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Discussion Starter #1
I have a 79 400m which has been adjusted to high altitude according to the sticker under the hood. I can't read everything that is one the sticker. I live in Houston (basically sea level) and need to un-do what ever was done to make this thing "high altitude". Can anyone help me in understanding what needs to be done? I am guessing timing and carb jetting.

Thanks!
 

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I would think it's just carb tuning, which could affect timing also. Have you had the truck a while? Tuned the carb yet? I'd start by setting the carb back to stock, then do your tuning from there. What carb do you have? Stock Motorcraft?
 

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IF you are running the stock 2 barrel carb, just do some JY crawling, and get a 2 barrel off a local 400, and rebuild it. I'm guessing your high altitude set up carb has been jetted down about 4 sizes.
 

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Discussion Starter #4
Yes, I have the stock motorcraft 2 barrel. I havent had the bronco that long but it does idle rough. I have the rebuild kit at home and was planning to do the full rebuild this weekend when I saw the sticker saying it "high altitude". If I have to re-jet the damn thing then I guess I should wait to rebuild. Anyone know the stock jet size for a standard carb?
 

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Yes, I have the stock motorcraft 2 barrel. I havent had the bronco that long but it does idle rough. I have the rebuild kit at home and was planning to do the full rebuild this weekend when I saw the sticker saying it "high altitude". If I have to re-jet the damn thing then I guess I should wait to rebuild. Anyone know the stock jet size for a standard carb?
Scott, look on the driver's side float bowl of the carb; there'll be
a circle with a number embossed in the metal. The number will
probably be 1.21. If so, then the stock jet size is #57.
 

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I went sea level to 6000+ elevation. so went with a Holley 2bbl for now till I get FI installed................


BTW, sea to high pig rich, high to sea way lean..............think about it
 

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Scott, look on the driver's side float bowl of the carb; there'll be
a circle with a number embossed in the metal. The number will
probably be 1.21. If so, then the stock jet size is #57.
I live at 7500 ft and went from 58s to I think 52 then to 48 and it is still a little rich. I believe stock at sea level is gonna be 58. You can re-jet right on the truck by pulling the top off no problem.
If you would, when you get those jets out, post back to this thread what size they are..
 

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Go down one jet size per 2000 feet above sea level.................per Holley

In your case, it could BE TOO lean, how do the plugs look?? tail pipes soot black ??? Should be clean grey look in the pipes............
 

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Discussion Starter #9
Well I pulled the carb...then said "hell, may as well pull the intake too"....holy crap it has ALOT of sludge. I mean a shit ton!. The plugs are black at the base (rich) and layers of ash on the ground. The truck was in the mountains of Cali all of its life and I just brought it to Houston. So I am not sure what to make of all this. i figure I will rebuild the carb, get rid of as much sludge off the rockers, springs, etc and button her back up. I will probably do the sea foam in the crankcase thing a few times to help breakdown the sludge I can't get.

She runs fine but with low oil pressure so I guess I just figured out why with all the sludge. I haven't disassembled the carb yet because I don't have the rebuild kit and I don't want the little pieces sitting around too long.

I will let you guys know what size they are when I pull them. Thanks for the help so far.
 

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Discussion Starter #12
Well I just got around to tearing into this carb and I owe you guys some info.

Yes my carb has 1.21 stamped on it, but it has #55 jets. Again I sit at sea levels so do you guys think these jets will be too lean?

Thanks in advance...

Scott, look on the driver's side float bowl of the carb; there'll be
a circle with a number embossed in the metal. The number will
probably be 1.21. If so, then the stock jet size is #57.
 

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Well I just got around to tearing into this carb and I owe you guys some info.

Yes my carb has 1.21 stamped on it, but it has #55 jets. Again I sit at sea levels so do you guys think these jets will be too lean?

Thanks in advance...
Since the carb's stickered for 'high altitude', the #55 jets are most
likely part (or all) of the modification. The accellerator pump linkage
may also be set lean.

I'd guess it would run better with #56 or #57 jets in it.
 

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Since the carb's stickered for 'high altitude', the #55 jets are most
likely part (or all) of the modification. The accellerator pump linkage
may also be set lean.

I'd guess it would run better with #56 or #57 jets in it.
The accelerator pump is touchy. If it was set too lean either by nozzle or cam he'd have noticed with bad stumbling problems.
 

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Discussion Starter #15
Well I have looked and called everywhere for replacement #56 or #57 jets...no luck. Hell I can't find anywhere that carries these old jets period! I guess I will take my chances on the #55 and just keep a close look at the plugs and listen for pinging.

Can someone verify my logic here: Since my carb was tuned to high altitude that means the jets would be tuned to provide less fuel to match the thinner air. So if I bring it down to sea level where there is "more" air then the smaller jets put in for the high alt tuning would be too small and I will run lean. On top of that I have EGR emissions on the truck which also means the truck is tuned to run slightly lean for the EGR system to do its job. But my EGR is blocked off meaning I have an engine that is tuned to run a slight bit lean b/c of EGR and then tuned to run even leaner from the high alt tuning. Now I am at sea level I really need to keep an eye on it because all car run lean at sea level.

Is that correct logic...or do I need to drink another beer and stop thinking about it so much?
 

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Well I have looked and called everywhere for replacement #56 or #57
jets...no luck. Hell I can't find anywhere that carries these old jets period!

I guess I will take my chances on the #55 and just keep a close look at
the plugs and listen for pinging.
Your logic is correct except for the part about just giving up and keeping
the 55's in it. LOL :) Lose the EGR! BTDT in March after getting my '75
F150 in February of '75.

Go around and find a wrecking yard that will -give- you some jets out of
a junk carburetor. Over the years I've collected 'em from the scrap yard.
And/or like someone said just buy another carburetor from the wrecking
yard if you see a nice one, won't hurt to have an extra.

If all else fails PM or email me your address and I'll send you a matched
pair of jets. I've got a set of jet reamers and I can make some 57's for
you. That 57 means .057" BTW. ;)

Alvin in AZ
 

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There's better instructions on the internet some place but here goes nothing...

Fuel is sluggish to change it's amount of flow compared to how quick airflow
can change so...

1) Drive along real steady like at 55 and then suddenly let off the gas as-if
you want to suddenly do 50mph. For a split second you've reduced the air
flow and not the fuel flow. :)

What does it do?

a) Exactly what you would expect? ...then it's just right.
b) Did it give you a tiny burst of power before settling down to the new speed?
If "b" it's running too lean.
It ran better than it was, for a split second, because the mixture got better.
c) a very slight sag in power and quickly revive? = way too rich

2) Ok now go the other way, 55mph then suddenly go to ~60mph.
a) Exactly what you would expect? ...then it's just right.
b) Tiny burst of power -before- settling down to the new speed?
If "b" ...it's running too rich.
It ran better than it was, for a split second, because the mixture got better.
c) If it's real lean there can actually be a slight stumble going this direction.

The main thing is to feel which direction your foot needs to move to make
it add a split second of power. :) That's the one to pay attention to.

...or something like that?

Do it at different speeds, 55mph is just to first one to try is all. :)

If it stumbles just-off-idle that's not the main jets, that's the "low speed
circuit" jets being too lean for "smog" reasons, fixed a few 2100 and 2150
carburetors for that problem way back when. :)

-------------------------

Accelerator pump is not the right cure for that unless you're a racer and all
you do is floor it every time. That's the usual recommendation. :/

The accelerator pump is for accelerating making it too-rich to get you past
the low speed circuit won't help you make a useful work truck out of it.
There are times you need a steady-state drive-ability just off idle.

Alvin in AZ
 
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