04 jun 2020
The small one-barrel carburetors Rambler sixes from the 1960's are now so old that rebuilt units are no longer available, never mind new replacements. It varies with model, but one of the most common, the Holley 1931, has no rebuild parts available that are modern enough to withstand ethanol in the fuel.
As a general warning -- do not use NOS (new old stock) carburetor repair kits. The rubber (plastic, actually) is too old, it will not last, and none of it was ethanol tolerant in the first place. You were warned.
Worse, the Holley 1931 is a bad design. Like most Holleys it has a vertical fuel bowl and therefore a "wet" bowl gasket. WHen it leaks, as is inevitable with age, it drips gasoline on the hot exhaust manifold, hich not only smells bad but is obviously a fire hazard. It also has a terrible acellerator pump design that also leaks fuel.
Often overlooked is the fact that the carburetor metal castings themselves wear out. throttle shaft bores are often so worn that they leak enough air to foul up idle adjustments. But carburetors are subtle devices that require the as-designed smooth cast finish within the venturi, air horns, and other seemingly-passive parts of the castings, and when these pit, corrode, and cake with crud and rust, fuel cannot properly wet internal surfaces and passageways and no amount of tweaking adjustments will make them work as well as they did when new.
These are most of variables to deal with when swapping in a carburetor not designed for/shipped with a particular engine:
In theory many carbs meet most of these requirements, but few are reliably available new.
For some reason the Carter YFA carburetor has been cloned and is being manufactured in large quantities in China. This is a very good thing. Ford used a lot of them, it's a fine design in it's own right, the Ford application is very close to AMCs. It meets nearly all of the criteria above, is available new ($125 when I bought one early 2020) and when purchased from a reputable domestic supplier, quality control is not a problem.
I bought mine from Mike's Carburetors. Out-of-the-box it meets all but the last three criteria.
The YFA bolts onto the engine as-is, with the following changes/accommodations:
1) The choke stove tube needed to be slightly re-shaped for the YFA; the port is about 1/8" higher than the 1931.
2) 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. It's not "correct" but works fine. I also have a 1979 air box, with the vacuum-operated flappers, that works as well.
3) The jetting will be wrong, and you need to fix it. My YFA was "supposed to" have a .092" jet installed, but it had something much smaller. No matter, it was wrong. I spent the time drilling out the jet incrementally, testing and pulling plugs, and ended up with a .120" main jet, and the stock metering rod which is .039"/.078" (tapered). The assortment of metering rods that in the old days were available to tune them precisely are no longer available. Doesn't seem to matter here. For my particular 199ci here in Los Angeles, sea leval and warm, I could probably use one drill size richer.
4) Last and definitely not least, the early Rambler Americans, at least, have a throttle operated by a rod that twists longitudinally, in the same axis as the carb's throttle bore. (Later cars use a cable that pulls a lever, much easier to work with.) This was the hardest single thing to adapt.
Here are photos of the installation.
The YFA clone has some (probably) Ford type linkage, and that's that. For cable type throttle adapting would be easy. The twist rod requires a funny part.
In the background of one photo you can see the prototype for this. I hacksawed the Rambler throttle adapter off the junk 1931, made a little "L" bracket and bolted it to an old Gremlin YF carb I had laying around. It worked great, but the carb needed rebuilding (I paid one dollar for it at an AMO swap meet I think in 1999). I decided that I wanted to work out the scheme for adapting the available new carbs instead, bougt one from Mike's (above) and worked this bracket out.
With a (now abandoned) idea of being able to make these in quantity, I fashioned an adapter out of a single piece of sheet metal that gets folded. For a one-off, it would probably be easier to make two pieces, drilled and folded, and screwed together. The adapter isn't rocket science, just an annoying arbitrary shape.
I suppose if someone were to ask me I would re-draw my sketch and post it as a printable template.
Astute persons will note that it's missing a spring and washer that takes up slack/slop. The twist linkage is a sort of U-joint, it allows the motor to wander around a wee bit on the motor mounts and not bind the throttle. It works OK without the spring, but "some day" I will find one and install it.