Carburetion and induction

17 apr 2023

This section covers induction, fuel and air intake: carburetion and EFI. There's a lot of work covered in these pages, some of it foolish (lessons learned) some work just great. The more exotic research is below, but the two solutions that have worked out best overall, the Carter YF/YFA and two small Weber two-barrels, are covered first. I have also used two larger performance-only Webers, the 40 IDF and 44 IDF, and a Sniper Autolite 1100 EFI system and have notes on each below.

Though most application-specifics here are for the Rambler 195.6 OHV inline 6 engine, much of the notes and details here applies to any application. There is much useful tuning knowledge in old VW flat four tuning that applies to the Rambler, vice versa, and otherwise. The methods don't change much, the principles not at all, only specific jetting and tuning will vary.

AMC had two general carburetor options for the (pre-1964) American: a single-venturi model (Holley 1931, 1904, 1906 or 1908), or a two-venturi Carter WCD. In Classics, Carter AS or RBS was used, depending on year and transmission. The WCD requires the matching two-hole intake trough cover plate. These setups are somewhat scarce, but available. The WCD provides somewhat more high speed performance.

Carburetor choices

Any of those OEM choices would be fine, if suitable examples could be found and if replacement parts compatible with modern fuels were available. Sadly neither is true today in 2022. The aluminum castings themselves wear (pitting in the venturis, and throttle shaft wear) and worse, lack of replacement parts, and often replacement parts that do not withstand modern fuels.

In my opinion the small Holleys are of poor design, and I will never run one again (easy to say because parts aren't available anyway). The float bowl design has fuel wet on the gaskets; they leak fuel onto the hot engine eventually. The new fuels are both more volatile and more corrosive and old gaskets not fuel proof.

In 2020 I obtained AS and RBS carbs and tried to find usable parts, and failed. Those too are no longer viable. NOS carburetor plastic and rubber parts and many gaskets are flatly not usable. I can state that with confidence: they will fail in short order. The rubber is old and degrades with the passing of time, no matter how well stored, and even if you get lucky there, they will not tolerate modern fuel formulations. I reserve the right to say 'I told you so'.

The practical choices for carburetors on this engine are the Carter YF/YFA, The Carter WCD for two-barrel versions of the engine, and the Weber 32/36 DGEV, the most tunable and best performing of the lot. Alas, these will need various mechanical adaptations to work.

Carter YF/YFA

Today (2023) for various reasons the Carter YF or YFA is available; used, new-old-stock, and new manufacture, mostly chinese-made. The imported new carbs vary in quality, but good ones from reputable importers seem just fine, there are lots of advantages to new metal, and man, are they cheap.

The YF and YFA (YFA is shorter (hood clearance) and has minor design improvements over the YF, but are interchangable for our purposes) were used on AMC sixes, but not the 195.6. It does bolt right on, but the throttle linkage is usually incompatible and requires fabrication. Most YF/YFA models will need re-jetting and slightly different adjustments. The YF may have hood clearance issues when used in the lower-beltline cars, 1964 and up. And many (all?) YFAs have a wider base pattern to accommodate sometimes-larger bores.

But the Carter YF/YFA is a very good carburetor choice, maybe the overall "best" considering availablity and tuneability and reliability. They are well made and of excellent design. Parts are available, new, including fuel-proof rubber parts. There is much knowlege of them about. It has no dangerous design flaws (fire!).

20 April 2022 Specifics on installation and tuning of the Carter YF/YFA has been moved to the Carter YF/YFA page.

Carburetor parts sources

There are many, but these are my favorites, for quality of parts and quality of business services.

Mike's Carburetors, carbs, parts, rebuild kits, documentation.

Quadrajet Parts, a broad range of parts including alcohol-proof accel pumps.

Top End Performance, I buy Weber carbs and parts from these folk, they're great. Webers are made in Spain. I'll buy cloned YFs but I will only buy genuine Webers -- here it matters, as they are tweaked by hand, jets are individually flow-benched, and the precision requires good quality control. Individual unit variations matter a lot less in carbs used singly. And matching two-venturi bores (and carbs, for multi- setups) really matters; see my notes and photos on the Weber regarding this.

