6 may 2019


this section covers induction, fuel and air intake: carburetion. i've been tempted to make/install an EFI system for this engine, and maybe i will, but for now, i'm still having fun with carburetion.

AMC had two carburetor options for the (pre-1964) American: a single-venturi base model Holley 1904, 1906 or 1908, and a two-venturi Carter WCD called the "Power Pak" option (relatively uncommon in it's time and now desirable.) in Classics, Carter AS or RBS was used, depending on year and transmission. all are very reliable, easy to maintain and un-fussy to adjust and drive, but are lousy performers. for the sort of driving I increasingly do, the WCD annoyingly starved out in every hard turn.

carburetor types, a summary

american car folk generally assess carburetors by size, in cubic feet per minute at some pressure drop, aka "CFM". european and sports car carbs are generally grouped by venturi ("choke") size. 99% of my driving is at part throttle. nearly all of my intentional driving is "sports car like", twisty mountain and desert roads and highways, where accurate fuel delivery over a wide range of air flow matters. but CFM is a common and reasonable measure; and for rough comparison, the 44IDF is approximately equivalent to 350 CFM, both venturis combined.

all of the Webers are "sports car" carburetors and will not starve out or act weird in hard turns. the WCD starved the main jet every turn.

each of the carburetors i've had on this engine are discussed below. in reverse order, latest carb first.

carburetor trough cover notes
Weber 38/38 DGVmodified dualmounted on fabricated plenum
Weber IDF 44modified dualfabricated plenum; carburetor too large!
Weber 38/38 DGVsinglelarger symmetrical version of 32/36
Weber 32/36 DGEV "Jeep kit"singleimproved performance and mileage, great for stock engine
Carter WCD (stock)doublesimple and reliable; starves in modest turns; hard to find

note that the single-hole trough cover carb pattern is the Holley/Carter YF pattern and spacing. the two-venturi (Carter WCD) pattern is the "small rochester", aka Rochester 2G.

carburetor selection

now that i've run three different Weber carburetors on this same motor i can summarize my experience with them.

32/36 DGEV in my opinion this is an ideal carburetor for this engine and the easiest to install and get running. 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 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; tight clearances.

38/38 DGV is identical to the 32/26 except for the larger symmetrical bores. for my modified engine and ported head this is probably the correct carburetor; the 32/36 was perceptibly limiting top-end (3200+ rpm) power. the first time i used this carburetor i installed it in place of the 32/36, and somehow overlooked that the adapter occludes the outside edge of the throttle plates when wide open. there is no interference, but it probably hurts maximum flow. i returned to the 38/38 after losing my argument with the 44 IDF (details below). this time i made a proper tube plenum/adapter to fit the modified two-barrel trough cover. the new plenum aligns the carb with each venturi favoring three cylinders.

44 IDF installed in december 2018, removed may 2019. battling this thing got me deep into carb and Weber theory of operation, i know know the Webers a lot better now, and i'm confident that the 44 IDF is simply too large. low speeds (idle mix and jets) tuned up just fine. however, the engine ran on the low speed jets (transition holes) up through 60 to 70 mph, the main jets not in use at all under 2500 rpm, and there was not enough airflow through the 36mm venturis for the main jets to work right -- i was up to 220's (2.2mm) and it was still crazy lean. i could probably get it to work with 32mm chokes but i stopped when i realized that even if i did, much of my driving would then be right in the middle of the low speed/high speed crossover region, a very difficult area to get perfect. it was at this point i reverted to the 38/38 with a real plenum.

installation details

below are some photos and descriptions of installation details.

Weber 38/38 DGV on tall plenum (2019)

this is the current installation and likely the final one. below are plenum construction photos. the plenum is tall so that it places the air filter base at the same location as the 44IDF installation, for which i had built an airbox system into the hood that meets and seals the air filter.

the trough head has an extremely short path from throttle plate to intake valve, even on the end cylinders. there is no intake runner "tune" to interfere with. and the height sure makes things easy to work on!

below are photos of the two trough covers i've used on this engine. on the left is the 1V cover used with the adapter, on the right the modified 2V cover with the plenum attached. the 1V cover and adapter were both bored larger (see below) and matched. i can't find my notes with the numbers but the area of the rounded rectangle in the 2V cover is within 5% of the area of the two 44mm bores combined, about 5.5 square inches. the runners interfered with the stock fastener location so i added four new tapped holes for Allen head screws, inboard front and rear.

the 1V cover was bored out to 1.625" from stock 1.5". the dirty cover below is dead stock taken from a 1965 engine. that .125" increases bore area by 8% (to 2.5 sq in). i'm guessing, from experience, that the enlarged 1V cover is more than adequate for anything this motor is capable of, but not worth the effort for a stock engine.

Weber 44 IDF (2018)

the IDF series are more or less downdraft versions of the DCOE without adjustable low-speed air bleed. they are more tunable (and demanding) than the DGxx types. the main jet, emulsion tube and air bleed are removable externally (very nice for tuning). IDFs have no choke or cold-start mechanism, so not a good choice for Wisconsin winter grocery-getting.

the plenum adapter is tall because that's the shortest i could make it with this fabrication technique. the runners are thinwall 1.75" ID steel, deformed and blended into the rounded-rectangle enlarged Rochester 2G opening. at the top carb end, a tube cut at an angle is oval, not round, and acceptable error limited the angle and therefore minimum height.

i became very familiar with the Weber system with this carburetor. here are my my Weber tuning procedures dealing with this device. most of the effort was in constructing the plenum/adapter to mount the carb to the trough cover and head.

(i took these photos the weekend i first got it running, with only the (so-called) tea strainers on the velocity tubes. a real Redline air cleaner was installed a week later. one of these centuries i'll take new photographs.)

Weber 38/38 DGV on small adapter (2017)

the 2017 build required more carburetor, and research showed that the Weber 38/38 ought to be adequate [may 2019: IT IS]. with this build i had also changed transmission (from two different 3-speeds to a custom T5 and carefully selected axle and tire size) and so RPM range and driving habits changed to match, which also allowed for much more aggressive spark advance as well, which in turn changed how and what RPM ranges i drove at. given all the changes it's impossible to assign improvement to any one change. in general, i'm running the engine at much higher rpms than the 2010, stock build. 3500 rpm was "scary", now it's routine. accordingly spark is set for these higher rpms and i no longer lug it like the stock engine.

the 38/38 is bolt-compatible with the 32/36, but see my note above on the possible restricted flow with the 32/36 adapter. of course with both carbs you have to fabricate throttle linkage. a small anoyance is that the left-side idle mixture screw interferes with valve cover removal; the screw needs to be removed to get the cover off. this is not the case with the smaller 32/36.

the 38/38 DGV 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.

The Weber 32/36 DGEV (2010..2017)

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); if you look closely at the picture to the right, you can see how close the carburetor is to the valve cover.

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.

the venturis are aligned perpendicular to the trough; the carb sits sideways. this solves the different flow rates of the progressive asymmetrical bores. shown also is the Redline adapter, dirty from use, but showing the adjusted holes and the tight clearance bolt issue.

here's the kit from Top End Performance that i used. link valid as of feb 2019; it's Kit 550.

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