Carter YF/YFA carburetor

 

03 jun 2022

Carter YF/YFA

Today (2022) the Carter YF/YFA is my go-to choice for replacing worn out one-barrel carburetors, maybe the overall "best" considering availablity and tuneability and reliability. The mechanical design is good, no fuel in constant contact with gaskets, throttle shaft bushings are replaceable, replacement parts and rebuild kits are available, new manufacture for seemingly all models including modern-fuel-proof rubber parts. Tuning ability is limited but easy enough. They were used in the millions by Ford, and AMC shipped them on the 232 and 258 sixes. They are available used, rebuilt, NOS (pricey) and import clones for under $100 new (I don't know what rebuilt kits fit those).

The YF and YFA (YFA is shorter (hood clearance) and has minor design improvements over the YF, but interchangable for our purposes) were used on AMC sixes, but not the 195.6. Ford used them on sixes and four cylinders. AMC or Ford models bolt onto the manifold as-is, but the throttle linkage is incompatible and requires fabrication. Most YF/YFA models will need re-jetting and slightly different adjustments. The older YF may have hood clearance issues when used in the lower-beltline cars, 1964 and up.

There are very old Carter YFs, from the 1950s and earlier, used on Willys and other vehicles; they are even taller and often have mechanical choke. Avoid those, but you probably would anyway.


Throttle linkage adapter

The early Ramblers use a throttle rod that twists longitudinally. The carburetor end has a U-shaped bracket with two holes and the L-shaped tip of the throttle rod hooks into the U, held in place with two springs. This comprises a "universal joint" that allows for misalignment the motor to shift in the chassis without goosing the throttle. Later cars mount the carb rotated 90 degrees or use a cable from the pedal. The cable sounds easy to adapt but in my experience it's easier to use the twist rod, since it's rotation of 90 degrees exactly matches the carb and with a cable you have to work out leverage to get full stroke and make all the brackets, cable, custom pedal, etc.

Here's the stock carburetor linkage on an old Holley 1904 (I think it is). Note that the "L" end of the throttle rod is at an angle when at rest (idle throttle closed). On the right is the fabricated adapter.

Click here for adapter drawings.

 


Air cleaner assembly, choke stove

The YFA requires an air cleaner box with a 2-5/8" hole, larger than the 1931's, which is about 3/16" smaller. I replaced it with an early 1970's air box. On my '60 I had nothing that fit, so I chopped some air cleaner I did have into a simple top and bottom hat.

If used, the choke stove tube needs to be slightly re-shaped for the YF; the port is about 1/8" higher than the 1931. An electric choke easily replaces the black thermostat, just plug the vacuum port.


Jets and metering rods

You can buy Carter jets easily enough, but unlike the Weber carbs feel free to drill them out to change size; it won't be precise, but since this has only one jet (and not a matched pair like in a Weber) the lack of precision won't hurt. I bought a couple too-small jets (common .094") and drilled them out. Use a regular 5/16-20 nut as holder for drilling. Jets from many other Carter carburetors are the same and the size number is the hole diameter in mils.

Metering rods are randomly numbered as far as I can tell, and you will need to work out for yourself how they work; the tip of the rod, which is stepped or tapered, sits inside the jet, reducing the effective diameter. A spring pulls the rod up by WOT or low vacuum, the tip has a smaller diameter hence more fuel. Tuning them is the usual art of trial and error and fundamental understanding of the mechanism. Your mileage may vary (literally).


Carter YF/YFA metering rods

An eBay seller i bought rods from was kind enough to point this table out to me. Only the columns marked "YF, YFA" apply. Click the image for the large version.

Metering rod lift adjustment screw access

One of the few internal adjustments is the metering rod initial position, which is set with a screw inside. Rod vertical position determines when (at what manifold vacuum/throttle position) the carburetor transitions out of light cruise into enrichment. If you want to fiddle with this it's a PITA to remove the carb top for each adjustment... it occurred to me to drill a hole in the cover to access it, and plug it when done.

Turns out Carter thought of this long ago -- there already is a hole there, with a plug in it. Tap the plug out from the bottom with a small screwdriver and tap it back in when done. I hand cut a fancy gasket to cover the hole while drivign around tuning it.

   





Specific installations

1960 American, OHV engine

I purchased this 1976 AMC Gremlin Carter YF (7112S) for one whole dollar in 1999, from the parking lot sale at an AMC meet, I think the Cactus Cruisers in Phoenix. There wasn't a lot of interest in one-barrel carbs then, and I bought it just to have around. Since then it's been the carb I make engines first run on, since it's so easy and reliable. So the hint was at hand this whole time.

If you find an AMC version of the YF carburetor it will probably be usefully jetted like this one was, but more likely you will find one for a Ford 300 and it's jetting will be way off. Use my settings below as a starting point. The 195.6 is a small bore long stroke motor with a funny intake system and every carb and the Sniper EFI needs to be set much richer at idle (13 AFR) than recommended.

Carter YF, stamped 7112
Mike's Carburetor rebuild kit K4073, float FL32.
Accel pump shaft is 2.645, gapped section .340".
quadrajetparts.com float 1445B, ethanol proof accel pump 1062AE.

JET rod CRUISE rod WOT Cruise AFR WOT AFT
.112 .078 .042 15.5 - 16 12
.110 .069 .045 15 - 15.5 14



This is a stock 1960 American linkage, the aforementioned adapter, on a yard-sale 70's YF.

         


1960 Rambler American throttle linkage in motion

Here I flooded my engine for you to make this video of the linkage motion. This might help clear up how it's connected.




1968 Rambler American, 199ci six

Here's the jetting I found to be "close enough". The carburetor was a new YFA clone, imported, purchased from Mike's Carburetors. Mike's does quality control and backs their products, so it's a fine choice. Finish quality and behavior was excellent. Out of the box however it barely ran. I drilled out the main jet and did test runs until it ran right. That car did not have an AFR meter on it, I tuned by performance and plug color. It got 22.5 mpg on the highway, plugs were a nice light color and it ran great.

Carter YFA clone

JET rod CRUISE rod WOT
.120 .078 .039

The metering rod was not stamped with a part number, but is quite close in specs to the 74-2171 part.

Here are photos of the installation.

     


Carter YF Service Manual

Here is the Carter YF/YFA Service Manual #3608B, with explanation of how the various circuits work (theory of operation) including the only description of the operation of the feedback version of this carb I've ever seen (briefly: 10 Hz PWM, ON leans the mixture).