets on holley's, the front four bolts in the front of the carb will drop the entire front bowl and the jets will be exposed.

 

I think it was rebuilt to stock specs except POSSIBLY less compression. The engine ran great at sea level, no pinging. I haven't heard any pinging and I got into it pretty good. It bogs down when you hit the gas at slow speed, but idles great and seems to have decent top-end power. Last, it smells VERY rich (ran the wife out of the house with exhaust fumes, and the interior garage door was closed! :D ).

 

I'll try tuning it and adjusting the curb idle, I'm hoping that leaning out the fuel-air mix will do the trick.

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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When tuning a carb most just change jets. BTW if you drill jets they may actually flow less due to surface finish disruptions. Jets are broached which eaves a very smooth surface, drilling leaves spiral lines around the surface. The other (and WAY more touchy) way to lean out the mixture is to increase air bleed size. This provides more air to the emulsion well and breaks the fuel into smaller droplets. Some times you can get a good cruise mixture but have a flat spot in transition, decreasing air bleed size can help the transition. It is ALL cause and effect, sometimes it is easy to get confused. There are some very good carb tuning (specifically Holley) articles I have saved. If you are interested shoot me a PM and I'll forward them.

 

As far as how far to reduce the jet size that is b'guess and b'gosh. Go one step at a time until your plugs look good. With ethanol in fuel you will run more jet than it seems like you should. We have noticed about a 5% increase in fuel consumption on E10.

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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When tuning a carb most just change jets. BTW if you drill jets they may actually flow less due to surface finish disruptions. Jets are broached which eaves a very smooth surface, drilling leaves spiral lines around the surface. The other (and WAY more touchy) way to lean out the mixture is to increase air bleed size. This provides more air to the emulsion well and breaks the fuel into smaller droplets. Some times you can get a good cruise mixture but have a flat spot in transition, decreasing air bleed size can help the transition. It is ALL cause and effect, sometimes it is easy to get confused. There are some very good carb tuning (specifically Holley) articles I have saved. If you are interested shoot me a PM and I'll forward them.

 

As far as how far to reduce the jet size that is b'guess and b'gosh. Go one step at a time until your plugs look good. With ethanol in fuel you will run more jet than it seems like you should. We have noticed about a 5% increase in fuel consumption on E10.

 

Thanks Jeff, PM sent!

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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most kits come with good instructions.you can talk to a tech and they can suggest a jet.best is to get a air fuel ratio gauge and see where you are at and see where you have to go.jetting a carb is very simple ,just follow directions and keep everything clean.

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Like Jeff said, go one step in jet size at a time, take her for a spin and then read your spark plugs. For 7000ft elevation, one step should be good. then do your fine tuning with the air/fuel mixture screw(s).

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Thanks again guys! I'll give it a shot...

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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2) Should I look at investing in a better carb? I've read very mixed reviews on the 4300 (it was rebuilt ~10 or 12 years ago and only has about 50 miles on the rebuild).

 

I would consider a nice Holley. If you move between high and low altitude and stay at either for a period of time, the jets are easy to exchange on the Holley.

 

mike

 

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2) Should I look at investing in a better carb? I've read very mixed reviews on the 4300 (it was rebuilt ~10 or 12 years ago and only has about 50 miles on the rebuild).

 

I would consider a nice Holley. If you move between high and low altitude and stay at either for a period of time, the jets are easy to exchange on the Holley.

 

mike

 

 

I'm really considering going to an aftermarket carb, when funds allow. I figure that rejetting is cheap enough that I can get away with doing that for the time being. I'm not going to drive much below 6,000ft elevation, so I should be okay.

 

I'll keep you guys posted, maybe this weekend I'll be able to do it?

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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Guest BigPinoyHunk

2) Should I look at investing in a better carb? I've read very mixed reviews on the 4300 (it was rebuilt ~10 or 12 years ago and only has about 50 miles on the rebuild).

 

I would consider a nice Holley. If you move between high and low altitude and stay at either for a period of time, the jets are easy to exchange on the Holley.

 

mike

 

 

I'm really considering going to an aftermarket carb, when funds allow. I figure that rejetting is cheap enough that I can get away with doing that for the time being. I'm not going to drive much below 6,000ft elevation, so I should be okay.

 

I'll keep you guys posted, maybe this weekend I'll be able to do it?

 

be caution when you do other stuff to the car that'll require different sets of jets. i went from 86 jets to 82 jets in the primaries because i changed my headers from 2 inch to 1.75 tubes. the 86 jets in my new headers was just just too rich and it would make my eyes water and smog up the house with the interior door closed as well. with the 82 jets its perfect.

 

my friend at work on his 69 GTO, he said he had to go UP 1-2 jet sizes just adding in his MSD dizzy and modulator box.

 

 

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If you ignore other factors, humidity and temperature, the jet size should decrease one number size for every 2000 feet of altitude gain.

 

Chuck

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If you ignore other factors, humidity and temperature, the jet size should decrease one number size for every 2000 feet of altitude gain.

 

Chuck

 

So we're talking about 3 jet sizes to get to the correct size for the altitude. I'm thinking of buying a kit so I don't have to make a bunch of trips to the parts store. That way I can experiment to get the correct setup.

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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If you ignore other factors, humidity and temperature, the jet size should decrease one number size for every 2000 feet of altitude gain.

 

Chuck

 

So we're talking about 3 jet sizes to get to the correct size for the altitude. I'm thinking of buying a kit so I don't have to make a bunch of trips to the parts store. That way I can experiment to get the correct setup.

I went back and read your original post saying you have a Ford 4300 carb. The info I gave you is for a Holley. It may be true for Autolite 4300. I just don't remember if the Autolite jets used the same flow/numbering system as Holley. Finding jets could be a challenge. The only sources I know of is possibly Pony Carbs or other rebuilder or find a stock or Super Stock Racer running the Autolite. Do you know if it is a 4300A (near square bore 600 cfm) or 4300D (spread bore 715 cfm)?

 

Chuck

 

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It's not the 4300D, so I guess it's a 4300A? I've never heard it referred to as a 4300A, just 4300.

Steve

 

[button=http://www.7173mustangs.com/thread-1972-mach-1--1800]The Stable[/button]

 

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It's not the 4300D, so I guess it's a 4300A? I've never heard it referred to as a 4300A, just 4300.

I'll check some local sources here tomorrow to see what is available, if any thing.

 

Chuck

 

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