4100 Carb for 1957 312

D

Dude81

Junior Member
My 57 Fairlane with a 312 Y-Block has a 58 Autolite 4100 carb rated at about 600CFM and originally designed for 332/352 engines. Engine runs very rich. Should I rebuild the 600 CFM for $300-ish or buy a rebuilt 500CFM Autolite 4100 designed for the 57 312 for $600-ish? Thanks
 
First question: Which 4100 carb do you have? Is it a 1.08 or a 1.12. One of those numbers will be cast on the left side of the front venturi. The 1.08 is the smaller of the two and is 480 CFM and the 1.12 is 600 CFM. While the 1.08 is the better choice, the 1.12 will work just fine. I have a 57 312 and use a 1.08 which runs great.


You say it runs rich. You say this because what, at idle, at cruising speed? If at idle, it could be as simple as replacing power valve.
 
Thanks. Pretty sure it's a 1.12 as the butterfly openings measure 1.55". I read somewhere that butterflys were 1.44 on the 1.08s, and 1.56 on the 1.12s. Couldn't find the stamping.
 
It may have an internal leak, or just need smaller main jets?
A local carb shop supposedly set up an original '57 Holley 4bbl for my not-quite-original '55 292 and it ran way too rich, when I checked them the jets were about 4 sizes too large.

.
 
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Since I have the same carb on my car (1.12) is there a way to 'detune' the carb so that it works right w/o fouling the plugs?
 
I haven't worked on 4100 carbs since the 70's or so but I have had a ton of 2100 2 blr carbs and venturi sizes from 1.02 to 1.21. all had the same sized main jets. I tried all sizes on our 302 and couldn't see any difference in idle, performance or richness in any of them.
maybe if I went to the dragstrip I would have seen a difference in performance but normal street driving I saw no difference.
 
A person can tune their own fuel system for best efficiency if they have an AF meter in the tail pipe. It may need smaller main jets if it's running rich as speed. Idle is controlled by the idle screws but a leak is definitely possible. Not all modern power valves fit older carburetors well if it has the vacuum type. I'm not well versed on the early 4100s but there are likely a few who are and may be of more help in this respect.
 
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If your carb is indeed running rich, it's not because it was made for a 332 or 352 FE engine.
Like robib said, it could be as simple as a ruptured power valve. If you take off the power valve cover (4 screws on underside of front carb bowl) and there is gas in the cover, then that would be a ruptured power valve,
Also, the '58 FE carb should work fine. It's not really too much airflow, sine it has vacuum secondaries, that open on engine demand, and are never really fully wide open.
An actual 1957 4100 carb being the first year for them, left a little to be desired, especially with the choke system.
I would be glad to rebuild your '58 carb, or any carb. I have over 55 years experience on all domestic carbs from about 1935 on up to the end of carburetors.

Sal
 
Those 4100 are very well regarded over at yblocksforever, both the 1.08 and the 1.12. Anything but race motors. Trust Sal. He’s got a great reputation here. I don’t have any other relationship with him, except the reviews.
 
miker98038 said:
Those 4100 are very well regarded over at yblocksforever, both the 1.08 and the 1.12. Anything but race motors. Trust Sal. He’s got a great reputation here. I don’t have any other relationship with him, except the reviews.
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Thank you, Sal
 
robib said:
First question: Which 4100 carb do you have? Is it a 1.08 or a 1.12. One of those numbers will be cast on the left side of the front venturi. The 1.08 is the smaller of the two and is 480 CFM and the 1.12 is 600 CFM. While the 1.08 is the better choice, the 1.12 will work just fine. I have a 57 312 and use a 1.08 which runs great.


You say it runs rich. You say this because what, at idle, at cruising speed? If at idle, it could be as simple as replacing power valve.
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That is not correct that a 1.12 is equal to 600 cfm. These were small carbs. Peop0le think they were around 600, nope!
 
Dude81 said:
Thanks. Pretty sure it's a 1.12 as the butterfly openings measure 1.55". I read somewhere that butterflys were 1.44 on the 1.08s, and 1.56 on the 1.12s. Couldn't find the stamping.
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Its not marked on the throttle plates. The venturi size is cast into the side of teh carburetor body.
 
PetesPonies said:
That is not correct that a 1.12 is equal to 600 cfm. These were small carbs. Peop0le think they were around 600, nope!
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:confused:​

Any chance of you explaining what it is you are trying to get across?

CFM RATING is determined by the CARB manufacturer with engineering and testing on a flow bench. How a CARB actually flows on any particular engine is determined by engine size and engine flow characteristics.

