4100 Carb for 1957 312
 

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

Re: 4100 Carb for 1957 312
 

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.

Re: 4100 Carb for 1957 312
 

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.

Re: 4100 Carb for 1957 312
 

Regarding run rich: lots of carbon out of tailpipe in idle and smell gas at cruise.

Re: 4100 Carb for 1957 312
 

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.

.

Re: 4100 Carb for 1957 312
 

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?

Re: 4100 Carb for 1957 312
 

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.

Re: 4100 Carb for 1957 312
 

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.

Re: 4100 Carb for 1957 312
 

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

Re: 4100 Carb for 1957 312
 

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.

Re: 4100 Carb for 1957 312
 

Thank you, Sal

Re: 4100 Carb for 1957 312
 

That is not correct that a 1.12 is equal to 600 cfm. These were small carbs. Peop0le think they were around 600, nope!

Re: 4100 Carb for 1957 312
 

Its not marked on the throttle plates. The venturi size is cast into the side of teh carburetor body.

Re: 4100 Carb for 1957 312
 

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

Re: 4100 Carb for 1957 312
 

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.

Re: 4100 Carb for 1957 312
 

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.

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..

Re: 4100 Carb for 1957 312
 

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

Re: 4100 Carb for 1957 312
 

Read This and Comment -

- INFO SOURCE - - https://thecarburetorshop.com/Carbsh...izesandCFM.htm

You have more experience than I do but what am I reading wrong?

Re: 4100 Carb for 1957 312
 

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