The Stovebolt Site Forums The Shop Area Engine Shop Evaluating Carter YF for rebuild

In this case it’s a YF 2100s (manual choke) that will go on a late 235 in a 51 pickup.

This is a continuation of the “Rochester B running rich” saga from the other thread. Can’t bring myself to tear the B down once again while I have this Carter that’s just been sitting and staring at me.. The B’s air horn to bowl gasket tore a little bit last time I was in it. I glued it back together with some permatex to get by. If I was to do some polishing of the power piston or what not....I’d be buying a kit again. It runs ok enough as-is to keep tweaking on some other things, but I’m moving on.

Anywoo, I bought this YF few years back on EBay and it was in a few pieces. I reassembled, disassembled, and compared to a diagram on the interwebs to see what was what.

https://www.carburetor-parts.com/assets/instructionsheets/k453.pdf

It looks to me like I have a complete carb worthy of rebuild, but before I go sourcing a kit...wanted to pick some brains here on a few oddities.

1) The top of the idle tube/ jet has been a little buggered up from some rough screwdriver work from a previous owner. I guess that’s Ok as long as orifice isn’t wollered out?

2) The little brass “discharge orifice” in the venturi also bears some screwdriver scars. Again, the orifice itself looks ok. Is this even a replaceable part?

3) The check ball weight in the middle section of carb looks to be very well encrusted in white crap. That might be interesting.

4) The upper spring on the accelerator pump shaft is actually TWO. A spring inside of a spring. Not sure if that’s correct? It’s certainly not on that diagram

I’ve gone from knowing nothing about carbs a few years ago to knowing slightly more than nothing. I can sorta get my head around how a B works. This metering rod setup in the YF is a mystery right now. Gonna need a tutorial on that at some point.

I think the Carter 2100s is ranked as a good one but CarbKing will probably chime in.

I rebuilt a YF2100S for my 52 with a 1956 235. It functions quite well. But so did the Rochester B I replaced. Only did the swap because the general consensus was that the Carter is a better carb. Maybe I was just lucky with a good Rochester. Can't comment on the condition of your parts. I had no trouble doing the rebuild on my Carter but I have done a great many carbs, but mostly 4 barrel.

I’m comfortable with doing the rebuild. I’ve done several carbs on various types of engines. Just wanted some insight on the handful of oddities I found inside this one.

Your questions: 1...the idle tube. Do you mean the low speed jet? It is brass, about 1.25" long, thin and screws in on top of the float bowl. If it is just buggered up at the screw slot, that may not have much impact on it. Couldn't say for certain without seeing it. 2...the little discharge orifice in the venturi should be left alone. This isn't replaceable, but some people seem to want to try to remove it. Maybe after a few beers. Please look at image 3 below for what it should look like. Hopefully whoever was "working" on the carburetor stopped before it was damaged. 3...the check ball weight in the middle section of the carb. On top of the float bowl, there is a pump check valve. Generally this is brass, about 1/2 inch long, square or triangular shaped with a point machined at the bottom (where it seals into a cone inside the carburetor body)--see first image below. Is this what you're talking about? It will need to be carefully freed up. Maybe with a needle, small dental pick or something like that. You may need to soak it also...possibly some sort of penetrating oil. There is a check ball down in the diaphragm retaining piece, but usually this isn't removed/serviced. 4. Yes, there are 2 springs on the upper shaft and 1 spring below that. Please see image 2 below.

As for the metering rod, here is what goes on there: first, please understand there is a mechanical rod action and when you're operating the carburetor at partial throttle, the diaphragm is pulled downward (by vacuum). At this time, the metering rod is more or less simply controlled by your accelerator rod because the movement of it (up/down) is dependent on the pump lifter link. What I just said assumes vacuum to the bottom of the diaphragm is stronger than lower spring on your rod. Now there is also a vacuum rod action and when the spring on the lower part of the rod is stronger than the vacuum under the diaphragm, then the metering rod will move upward (opening up the jet to allow more fuel to the engine). Simple as it may seem, this is the power circuit of the YF. When the vacuum drops (as would normally happen when your throttle is opened up...engine under load), the carburetor will provide more fuel to your engine. When vacuum stabilizes, the metering rod returns to normal operation. If you study the rod, you'll see it has either 2 or 3 steps machined into it. This allows the main jet to either be more or less restricted and please remember in the YF, fast (unrestricted jet) does not mean down...it means up. Slow (restricted jet) happens when the rod is down. There is a diagram in image 4 below. The rod is adjusted this way: close throttle valve and press down on the accelerator pump shaft all the way. The metering rod should bottom out (contact the bottom of the float bowl) at the same time. The metering rod arm should touch bottom at the same time the metering rod arm touches the pump lift link (between the springs). If the metering rod doesn't hit bottom at the same time as the accelerator rod or if it hits bottom first, then you can adjust by carefully bending the metering rod arm up/down.

