This article deals with the most commomly upgraded-to '02 downdraft
carburetors, the Weber 32/36 and the 38/38. The carbs are named for
their throttle diameters in millimeters. The 32/36 is also a sequential
carb, meaning that first the smaller 32mm throttle butterfly opens,
then as the throttle is opened further the second, 36mm throttle opens.
On the 38/38, both 38mm throttles open simultaneously, giving smoother
overall response and more power.
There are three popular types, designated by the : manual choke (designated
by the model suffix DGx), water choke (DGAx), and electric choke (DGEx).
Because fuel does not atomize well when the motor is cold, the choke's
purpose is to enrichen the mixture during those cold starts by cutting
off the supply of air. On the 32/36 and 38/38 downdraft this is accomplished
by the square butterfly valves at the top of the carb inlet. The throttle
plates themselves are at the base of the carb.
Manual Choke: The manual choke has a cable-actuated mechanism
allowing the driver to manually set the amount of choke they need for
cold startup. Many people prefer them for their simplicity and the level
of control. When installing or adjusting a manual choke cable, obviously
the idea is to allow for a full range of movement of the choke plates,
from wide open to snapped shut. The only connections to a manually choked
carb are a vacuum takeoff for the distributor (optional), and the choke
cable itself. There is a kit available to retrofit the 32/36 either
electric or water choke with a manual control, and this kit also works
with the 38/38.
Water Choke: The water choke is semi-automatic. To activate
it, you depress the accelerator all the way which causes the choke cam
to catch with the butterflies closed. Water from the engine block is
run into the choke housing which heats the mechanism and allows the
choke to open as the engine warms up. The water choke requires a small
water hose to the engine block and the manifold. There are an optional
electrical connection to a solenoid which goes to a temperature sensor
in the manifold, and a vacuum port for the distributor.
Electric Choke: The electric choke is essentially a timing circuit
made of a bi-metal coil attached to the choke pivot. When you turn on
the ignition on a cold morning, the choke is normally closed. As the
electricity begins to heat the coil, it slowly contracts and opens the
choke plates. Thus it is important to avoid leaving the ignition on
on a cold morning without starting the car because you will open the
chokes without realizing it, and thus make it harder to start when you
finally do try to start it. The electric choke requires, not surprisingly,
a 12-volt ignition-switched and fused source. The easiest places to
get + 12v for a car not normally equipped with an electric-choked carb
is from the + side of the coil, or from the #12 fuse (later 12-fuse
cars) or #11 fuse (early 12-fuse cars) or the + terminal of the coil.
There is a vacuum advance port for the distributor (optional).
To adjust the electric choke, with the air cleaner off and the car
cold, loosen the 3 screws surrounding the plastic cover of the electric
choke mechanism. Rotate the plastic cover until the choke butterfly
valves are just closed. This is the choke position. Start the car and
get it to idle. As the choke and the car warm up, the butterflies should
open to a straight up position - takes about 5 minutes. If they open
all the way, then tighten down the 3 screws and you are good. If not,
turn the plastic cover until they are straight up, then tighten.
There were two types of double-barrel manifolds that originally came
on '02s: the "Peanut" manifold and the regular two-hole one.
These were both designed for the 32/36. In addition, there was a performance
manifold made for these cars back in the day by Cannon for people who
upgraded to 38/38s or 40/40s. This manifold was "Hogged" out
so that instead of two seperate holes, or a single peanut-shaped hole,
there was a single large oval hole for better airflow. It is pretty
easy to make this modification to either of the stock-type manifolds.
Any type of carbide metal cutting tip for a die grinder will do the
job, or a jigsaw/carbide grinder tip combination. Just cut the basic
shape clean with a jigsaw (make sure the blade wont bottom out inside
the manifold!) and then use the carbide grinder to clean up things.
"Hogged" intake manifold and stock two-hole.
If you dont want to "hogg" out your manifold, you can simply
enlarge the holes with a carbide-tip grinder, as mentioned above. The
easiest way to do this is by tracing around the gasket with a marker.
Glenn used abrasive cylinders and cones on an air-driven die grinder.
You could also use a drill or a dremel. Make it a smooth, but it does
not have to be polished. There is actually some disagreement about polishing
the intake tract of a street car, because the turbulence created by
the uneven cast surfaces can actually enhance the mixture of fuel and
air, and therefore give you better combustion. This is a controversial
point, so you may want to come to your own conclusions about it after
doing your own research. While you are at it, you may also want to bore
out and/or polish (?) the cylinder inlet runners to match the size of
the holes in the head. Using the gaskets as templates is, again, the
easiest way to do this.
Stock "peanut" style manifold.
In addition, 1974 and later vehicles have a hole in the end of the
manifold for an EGR valve. Depending on your local smog laws, you may
decide to remove the valve and plug the hole, either with a threaded
plug or a metal plate. A 58mm circular aluminum plate works perfectly.
