Maybe someone of the "airflow intelligentsia" can help me understand the "why" of different carb. set-ups on the same engine size.

I have an '84 351W that originally had a feedback carb. before a Duraspark conversion/computer removal. This 2350 carb. had 1.08 venturi size and jets marked 55F. Truck ran O.K. above idle but never would idle right, even with professional adjustment.

To eliminate the feedback system, I obtained a carb. that was from a 351M truck engine, if I remember correctly. Obviously this carb. was from an earlier year. This 2350 carb. had 1.21 venturi size and jets marked 53F. Truck idled well and ran fine above idle. Just being able to set the idle correctly was worth the swap.

Now as I understand it, a larger venturi flows more air and larger jets flow more fuel. So why would the 1.08 venturi feedback carb. have larger jets than the 1.21 venturi carb. especially for the same engine size? I can understand the smaller venturi size for possible emissions reduction but why the larger jets?

I'm also wondering if the 1.21 venturi carb. might benefit from a swap to the 55F jets.

Any thoughts?

 

What you have is a Motorcraft 2150 carburetor, not a 2350.

First, keep in mind that you are dealing with OEM Ford carburetors that Ford specifically designed and engineered to work with *specific* engines and *specific* vehicles. These are not a "one-size-fits-all" universal type carburetor. The universal aftermarket carburetors have the same basic design and boosters, but they have different CFM ratings and use different jets, etc. to work for ALL sizes of engines for ALL makes and ALL models.

Ford went much further than that when they designed their Autolite/Motorcraft carburetors...

The internal passages of a carburetor Ford designed to use on the "M" series is entirely different than the internal passages on the "W" series. And that is because these two engines have entirely different flow characteristics. So that means the venturi boosters used for each engine family are also different. And the different venturi boosters on Ford carburetors combined with the carburetor size and application determine the correct jet sizes.

For example, the 1.08 carburetor has different boosters than the 1.21 carburetors. Further, the 1.08 model used on trucks has completely different boosters (and jets) than the 1.08 model used on a lightweight car.

Further still, the jet sizes are also different for different vehicles sizes and even different transmissions!

That is why no aftermarket carburetor will ever run as good a an OEM Ford carburetor on a stock engine.

This site is specifically dedicated to Ford carburetors and can answer your questions better than I could:

Ed's Carburetor Forum. Carbs rule!!!!

 

Thank you for the reply. I appreciate any additional knowledge I can get.

 

Well spoken ! The point couldn't have been professed any better !
DON'T FIX WHAT AIN'T BROKE...
There should be a Stickey...'If it ain't broke...don't mess with it !"

 

Jet down till it bucks aka "lean surge" and go up two sizes from there. Be careful. Check plug "color" or better, an O2 sensor or air/fuel exhaust analyzer.

 

All of the OE carbs are built to meet EPA standards, read run lean but maybe not the best for performance.
The OP said it would not idle right no matter who or what was done to the OE carb. That would a vary good reason in my book to replace the OE carb with something you can work with.


As for the different venturi size and jets sizes between the 2 carbs you need to know the different working RPM of carbs.
Idle/mid range/high speed.
The 1.08 venturi size and jets marked 55F has a leaner idle and richer high speed where the other has 1.21 venturi size and jets marked 53F has richer idle & leaner high speed.
The richer idle bleeds over a little to the high speed side to richen it up and why the smaller 53 jets where it could be the same AFR as the leaner idle/richer high speed at higher RPM.


As for the 55F jets in the 1.21 venturi size carb you can give it a try and see what the out come is. It may run better or may run worst but you will never know till you give it a try.
That is my story and I am sticking to it.
Dave ----

 

It's great info, but the software ... threads like the old Usenet FTP from 1988 or something, I just can't.

 

Edelbrock's owner's manual for their carbs has a good explanation for how carbs work. Yes, it is for 4bbl carbs and the 2150 is a 2bbl, but that really doesn't matter and you can ignore the Secondary Systems.

What does matter is that most of the how-it-works is applicable to the 2150. The only real difference is in the way the enrichment system works, where the Edelbrocks use a metering rod and the 2150 uses a power valve.

But, I take issue with the statement that "no aftermarket carburetor will ever run as good as an OEM Ford carburetor on a stock engine". I've seen plenty of situations where the stock carb was jetted to meet emissions and not for running well. I'll give you two examples - my 1969 Super Bee and my 1972 F250. Both were jetted so lean from the factory that the Bee had the lean surge Tedster spoke of, and the pickup wouldn't accelerate at all until the power valve opened on the 2100. In the case of the Bee rejetting the Holley fixed the lean surge but not the leaking like a sieve. (I should have put a Carter on like Chrysler did for the hot Bees.) In the case of the pickup, a 4bbl intake and a QuadraJet fixed it - the gas mileage went up and the acceleration was much better.

