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Inglese 03-15-2009 08:19 PM

Weber Induction FAQs
Inglése™ Question Index

1. What makes Weber carburetors so special?
2. How does Weber carburetor terminology work?
3. Are Weber carburetors reliable?
4. How can a single Weber carburetor model be adapted to work with radically different engines?
5. How does a Weber carburetor work?
6. What’s with that “flat spot” I keep hearing about with 48 IDA carbs?

What makes Weber carburetors so special?

Weber carburetors have been standard equipment on the finest racing and street machinery to come out of Europe for over four decades. Maybe you've been lucky enough to get a ride in a Ferrari or a Weber carbureted 289 Cobra; if you have, chances are it's a ride that you've never forgotten! Weber-carbureted engines all have one thing in common: they assault the senses with a rush of torque that is generally unmatched among carbureted engines (and they have a sound all their own...go to a Shelby American convention on open track day and you can pick out the Weber-carbureted Cobras just by their sound; there is no mistaking it!). The world's most beautiful, exotic, and powerful engines have traditionally been fed through Weber carburetors. But why Weber?

One of the primary features of Weber carburetors is their modular design. They are produced in a wide variety of styles which incorporate different features, enabling the user to select exactly the right design and size for the intended use. You can even change Weber carburetor airflow capability to suit your needs –making Weber carburetors adaptable to more applications than any other.

Now, if you've always had trouble accepting the idea that Webers can be a terrific street carburetor, consider it this way: Weber carburetion is like an expensive musical instrument. If it is not tuned properly, that instrument will never make beautiful music -no matter what! And therein lies the secret of making Weber carburetors perform to your expectations -tuning.

Far more than merely offering good looks, Inglése™ Weber carburetion also delivers the performance that you crave. Every time you take your Weber-powered vehicle down the road, you’ll become more aware of your engine's ability to do everything it should do with a minimum of fuss. Excellent throttle response, quick acceleration and overall flexibility are the constant reminders that Weber carburetion is the ultimate induction system!

How does Weber carburetor terminology work?

Every Weber carburetor has an alphanumeric model number stamped at the base of its mounting flange. This model number, is made up of a numeric prefix that indicates the carburetor bore and throttle plate diameter, and an alphabetic suffix that indicates what type of carb it is.

Probably the best known is the Weber 48 IDA, a masterpiece of design and precision that has been around since the early 60's with only minor revisions. In this case, the model number tells us the carburetor has a bore and throttle plate diameter of 48mm, while the IDA suffix tells us that this is a high performance twin-throat downdraft carburetor.

There is also a 40 & 46 IDA/3C (“3C” denotes "3-choke", or three barrel). Again, a high performance downdraft, available in 40 and 46 mm sizes. The IDA/3C is an inline three-barrel design often used on carbureted V12 engines such as those manufactured by Ferrari and Lamborghini.

The 40, 42 and 44 DCNFs are compact twin-throats with a cold-start feature. As the prefix numbers indicate, they are available with bore diameters of 40, 42 and 44mm.

The 44 and 48 IDFs are 2-barrel downdrafts that were designed with the street performance enthusiast in mind. Weber IDFs feature easier tuning, improved streetability, and a compact design that makes them easier to fit into a tight engine compartment than nearly any other Weber carburetor.

Then there are the sidedrafts - all Weber sidedraft carburetors carry the suffix DCOE, their prefix numbers (sizes) range from 38mm all the way to 55mm (that's close to 2 1/4"). Although they were first made famous in European Grand Prix racing, DCOEs make an excellent street carburetor, both on supercharged and naturally aspirated engines.

Are Weber carburetors reliable?

Weber carburetors have an extremely simple and reliable design with few moving parts. There are no metering rods, power valves, rubber seals or plastic parts. The accelerator pump on the 48 IDA and DCOE is a brass piston. The throttle shaft rides in a set of precision roller bearings. Webers also use brass floats, which cannot become fuel-logged and sink with age.

Quite simply, Weber carburetors are a superb example of simplicity, precision machining, and beautifully-fitting components. That's another reason why they're well suited to street use and long-distance cruising - they are extremely reliable.

With the infinite tuneability of Weber carburetors, there is no need to compromise drivability or road manners. If you know someone who suffers from drivability problems with such a nice carburetion system, he is doing so unnecessarily. A Weber unit should be crisp, responsive and smooth. If it is not, something is wrong - let's just say he's not through tuning it yet, that's all!

How can a single Weber carburetor model be adapted to work with radically different engines?

Weber carburetors were designed to be totally adaptable to any size engine, for any purpose, at any altitude. There is no such thing as taking four of these out of their boxes, bolting them to an intake manifold, and going for a spin around the simply isn't done that way. These carburetors were intended for serious performance enthusiasts who want the most that their engine can give them and are willing to put in the tuning time to get it.

Somewhere along the line, you may recall seeing four 48 IDAs on a Big Block Chevy in a street rod. You've may have also noticed the same 4 x 48 IDA setup installed on a 289 Small Block Ford in a Cobra or Shelby GT-350. You may have wondered how the same carburetor setup could work on two such vastly different engines, as it would seem that one engine would have to be either over or under carbureted. The reality is that neither engine is running the same set of carburetors as the other. Assuming that the Webers are set up properly, the only thing the two systems will have in common is their outward appearance.

A Weber carburetor’s most interesting design feature is its removable "choke" or venturi, which allows it to be instantly converted from a large-CFM carburetor to one of small CFM, and vice-versa. The choke size plays the single most important role in determining drivability, throttle response, and torque output. The appropriate choke size for the application depends on many factors, among them engine displacement, compression ratio, and your intended use for the motor. Once the correct size choke has been selected for your application, the jetting for all the rest of the circuits can be established around that choke size.

