28 May 2010
AMC called this engine the "Economy Six". It was designed for utter simplicity, reliability, smoothness and driveability, not high performance. It also had to fit in the very narrow engine compartment of the first American production car semi-monocoque chassis -- the hole for the engine is only 20 inches wide!
This engine, like all of the Nash Six overhead valve line, has an integral intake manifold cast into the head, the "trough". Besides cost and fit issues, performance-wise it means that the intake is at a constant temperature and carburtetor heat is built-in. (See the exhaust modifications page for changes to that system.)
It has an interesting advantage too in that it delivers perfect mixture distribution to all cylinders, a problem on long inline sixes. If you click on the picture to the left, between cylinders 2 and 3 (and 4 and 5), adjacent to the second head stud from the front of the engine, you will see within the right hand trough wall a ramp-shaped protrusion cast into the trough. It pinches mixture flow at that point -- it is an anti-reversion device, preventing back-flow of intake mixture pulses. All six plugs burn to the exact same color. Very nice.
Luckily the trough shape and dimensions are not particularly bad; it's adequately sized for the flows involved in an engine unlikely to see much past 5000 rpm. The runners off the trough are straight rectangular shots into the valve pocket.
The trough is covered with a cast aluminum trough plate, a very handy design for hacking induction. It's flat, and trivially easy to fabricate one from scratch. Even that wasn't necessary in this case.
AMC provided two carburetor options for this engine: a single-venturi as the base model (Holley 1904, 1906 or 1908 in Americans, or Carter AS or RBS in Classics, depending on year and transmission), and a two-venturi Carter WCD which AMC called the "Power Pak" option (relatively uncommon in it's time and now a highly desirable option). All are very reliable, easy to maintain and un-fussy to adjust and drive, but are terrible performers. And for the sort of driving I increasingly do, the WCD annoyingly starves out in every single hard turn.
Anecdotally, the one-barrel carb "runs out of steam" about 3000 rpm wide open throttle; the WCD will get you about 3500 rpm before it flats out. They are allegedly 200 and 300 cfm flow, respectively, which ought to be enough for more power than that, but wide experience says otherwise.
Initially I searched for carb options to bolt onto the two-barrel trough plate; but I soon realized that the dirt-common one-barrel plate with it's standard "YF" bore was far easier to adapt.
Ditch the stock carbs and give them to restorers. They're worthless for any sort of performance.
My research ended when I realized that Redline makes a "Jeep Weber kit" that simply bolts onto the 195.6 OHV's trough plate. The necessary adapter also raises the carb enough to clear the valve cover (which can be removed easily enough 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. I've installed a Weber DGEV 32/36 on an AMC 258 six before -- it was ideal. With a primary bore not much over an inch in diameter, low speed performance is great, which is exactly what you want for around-town luggy driving with a long-stroke six -- yet more than enough flow for "high speed" operation.
With the mods below and the DGEV 32/36 in place, this engine pulled hard and fast to 4200 rpm before I upshifted -- until I finish engine break-in I can't do more, tantalizing as it is.
Here's the overall relationship of head with the trough plate, adapter and carburetor installed:
Here's the jetting I ended up with after a month of tuning. There might be further changes but it's pretty close to correct. Plugs run lots-of-milk-in-coffee color.
Old inefficient engines like this one usually want richer idle jetting. The idle jets in a Weber carb determine the A/F ratio during progressive-circuit operation -- just-off-idle operation. With this engine's long stroke and small bore it spends a lot of time running within the progressive circuit so it's worth getting it right.
Stock out-of-the-box jetting was idle primary 60 (fat), secondary 50 (skinny); main secondary jet was 140.
It found it quite easy to blend the adapter to the trough plate and to remove all the right angles and sharp edges in the flow. The first thing I did was to add locating pins to the plate and adapter so that it would stay in alignment after it was blended. (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. Basically I picked a likely place for two 1/8" pins on the plate where the carb goes and drilled holes about 3/8" deep. Into these holes I dropped nipped-off wire brads with the points sticking up slightly. Then 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. Then I bolted the adapter to the plate and filed them 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 throttle linkage worked out neatly. I am very picky about throttle feel, and I truly love AMC's simple system. It allows a feather-light throttle with short travel and great feel. The floor-mounted pedal pushes a bellcrank that rotates a shaft that is coupled directly to the carburetor throttle shaft. Simple!
I retained the original design, and most of the original parts. The Weber's throttle shaft was longitudinally in line already. Due to the adapter however it was almost two inches higher.
I found it easiest to cut the firewall-mounted pivot bracket and weld in the two inches of height. (I got it a bit too high, but it's fine.) However, the Weber throttle rotation direction was opposite the stock carb. I made a new pivot rod from an old 232 six pushrod (very handy, those) ground square on one end to fit the slot on the carb; the other end was the correct size for the stock pivot bracket. I welded on a tab for the bellcrank linkage, on the opposite side to accomodate the reverse rotation, and made up the linkage from 10-32 threaded rod and tiny ball ends from my junkbox.
This retains the original look and feel of the factory system, and the mechanical progressive secondary can easily be felt. Sweet!