Holley Sniper Autolite 1100

Fuel injection is superior to carburetors in ways too many to recount here. I'd put a GM TBI system on my 232-powered 1963 Rambler Classic around 2005, it worked great, though then it was difficult to modify, it was straightforward enough with Moates's chip gear.

My hopes of joy were dashed when the thing failed on the road.

After driving my 1960 American wagon with the Carter YF (7112s) for a while I sprung the money for the Holley Sniper Autolite 1100 system, in 2020. It had been out about a year and hoped the bugs would be worked out.

It installed by-the-book, I used the Holley "Jeep" in-tank kit, that fit into the 1960 Rambler gas tank (which is the same as 1950s Nash Rambler gas tank...) with one easy modification, replacing the fixed pickup tube with a foot of submersible hose to accommodate the Rambler's different tank shape. Worked first time. The Holly Jeep system does fuel pressure regulation in-tank, requiring no return line, but this induced problems with fuel pressure pulsations that were 100&percent; fixed with a Radium Engineering pulse damper (another hundred bucks). End to end the system cost about $1300.

The Sniper tuned up also by-the-book but you really do need a laptop, Windows only, to properly tune it. The built-in LCD dongle is not sufficient to do more than "make it run". (I ran the tuner program on a Macintosh laptop using Virtualbox virtualization running a free Windows install.)

Installation details and documentation

I took notes and photos of the Sniper Autolite 1100 installation process, and worked up tuning configuration, and especially details of the Holley in-tank fuel pump system, probably the biggest single complexity of installation. However I never properly wrote up the process after the failure.

I had the old Carter YF tuned to perfection and the Sniper matched then showed signs of exceeding it when I ran into problems with their hardware. After a month of use and on the morning of imminent departure on a three-day tour with a bunch of vintage car folk, the engine began dying at stop signs (idle). The entire time I ran it it exhibited sporadic lean spikes, which I assumed were misfires. I first assumed it was my tuning, but long story short, the unit stopped delivering fuel entirely. It eventually ran again, I abandoned the tour, limped home, and with the hood open diagnosing the problem, assuming the cause was my own, it died one last time.

A call to Holley tech support revealed a failed unit, I sent it back for repair, two weeks later I got it back. No charge for the repair. "An internal connector failed" was the diagnosis.

What this told me was that Holley's system is not a daily-driver reliable system. Driving a 63 year old car on long trips requires extreme attention to reliability and likely failure points and weak spots. Dead on the side of the road and two-week repair time is out of the question. The old Carter YF went back on.

On my '63 Classic, all but the modified chip was dead-stock GM TBI parts, and in the 2000's, parts were readily available from junkyards or NAPA. And I carried a spare chip. The fuel pump was external and easily replaced and I carried a spare.

Carburetors essentially do not fail on the road and cannot leave you stranded, with the following exceptions: floats can sink, needle and seat can clog... fuel filters and fuel pumps can fail. I routinely carry spare float, needle and set, a Carter fuel pump and a few feet of hose.

OEM EFI is reliable. Hobby EFI doesn't have the same attention to reliability as OEM gear.

Electronic Carter YFA ("feedback carb") with modern controller

After the debacle with the Sniper system I drove for another year on the old Carter YF. With adaptable OEM EFI gear no longer available (the GM TBI stuff is now too old) I made another effort to find documentation on the old dreaded feedback carburetors. I finally found what I was looking for: a technical description of their operation.

Feedback carburetors were fairly roundly hated, for mainly good reason, though it wasn't the carburetor that was the problem, but the primitive control systems. The feedback Carter YFA, at least, is a fine device; this last YFA has performance improvements over the already-good YF.

The "feedback" mechanism is a solenoid that when pulsed, leans out the main circuit. With the wires unplugged the YFA falls back to being a solid, reliable carburetor. With modern control circuits and software -- microcontrollers, 40 years later, are much advanced! -- the inherently reliable carburetor is now a fine closed-loop fuel controller.

