|Weber Downdraft Carburetor Installation Guide|
|Written by Glenn Stephens and Rob Shisler|
|Saturday, 03 September 2005|
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.Choke Types:
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.Manifolds:
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.
Stock "peanut" style manifold.
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.
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
Tuning a carburetor can be a maddening and very rewarding experience. As of this writing, we can only offer advice on how to diagnose and suggestions on how to tune your Weber downdraft. Eventually, the FAQ will have the aforementioned Setup Database that will allow you to simply match your engine/exhaust specs to a table, and chose the Jetting setup that looks like the best match. Of course all engines are different, but this will help to give a lot of people a head start in fine-tuning their performance.
Understanding how to tune a carburetor begins with understanding the basics of air-fuel ratio as it relates to car engines. Gasoline vapor burns most efficiently at a ratio of 14 parts air to one part of fuel. This "ideal" ratio is often referred to as "Lambda." However just because this is the most efficient ratio, doesn't mean it is the TRUE ideal for your particular application. Modern fuel-injected cars often run at VERY lean settings of 16:1 or more while just cruising along, but can also instantaneously adjust themselves to much more powerful settings of as little as 12:1. (More fuel per unit of air produces more power, up to a certain point).
With carbureted cars, this level of electronic adjustability isn't available so we have to set up our cars based on our usual driving style or, for enthusiasts, to produce the most power (or some trade-off therein). It is important to know that a carb that is running too lean will tend to "race" or "surge" as you accelerate through the powerband, whereas one that is set up to run too rich will tend to "bog" or "stumble." Knowing those characteristics will help you to diagnose and tune your own setup.
If your carburetor was shipped from a dealer that knows '02s, it is highly likely that it is set up for at least a stock '02 configuration. This can provide you with a good baseline for making subsequent adjustments. Certain vendors can in fact set up a carb based on their experience for whatever setup you happen to have in your car. Carbs "rescued" from a junkyard, e-bay, etc., however, may have been set up for any number of different situations and may require extensive tuning. Furthermore, enthusasts who change other aspects of their engines' performance hardware will always want to re-jet for the new changes.
Proper kink-free installation of Fuel Line from stock Mechanical Fuel Pump
What follows are links to several online sources of MUCH more extensive info on how to tune a carburetor for various setups. Also please note that the FAQ Store now carries a full line of NEW Webers, parts, jets, RamFlo Air Filters, etc., at super-low prices, and the profits from any sales help to keep the FAQ site going! And, as always, if you have any questions or comments, please feel free to post them to the Message Board!