no doubt some of you will have seen some of my posts re the Weber IDA. Here is a beginner's guide - basically the result of my research so far on this brilliant carb

enjoy !


The Weber's most interesting design feature is it's removable "choke" or venturi, allowing it to be instantly converted from a large-CFM carburetor to one of small CFM, or vice-versa. By installing a smaller choke, the carburetor is constricted and it flows less CFM, to make it perform in the midrange, or to make it suitable for use on a low-compression small block engine. Pull out those small chokes, drop in some large-diameter ones, which may be nothing more than thin-wall "sleeves", and you've got a set of 48 IDA's that will flow enough CFM to make a big block scream. But don't try putting those "big" carburetors on the small block motor! It will fall flat on it's face, lack throttle response and become a complete nightmare in traffic. In order to get drivability, throttle response and lots of torque from the Weber-carbureted engine, the choke size, therefore, is the first consideration. How big is the motor, what's the compression ratio and what do you want to do with it, 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. As an indication, for street port 12A RB recommend 37mm, street port 13B RB recommend 38mm. 12A bridgeport RB recommend 42mm

For the sake of simplicity, let's look at the 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 during the low rpm operation, while making very fine adjustments to either, if necessary. The idle mixture is delivered as a proportioned mixture whose 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 will hold true 100% of the time, no matter what anyone else tells you. If you have to go more than that, you'd better heavy-up the idle jet. Even if you get it to idle, going more than 3/4 turn tells you 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 the Weber isn't just an idle circuit - it does more than that. It is actually the circuit which must carry the engine all the way up to about 2,800-3,000 rpm, where the transition to the main circuit take place. That means if you don't drive over 3,000 rpm, you're only running on the idle jets. After 3,000 rpm or so, the idle circuit is entirely bypassed and no longer has anything to announce. So, if you have a tuning problem that "goes away" after about 3,000 rpm, that tells you to play with the idle circuit. Or maybe the opposite is true. Either way, it's very cut and dried as far as the two circuits are concerned - so isolating the problem is a breeze

One the most frequently experienced "gremlins" with Weber carburetors is a seemingly incurable and very annoying flat spot which rears its ugly head at about 2,200-2,800 rpm. This condition is generally caused by one of two things - 1) 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
2) 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. 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 you 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. The point here is that these carburetors are designed to come off idle and run smoothly all the way up. Your problems can be solved with a little tuning on your own or by relating the symptoms to someone who is knowledgeable enough to help you.

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 circuit also has two basic elements. These are the pump exhaust valve and the pump jet. The pump exhaust is nothing more than a bypass valve and this is located in the bottom of the float bowl. This is the piece that regulates how much fuel you want to make 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 shooters. The smaller the hole, the more fuel you're making available. You can even put in a "closed" bypass for drag racing, when you need all the juice you can get in order to get those slicks turning. Obviously, there is nothing complicated about a simple bypass system. The duration of the pump shot is varied by installing a larger or smaller pump jet (shooter). 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 you should concern yourself with - the main jet itself, 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 "depression" in the carburetor increases to the point where it's strong enough to pull it 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,

a) if you want to implement a change over the entire rpm range, you play with the main jet.
b) if you want to change the way the car feels at the high end, that's where the air corrector comes in.

Also, you should keep in mind that the air corrector is a finer adjustment than the main jet. Example: One step upward in the main jet (richer) equals about the same as three steps down on the air (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 she's going lean on you up top; 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 mains only.

 

Someone had the Mazda carbmanual.pdf file on here some time ago.

 

Thank you for that break down. Actually reading it made me understand what iv been getting told when building my car and your explaining was on point. So again thank you

 

this is a 16 yr old thread and neither member has been on the site since 2008.