The truth is that Detroit had to jet the carbs to meet emissions. That's the law, and the law doesn't state that they have to drive well nor have good performance. In many cases factory carbs can be tuned to run better, but not in all cases or without great difficulty as many of the emissions carbs had restrictions in the passages that made them difficult to tune. But an aftermarket carb has no such problems and can be much easier to tune.

 

Here's another useful site

Ford Motorcraft 2150 Theory and Operation

 

Thanks for the replies and sites to visit for more info. Sorry for the mis-numbering of the carb. in the OP. I thought at the time I might have gotten it wrong but just got lazy.

As to the original carb., it was rebuilt by one who was acknowledged to be THE carb. guy in the area and it still would not idle at a reasonable RPM. Had several other good shops try to adjust it without much success. Adjusted so it would not stall randomly at idle, you had to really stand on the brake to keep it from moving forward in gear at a stop. With the replacement carb. there is no such problem.

I may eventually go to an aftermarket 4 barrel, but have to decide which one. I really like the looks of, and reports on the Summit carb., which is said to be a new iteration of the old 4110 Autolite/Motorcraft. We shall see. But first I may try re-jetting the 2150 and see what happens.

Thanks again and all additional comments are welcome

 

Ford actually calls it an economizer valve. And don't forget the choke system is completely different, the booster design is completely different, and the air bleeds are completely different. Ford's engineering design on these is superior than any aftermarket carburetors out there, even today.

Well, the 1969 Super Bee is a Chrysler product, something I don't know all that much about. And then it had a Holley: a carburetor I don't personally care for on a street-driven vehicle.

And in the case of the 1972 F250, well of course gas mileage will go up when you replace a 2-barrel carburetor with a 4-barrel carburetor with much smaller primaries. And the acceleration would certainly go up with the HUGE secondaries provided by a 4-barrel QuadraJet carburetor. A win/win for most people. But at what price? Simplicity, reliability, and driveability, that's what. You could have simply replaced the jets on the Autolite 2100 (like you did with the Super Bee) and that may have solved the problem a lot cheaper than dismissing it in favor of a 4-barrel. (I can't say I blame you though. I bet the QuadraJet made that truck a LOT more fun!)

My earlier statements in this thread were in reference to OEM Ford Autolite/Motorcraft carburetors, specifically the earlier Autolite 2100/4100 and the later Motorcraft 2150 carburetor. When I said that "no aftermarket carburetor will ever run as good as the OEM carburetor on a stock engine", I meant that in overall reliability and driveability, not necessarily speed and/or performance. With exclusive features such as the thermostatic choke, annular booster design, and variable air bleed system, the Autolite 2100/4100 and Motorcraft 2150 carburetors are as close to EFI as you can get in carburetion.

When Ford had to jet their carburetors to meet strict emissions regulations, it is true that performance was sacrificed for cleaner air, especially in the earlier years. That is where the aftermarket carburetors clearly had an advantage. But overall driveability and reliability (and now emissions!) was still better than the aftermarket offerings, at least in the case of the Autolite 2100/4100 and the Motorcraft 2150. There is a reason why this carburetor design started back in the 1950s and went all the way to the 1980s!

 

Rick - The main thing we agree on is the dislike of a Holley on the street. In my experience you will have blown power valves and leaking accelerator pumps, it is just a matter of when. And, while the 2100/2150 and 4100's are fine carbs, they run Holley accelerator pumps and power valves - or "economizers" if you want.

I prefer my carbs to keep the fuel in with metal rather than cloth, so like the Carter/Edelbrock carbs as well as the Rochester. Or, even the new Street Demon. Piston-style accelerator pumps and metering rod enrichment systems don't leak.

Having said that, I do like the hot-air choke of the 2100/2150/4100's.

 

I could not get a 1975 MC 2100 v2 carb to run right on a v8 motor no matter what I did to it. I replaced it with a Holley 3200 v2 and it was a night & day difference how the motor ran.


This Holley was used on a street car with a factory stock motor (AMC 304) and other than choke high speed settings that all Holleys I have had were the choke high speed would be too hi or too low. Other than that once up to temp ran great.


On the older Holleys if it back fired up thru the carb it would blow out the power valve (PV) new Holleys have a check valve to stop this.


Only time I have had the acel pump leak is if they have not been used for some time and dry out then you start using the car again.


For performance even on the street I would go with a Holley carb. Besides I have a small tool box full of Holley tuning parts - jets, gaskets, pumps, pump cams, squirters, vent tubes & whistles, etc. to dial them in.
Dave ----