There are three studs that can be tapped down and cut off for mounting
the plate. Put some RTV around the hole for good measure.
Manifold Connections:
The manifold has two large water inlets, one the bottom and one on
the firewall side. These are used to circulate water through the manifold
which are designed to help with fuel vaporization during engine warm
up and in cold weather. Hot-weather enthusiasts often just bypass them,
but early-risers who have to drive to work in the cold would do well
to keep them connected. Typically there is also a small water nipple
on the end of the manifold that is used as one of the connections for
a water choke. In other applications you can cap this with a bolt and
copper washer or run a hose to the connection on the rear of the engine
block.
There are lots of little nipples on the later manifolds used for vacuum
connections for the emissions systems. Again, depending on your local
smog laws, you may want to remove all that stuff and block them off
with rubber caps or plugs. There is a large nipple that should be connected
to your brake booster with a check valve in the middle. The valve should
be situated so that vacuum can be held in the booster after engine shutdown.
Between the 3rd and 4th intake runner is a threaded hole where the sensor
for a water choke solenoid can go. Other applications can remove the
sensor. On the bottom of the later manifolds is a temperature switch
which is used for emissions. It is a wet connection so if you are desmogging,
either leave the sensor unconnected or plug it with a bolt and copper
washer.
Later-model Emissions Loom that is replaceable by
single wire from #12 fuse to + side of Coil.
To remove the manifold, disconnect all the hoses and remove the 12mm
nuts from the studs. There is another bolt that attaches to the starter.
There is no need to pull the starter. Some of them are tough to reach
but just be perisistent. LABEL all the wires and hoses you intend to
keep, and once they and the fasteners are removed, the entire manifold/carb
assembly will pull right off.
Linkage:
Most carb kits come with a proper linkage for your vehicle, but you
may find yourself adapting your old linkage or using or making your
own cable linkage.
Stock-style rod linkage installed on 38/38 carb.
For a stock shaft-style linkage, adjust the throttle control shaft
(carb to firewall shaft) so it is level and straight - inline with the
carb. This is done with the pivot bracket on the firewall. Then adjust
rod end (on rod coming up from pedal box to control shaft) to achieve
full throttle opening when depressing gas pedal. You can look through
the windshield at the rear of carb to confirm full opening. Place the
spring as shown with the long arm down and hook toward the engine bay
so that it does not catch on the heater hose.
Rod-type linkage at firewall.
Rod-type linkage at carb.
If you are replacing the stock shaft linkage with a cable setup, Rob
suggests making your own from commonly-available bicycle cable parts
and perhaps some hand-made cable "stays." What you need is
about two feet of cable including the shroud (get high-end teflon-lined
cable), two metal brackets that you can make yourself, and two adjustable
cable housing retainers such as you find at the end of the cable shroud
on a bike brake cable, for example. You will also need the two cable
clamps that hold the actual cable itself to the end of the lever arm
attached to the throttle pedal, and the one that attaches to the throttle
mechanism on the carburetor itself.
38/38 Carb and Cable Linkage Detail
Fuzzy pic of Lower Cable Stay Bracket.
The lower cable stay is bolted to brake booster bracket via the large
main bolt that holds the upper pedal box "arms" in place.
A simple 90-degree angle and hole set the cable end above the arm attached
to the throttle pedal. You will need a similar type of rotating cable
end clamp to fit in the hole at the end of the throttle arm as you have
for the throttle mechanism at the carb itself. Please note in the picture
of the lower stay (above) that the cable exits the lower stay at a severe
angle. This is the way the thing was installed in my parts car, and
it, among other problems, caused severe binding.
When I moved the cable setup to my project car, I fixed all these details
and "tuned" the operation of the cable (shortening it, and
chamfering the outputs of the cable housing end holders) itself so that
only the carb spring itself would pull the cable up and close the throttle.
I also actually used two more springs (one at the pedal box and an additional
one at the carb) as failsafes, however, but its nice to have a nice,
smoothly operating cable without any binding. Overall, the kit produced
by a well-known Southern California (non-'02-specific) performance tuner
that I inherited in my parts car, and its subsequent installation by
a well-known local independent BMW shop, were of such alarmingly poor
quality that the throttle would often bind up and not close fully even
with ALL THREE springs installed. No more of that!
Jetting and Tuning:
Carbs must be jetted to match the performance characteristics of your
other engine and exhaust modifications (compression, header, cam, etc.)
There are 3 kinds of jets on a downdraft Weber: Idle, Main, and Air
Correction Jets. The idle jet can be changed easily but main and air
correction jets require you to unhook the choke lever (small circlip),
remove the 6 screws on top of the carb, and pop the top.
Idle Jet Location for Weber Downdraft Carbs
Main and Air Jets Locations Inside Carb Assembly