I've worked up a controller and software that makes the feedback YFA a near-ideal fuel-delivery system for a daily driver. Performance and mileage tuning from the panel, and automatic altitude correction.

The project is covered in depth in my modernized electronic carburetor project.

The Weber 32/36 DGEV

In my opinion this is an ideal carburetor for this engine and the easiest to install and get running -- but it requires rejetting and tuning to be usable. The tiny primary venturi means that this high-torque, long-stroke motor has an accurately metered and tunable mix at speeds below 2000 rpm. The larger secondary is progressively coupled, begins opening at half-throttle. At wide-open-throttle the two venturis are larger than the Carter YF. I found it to be well behaved under all circumstances and easy to tune.

Because the two venturis are different sizes the carb mounts to the trough rotated 90 degrees. Looks funny, works great. There are two adapters available for this carb; both bolt to a "Carter YF" type flange. The one I got from Top End Performance, and the Trans-Dapt version (nearly indentical) need work with a file to fit. The carb-end holes are off and need to be filed/notched to accept the bolts. The large oval hole accommodates the bores fine. Make sure that the gasket seals correctly. I had to remove the manifold studs and use 3/4" bolts to mount it due to tight clearances.

There will be a fair amount of work required to couple the throttle linkage. This is not a bolt-in project. The carb on it's adapter puts the throttle higher than the twist rod. I extended the rod pivot at the firewall and welded up an adapter for the carb end. You could convert to a modern cable throttle, but if you have an automatic with transmission cable you will have to retain that which is tricky.

Redline makes a "Jeep Weber kit" that bolts onto the 195.6 OHV's most common single-barrel trough cover. The adapter raises the carb enough to clear the valve cover (which can be removed with the carb installed).

The 32/36 DGEV will fit under the hood, with air cleaner attached, on the pre-1964 Rambler Americans. I am told it hits the hood on 1964-up Americans.

Here's the kit from Top End Performance that I used. Link valid as of April 2022; it's Kit 550. There's a hilarious note in red that says "this carb is too small buy larger" which is certainly aimed at the sports car crowd. It is the correct carb for our 50, 60 year old, slow-turning engine.

Pressed-in fuel inlets have been known to pop out and pour gasoline all over the hot engine; remove, tap 1/8" NPT, install brass nipple.

Out of the box this carburetor will run terribly. The float level is not set correctly and the jets are chosen for some hypothetical small four-cylinder with big bores and short stroke. The procedure to tune them is very straightforward, but utterly alien to people used to American type carbs. Follow the procedure exactly and you'll get great results.

Here are my Weber tuning procedures.

Here are my settings, use these as a starting point.

low speed jet 55
idle stop screw 1/2 turn
idle mix screw 1.5 turns out
main jet 140
air bleed 170
float level 35mm

Weber 38/38 DGV

The 38/38 is almost identical to the 32/36, but the two venturis are the same size, the carb is not progressive (both open at once). Though intended to be used as one venturi feeding one half of an engine, on the 195.6 and the adapter it will be plumbed as a "one barrel" carb and both V's will feed all cylinders at once. This and the lack of progression make it less attractive for stock engines. If you are absolutely certin you will be keeping RPM up and throttle open in a range that the engine will flow the CFMs, it's a fine choice. It's the carb currently on my roadster. The 38/38 DGV is bolt-compatible with the 32/36 DGEV.

Pressed-in fuel inlets have been known to pop out and pour gasoline all over the hot engine; remove, tap 1/8" NPT, install brass nipple.

Here is a copy of my 38/38 tuning notes if you care. Please refer to my notes on Weber carburetor tuning for details on these carburetors.

Weber IDF series

Less practical Weber carburetor choices are the 40 IDF and the 44 IDF. I have used these on my highly modified roadster engine, which currently has the 38/38 DGV installed. The 40 IDF is great at WOT above 3000 rpm; you will find it undrivable on a stock engine. The 44 IDF was far too large to be tuneable on even my roadster; thr 40 IDF replaced it.

Please refer to my notes on Weber carburetor tuning for details on these carburetors.