Installing a smaller choke in a 48 IDA restricts the carburetor, causing it to flow fewer CFM for better torque and midrange –especially on a low-compression small block engine. Pull out those small chokes, drop in some large-diameter ones, and those 48 IDAs will flow enough CFM to make a big block scream. However, as with a large-CFM conventional 4 barrel carburetor -don't try putting those "big" carburetors on the small block motor, as doing so will result in a loss of throttle response and poor drivability, especially in traffic.

How does a Weber carburetor work?

For the sake of simplicity, let's look at a Weber carburetor as having three basic circuits- the idle circuit, the accelerator pump circuit and the main circuit.

Idle circuit

The idle circuit is comprised of two components: the idle jet and the idle jet carrier. With these two pieces, the tuner can select exactly how much fuel and how much air he wants to provide the engine at idle and at low rpm, while making very fine adjustments to either. The idle mixture’s total volume can be further regulated with the idle mixture screw, which is located on the lower part of each carburetor barrel.

On a correctly-jetted idle circuit, the mixture screw on a 48 IDA is never more than 3/4 of a turn out. This holds true 100% of the time, no matter what anyone else tells you. If you have to go more than that, your idle jet is too small. Even if you get it to idle, adjusting more than 3/4 turn tells you that the jet is lean and you're going to have other drivability problems, which brings us to the next part of the idle jet's function.

The idle circuit in a Weber carb isn't just an idle circuit -it is actually the circuit that carries the engine all the way up to about 2,800-3,000 rpm, where the transition to the main circuit takes place. After 3,000 rpm or so, the idle circuit is entirely bypassed in favor if the main circuit. So, if you have a tuning problem that "goes away" after about 3,000 rpm, take it as your cue to look for the problem within the idle circuit.

Accelerator pump circuit

The accelerator pump circuit, just like on any carburetor, is responsible for eliminating "bog" and making a passing maneuver without a hesitation or stumble. The accelerator pump circuit has two basic elements: the pump exhaust valve and the pump jet.

The pump exhaust is nothing more than a bypass valve located in the bottom of the float bowl. This is the piece that regulates how much fuel is made available when you need that pump shot. Putting a bigger bypass hole in the valve allows more fuel to bleed back into the float bowl instead of out of the accelerator pump jets. Putting a smaller hole in the bypass valve causes more fuel to squirt out of the accelerator pump jets. You can even put in a "closed" bypass for drag racing, when you need all the fuel you can get in order to get those slicks turning.

The duration of the pump shot is varied by installing a larger or smaller pump jet. Larger pump jets give a heavy blast over a short period, while the smaller ones will give a finer, longer-duration shot. As long as you leave the bypass valve alone, you're still getting the same overall volume. In most cases, the stock pump jets can be left alone.

Main circuit

The main circuit is the easy one. This is where you make your power. This circuit has three primary elements that you should concern yourself with - the main jet, the emulsion tube, and the air corrector.

The main jet is stuck into the bottom of the emulsion tube and sits in fuel. As the carburetor begins to work, the main jet meters the amount of fuel allowed to pass through it and up into the "main well" around the emulsion tube. Air enters the top of the emulsion tube through the air corrector which meters the amount of air to be mixed with the fuel. The air blows out of the emulsion tube through a series of holes along its length and aerates the fuel that is rising up the well around the tube. This emulsified mixture is then sucked out of the main delivery nozzle as the vacuum in the carburetor increases to the point where it's strong enough to pull the air/fuel mixture out. This occurs by 3,000 rpm or so, and you're down the road like a shot.

Tuning the main circuit for maximum power is something that can be done by a series of road tests and a handful of jets. The simple rule of thumb for jetting Weber carburetors is, if you want to implement a change over the entire rpm range, you change the main jet. If you want to change the way the car feels at high rpm, you change the air corrector. Keep in mind that the air corrector is a finer adjustment that the main jet. As an example, one step upward in the main jet (richer) produces the same change in engine behavior as three steps down on the air corrector (less air: richer).

A change of air corrector would be appropriate, for instance, if the engine pulls strong to 5,000 rpm and then goes flat. This would mean it's running lean at the top end; drop the air corrector three sizes or so, and you'll probably be able to buzz that engine right up to 7,000 rpm. If the motor feels sour all the way up, go one or two sizes heavier on the main jets.

What’s with that “flat spot” I keep hearing about with 48 IDA carbs?

One the most frequently experienced tuning problems associated with Weber carburetors is a seemingly incurable and very annoying flat spot that occurs at about 2,200-2,800 rpm. This condition is generally caused by one of two things - you either have the wrong emulsion tube in the carburetor, which is causing a rich stumble due to an under-emulsified mixture at that particular rpm range or the idle circuit is falling off too early to carry the engine up to the point where the main circuit can take over, leaving a "lean hole". In simple terms, the idle circuit is going lean too early. Either condition is easily rectified.

In the case of the emulsion tube, there are really only a few that work really well for V8 applications; and if you aren't using one of them, it is certainly a big part of the problem. If the flat spot is still there even with the correct emulsion tube, then you'll need to richen up the idle circuit. This is sometimes a tricky area, because the first thing that you may want to do is throw in a bigger idle jet, but sometimes playing with air bleeds, mixture screws, or choke sizes can accomplish the same thing while sticking with the original jet size. Seeking a little bit of sound advice here can save a lot of time and hassle.

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*13* 10-18-2009 07:06 PM

Thanks for posting! My Webers were a vast improvement over my Holley Double Pumper. I love toying with, figuring out & tuning carbs!

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