How one chosen to work properly on one application may not do well on others (of similar engine size). The reason for CARB calibration(s) and running in on an ENGINE DYNO.

There is much more to consider other than the CFM RATING/engine size.

ADDENDUM -

Flow Rate - http://fordfe.info/Autolite4100Test.htm
 
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To put it simply, when one either tries to rebuild/calibrate either a carb that is on an engine (if it was not swapped previously at some point) or has one that has been acquired, one needs to ID the CARB and either go into the appropriate model year WORKSHOP MANUAL, correct period PARTS MANUAL or have a REBUILD KIT SPEC SHEET to return it to stock spec and then modify to suit the application.

Same on HOLLEY or CARTER. It is just like modifying an engine, you have to know and understand what you are working with.
 
To put it simply, when one either tries to rebuild/calibrate either a carb that is on an engine (if it was not swapped previously at some point) or has one that has been acquired, one needs to ID the CARB and either go into the appropriate model year WORKSHOP MANUAL, correct period PARTS MANUAL or have a REBUILD KIT SPEC SHEET to return it to stock spec and then modify to suit the application.

Same on HOLLEY or CARTER. It is just like modifying an engine, you have to know and understand what you are working with.

Its not marked on the throttle plates. The venturi size is cast into the side of teh carburetor body.
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Not all are marked and if the ASSEMBLY ID TAG is missing, you would have to calibrate the carb to the application you are working with and then modify from there to get it to perform correctly..
 
I disagree with what it says above the flow rate chart. It says Autolite 4100, Holley's, Edelbrock and Carter were all flow rated at 1.5" of Mercury. I know for a fact that Holley 4 barrels were flow rated at 2" of mercury, which whould result in a higher CFM rating. I have no idea what Edelbrock flow rated at, but I would lean towards 3" of mercury.
If the 4100 was flow rated at 3" of mercury, it would have a higher CFM rating than indicated on the chart.

Sal
 
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Read This and Comment -

CFM ratings are more accurate than physical carburetor sizes, as the CFM rating takes into account the venturi size of the carburetor. It is not uncommon for a given physical size (see Carburetor sizes paragraphs) to have many different internal venturi sizes. Early Stromberg and Zenith carbs could have as many as 9 different venturi sizes for a given physical size. Carburetor CFM ratings have been around since at least the 1920’s; however many O.E. (original equipment) carburetors never had published CFM ratings. I have not seen any actual agreement that stated that it had to be this way, but the early published ratings that I have seen for 1-barrel and 2-barrel carburetors were measured at 3 inches of mercury. This rating was about the amount of vacuum available on engines of the period under wide-open throttle conditions.

Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 ½ inches of mercury was chosen for rating 4-barrel carburetors. The ratings for 1-barrel and 2-barrel carburetors were left unchanged.
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- INFO SOURCE - - https://thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

You have more experience than I do but what am I reading wrong?
 
I used to flow test carburetors at the old Zenith Caruretor engineering lab in Detroit and they used 2" of mercury as the standard. I worked with people that came from Holley engineering in Warren, MI and they confirmed that Holley four barrels were flowd at 2", and Holley two barrels were flowed at 3". That's why you see some Holley ywo barrels being rated at 500 CFM. To me, that's an apples and oranges comparison, and gives customers the wrong impression. Kind of like a marketing trick.
I agree that to get an engine to have 3" of mercury at wide open throttle would have to be at a screaming high RPM.
I beleive the chart you posted was created based on a national standard 1.5" and not what all carb companies rated flows at.
I think if the 1.12 Autolite 4100 was flowed at 3" it would be closed to 600 CFM.
Everyone can have their own opinions on this stuff, but that's mine.

Sal
 
KULTULZ said:
:confused:​

Any chance of you explaining what it is you are trying to get across?

CFM RATING is determined by the CARB manufacturer with engineering and testing on a flow bench. How a CARB actually flows on any particular engine is determined by engine size and engine flow characteristics.

How one chosen to work properly on one application may not do well on others (of similar engine size). The reason for CARB calibration(s) and running in on an ENGINE DYNO.

There is much more to consider other than the CFM RATING/engine size.

ADDENDUM -


Flow Rate - http://fordfe.info/Autolite4100Test.htm
Click to expand...

4100s with 1.12 venturis are not 600 cfm. They are low 500s. From about 506 to 528 as I remember. The only 600 cfm 4100 is marked with 1.19 venturis and is very rare. The 1.08 is about 441 cfm and was used on the 289s, etc
 
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