Hope this helps. Jon (carbking) posted some good info on the Carter YF just a few weeks ago...

Oh, sorry...you asked about the 2 springs on top. What I understand is those are there for two reasons: one---deceleration bumper and two---a delay provision for acceleration.

Here is an image showing the low speed jet (turquoise arrow).

And here is where the pump check goes (red arrow).

And one more thing...when you reassemble, please remember to add the rubber piece and the odd-shaped metal collar on the pump lifter link. This little thing keeps fuel from sloshing out when you go around a turn, curve, up or down a steep hill, or whatever may cause gas to run up to the top of that link. See first image below. There is a right way to install this and a wrong way. You'll see one tip of it is cut diagonally. Image two below is the right way to position it.

All makes good sense. Thanks for that very complete treatment.

I guess someone assumed the slots in the discharge orifice were for a screwdriver. I can send a pic later, but I don’t know why it wouldn’t still function as it is. I’m not 100% certain I installed the metering rod correctly in relation to the pump arm. But your pic will help me confirm.

One other thing for today, the needle/seat on my carb has a spring in the works. Not sure if that’s included in the kits or not.

Only in the best of kits will the spring-loaded needle be found, but it is important for the float to work as designed. Another very important thing is the orifice size of the seat (the fuel inlet orifice). It ought to be about .80~.81" in diameter from everything I've read/seen. If you bought a cheap rebuild kit, the orifice will be made for Fords, and it could be as large as .93~.95". This may (probably will) cause flooding. What happens is the pump keeps pressure on the needle/seat. The float drops to the point where it allows gas in, however if the inlet size is too large, more fuel will shoot inside before the float can react and lift the needle back to a sealing position. So with a "too-large" inlet, you'll always have too much fuel in the float bowl.

First image is what the original needle looked like. Second image shows where I sleeved a "too-large" seat so that it would match what Carter wanted.

(1) If the orifice is OK, should be fine.
(2) The main discharge nozzle is not a normally replaceable part. It can be replaced, depending on the skill of the restorer, and the tools in the possession of the restorer. Yours should be OK.
(3) See below
(4) The original valve was spring-loaded, with a small orifice, They come in the better rebuilding kits.

(3) The original issue of the Carter YF's used a small check ball, topped with a cubical brass weight for the pump discharge valve. After a few years, Carter obsoleted the ball/weight in favor of a pointed square brass needle of equivalent weight. The reason, folks would turn the carb upside down, and lose the brass weight. The pointed square brass needle comes in most rebuilding kits, and supersedes the ball/weight.

The metering rod.

Carter began using metering rod technology in 1929, and is one of the MAJOR reasons to use Carter carburetors when possible.

Think of the metering rod/jet combination as an automatically adjustable main jet.

Here is a link to the article to which Jon G. referred: Carter YF 235

Check the chart in the link, and you will find the 2100 has a three step metering rod.

The metering rod is inserted THROUGH the main metering jet. The effective metering AT ANY VALUE OF VACUUM is the area of the jet less the dynamic area of the diameter of the rod then in the orifice of the jet.
So the effective area of the main metering circuit varies at three values of vacuum, whereas the effective metering on the Rochester B has only two [(1) the area of the main metering jet, and (2) the area of the main metering jet PLUS the area of the power valve].

One can see from the chart that the metering rod has three different diameters: (1) 0.067 at the highest vacuum, (2) 0.060 at a midrange vacuum point (without pulling the print, guessing 10~12 inches of vaccuum), and (3) 0.050 at wide-open-throttle.

Because of the extra step, it is possible to keep a more consistent A/F density over the full range of engine vacuum, which improves economy, sometimes significantly, assuming the metering rod is calibrated for the engine.

What kind of economy is possible? I no longer own a Chevrolet, so cannot give an exact answer, but my non-Chevrolet shop truck has 390 CID engine, and two 625 CFM Carter AFB's (with metering rods), and gets 22 MPG at 70 MPH.

Jon

Automatically adjustable main jet.....I can understand that.

The article was very helpful as well. But one thing I can't envision....how does one install the metering rod AFTER all castings are in place?

On the YF, the metering rod is installed with the top casting off.

Many of the Carter carburetors have the metering rod(s) inserted THROUGH THE TOP CASTING. Trying to leave the metering rods in the top casting and trying to set the top casting onto the next casting invariably results in the power tip of the metering rods bending. Since I have the capability of machining metering rods (it is APITA!!!!!, and thus EXPENSIVE) this does not ruin the carburetor, but it doesn't help one's wallet!

On the WCFB, AFB, and others where the rods are inserted through the top, just better to do it after the castings are assembled.

I am so used to telling people NOT to try setting the top on with the metering rods in place on the 4-barrels, the thought crept into the YF article. The article has been changed. Thanks for bringing that to my attention.

Jon.

Thanks! I really thought maybe I was going nuts. Couldn’t for the life of me think how I’d do it otherwise...