06 january 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 when this engine installed in 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 a desirable option). 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.
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.
each of the carburetors i've had on this engine are discussed below. in reverse order, latest carb first.
|Weber IDF 44||modified dual||much better flow; no choke; fabricated plenum; ultimate tunability|
|Weber 38/38 DGV||single||larger symmetrical version of 32/36, better performance|
|Weber 32/36 DGEV "Jeep kit"||single||easy install, improved performance and mileage, great for stock engine|
|Carter WCD (stock)||double||simple and reliable; starves in modest turns; hard to find|
note that the single-hole carburetor is a small Holley/Carter YF pattern and spacing. the two-venturi (Carter WCD) base pattern is the "small rochester", aka Rochester 2G pattern.
Weber carburetors work great on this engine, and are far better performers and the same or better fuel efficiency thatn any of the stock carbs. however the common lore on how to tune them is fundamentally flawed, as nearly all of it has been reduced to rigid rules more appropriate to "sports car" engines -- big bore/short stroke, high RPM, and importantly, one venturi per cylinder or pair.
here are my Weber tuning procedures that take into account the characteristics of this, and similar, multi-cylinder long stroke manifolded motors.
(i took those photos the weekend i first got it running; here, there are only the (so-called) tea strainers on the velocity tubes. a real Redline air cleaner was installed a week alter. one of these centuries i'll take new photographs.)
the IDF series are so-called "race" carbs, more or less downdraft versions of the DCOE and other side drafts, with one fewer circuit to tune (no adjustable low speed air bleed). they are more tunable that the DGxx types (all jets including main jets changeable from outside the carb). IDFs have no choke or cold-start mechanism.
the DGV and DGEV carburetors, below, while not "OEM replacement" simple do bolt on directly (with adapter) and you'll need to only fabricate throttle linkage.
the 44IDF this is not a "stock replacement" carburetor. it will definitely poke up through the hood, after you make your own adapter from scratch. also there's no choke so it will not be a good grocery-getter in northern Wisconsin, let's say.
most of the effort was in constructing the plenum/adapter to mount the carb to the trough plate and head.
the carb itself got some working over. the booster venturis were fine out of the box but the aluminum castings benefit from some hand attention not feasible at the factory. the booster venturi castings were rough on the leading and trailing edges, with ground-off sprue stubs and other rough stuff. not visible here is that the internal passages in the fuel feed from the bowls are internally rough, with flash partially occluding the port. these got attention via magnifier and tiny needle files.
here's before (38/38DGV) and after (44 IDF) compared.
the 2017 build required more carburetor, and research showed that the Weber 38/38 ought to be adequate. 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 a drop-in (hence no install-specific photos), bolt-on compatible with the 32/36 and an immediate improvement. 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.
(in my opinion, this carburetor plus an ignition upgrade would be a noticable improvement for a stock engine at least cost and increased reliability.)
the 38/38 got a lot more tuning attention once it was installed on the 2017 motor. my spreadsheet notes are here. but i ended up with the following jetting:
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.
|low speed||high speed||air bleed|
the remaining off-idle flatspot was the usual weber transition hole vs. throttle plate thing i never dealt with.
Redline makes a "Jeep Weber kit" that bolts onto the 195.6 OHV's most common single-barrel trough plate. 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.
here's the kit from Top End Performance that i used. link valid as of feb 2019; it's Kit 550, though i recommend the "upgrade to 38/38" option for 30 bucks.
the 32/36 also runs out of flow (subjectively) around 3000 rpm. however it's tiny primary bore is a great match for low speed luggy stock engine and gears, and is far more tunable than stock carbs. it also does not starve out in turns.
(the 38/38 and 32/36 venturis are aligned perpendicular to the trough; hence the carb sits sideways. on the smaller carb this solves the different flow rates of the progressive asymmetrical bores.)
here's the overall relationship of head with the trough plate, adapter and carburetor installed:
jetting the 32/36 DGEV was easy, because i didn't sweat it much. i got mid 20's MPG it ran great and i wasn't looking for performance then.
it was easy to blend the adapter to the trough plate and remove all the right angles and sharp edges in the flow. The first thing was to add locating pins to the plate and adapter so that it would stay in alignment after blending. (the adapter has slotted holes that let it slide around.) unfortunately I neglected to take any photos of this, but it's not rocket science. I picked a likely place for two 1/8" pins on the plate where the carb goes and drilled two 1/16" holes about 3/8" deep. into these holes I dropped nipped-off wire brads with the points sticking up above flush approximately 1/32". I carefully aligned the adapter by hand, set it onto the plate (sitting on the brad tips) and whacked it hard with my hand, pricking the bottom of the adapter. I used the prick marks as guides to drill 1/8" holes for the pins, and replaced the brad tips with 1/8" wire. the adapter was to the plate and filed to match.
with the adapter joined to the plate, I smoothed them where i believed the flow would actually go. the big hole in the bottom of the plate had sharp 90-degree edges; I smoothed these as much as I dared, probably a 1" radius. the carb end of the adapter I made slightly (1/32" or so) larger than the carb bore. the inside of the adapter itself was cast lumpy; I smoothed that out substantially, left it with a faintly venturi shape, and increased the overall diameter about 3/16".
the WCD was part of the "Power Pak" option (relatively uncommon in it's time and now a desirable option). they are reliable, easy to maintain and un-fussy to adjust and drive; they aren't good performers, and the design starves the main jets in every hard turn. it's not easy to find a good one, and the trough plate with it's Rochester 2G base pattern will be even harder to find.