Weber Tech

World's most tunable carburetor

Before modern electronic fuel injection was embraced by car manufacturers, the world's most exotic European cars had one thing in common: they all came with Weber carburetors. Ferrari, Porsche, Maseratti, Lamborghini...all of them. And here is why:

Every Weber performance carburetor is of modular design, produced in a variety of styles, each incorporating a number of features that enable a tuner to change anything inside for the intended use. You can even change the CFM in the carburetor, which begins to explain Weber's superior adaptability to any application.

If you're one of those people who has always had trouble accepting the idea that Webers are terrific street carburetors, consider it this way: Weber carburetors is like expensive musical instruments. No matter how good the instrument is, if it's not tuned properly, it isn't going to make nice music.

With Weber carburetors, it's all a matter of tuning. On this page, I'm going to explain to you how they work, and most importantly, how to get them to make the music you want to hear. And, as you'll see, none of this is rocket science....once you understand it.

I'll explain what causes them to run badly, and how to troubleshoot some of the most typical problems. You'll see that some so-called "Weber carburetion problems" actually have nothing to do with the carburetors at all, and you'll see why they get the blame.

You're about to learn more about Webers in one gulp than everything else you've ever read on the subject all put together.

Weber Tech 101

As already stated, Weber carburetors have been recognized as standard equipment on the finest racing and street machinery ever to come out of Europe. Maybe you've been lucky enough to get a ride in a vintage Ferrari or a Weber-carburetor Cobra; if you have, chances are, it's a ride you never forgot. 

Weber-carbureted engines deliver explosive acceleration and torque unmatched by conventionally-carbureted engines, and they have a sound all their own. The world's most exotic and powerful engines have traditionally been fed through Weber carburetors, and there is a reason for it. 

The Weber carburetor was 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 and bolting them onto a V8 intake manifold, and away you just isn't done that way. This carburetor was intended for serious tuners and performance enthusiasts who want the most out of their engine and know how to work with them. And, best of all, they're not hard to understand.

All Weber carburetors carry a model number stamped at the base of the carburetor on its mounting flange or on the top cover. The most well-known is the good-old "48 IDA", a masterpiece of simple design and a marvel of precision machining that has been around since the early 60's with no major revisions. The number 48 indicates the carburetor's size. The carburetor has a bore diameter and throttle plate size of 48 millimeters (about 1 15/16"), while the IDA suffix means it’s a high performance downdraft carburetor. There are also 40 & 46 IDA/3C’s. These are high performance 3-barrel downdrafts, available in 40 and 46 mm sizes. The 3C means this one's a "3-choke" (in-line three barrel). The chokes are in the throats, but not the kind of choke you are thinking of. It's a removable venturi "cylinder", which determines the carburetor's CFM.

Another popular downdraft model is the more compact (and very potent) IDF, which comes in 44mm and 48mm versions.

As for the sidedrafts, all Weber sidedraft carburetors carry the suffix DCOE or DCO, their prefix numbers (sizes) ranging from 40 mm all the way to 55 mm (close to 2 1/4"). So, all those numbers and letters really mean something and it’s pretty simple. Now, the next time someone mentions he's running Weber's, ask him whether he's running DCOE's, IDF's, or IDA's and pick up a few bench-racing points. But, you may find that a lot of guys don't know what they have. 

Everybody knows a lot of the 289 Cobras ran four 48IDA’s. But somewhere along the line, you can probably recall seeing four 48 IDA's on a bigblock. Did you ever wonder how the same carburetor can work on either a small or large engine? 

Both engines are running the same carburetors, but if the Webers are set up properly, the only thing the two systems will have in common is their outward appearance. The Weber's most interesting design feature is it's removable "choke" (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 will flow less CFM. This will make it responsive in the midrange and allow it to work on a low-compression small block engine. If you were to pull out those small chokes and drop in some large-diameter ones (which may be nothing more than thin-wall "sleeves"), now you've got a set of 48 IDA's that will flow enough CFM to make over 600 hp on a bigblock. But don't try putting those "big" carburetors on the small block, or it will fall flat on it's face, lack throttle response, and probably be a complete nightmare in traffic ("....My buddy had a set of those Webers on his engine, and, boy…did that car run badly"). Now you know one possible reason why.

A lot of people ask about CFM rating. It doesn’t work the same, so it’s a lot like comparing apples to oranges. Remember, these are designed to be mounted on an independent-runner manifold. Right out of the box, with factory-fitted chokes, a Weber 4x48IDA carburetion unit flows 1936 cfm. But that means 242 cfm flows to each cylinder. That's an extremely efficient system. 

The only way a smallblock Ford can handle that kind of CFM is with an "independent runner" manifold.

So, in order to get drivability, throttle response, and lots of torque from the Weber-carbureted V8, the choke size is the first consideration. It’s the heart of the system. How big is the motor, what's the compression ratio, and what are you going to do with it? Once the correct size choke has been selected for your application, the jetting for all the circuits can be calculated around that choke size. If you change the chokes (which changes the cfm), you must also change jetting.

The Weber carburetor has three basic circuits: the idle circuit, the accelerator pump circuit, and the main circuit. 

The idle circuit is comprised of two components on an IDA, the idle jet and the idle jet carrier. On the other carburetors, the idle jet does it all. With the fuel and air that is measured in the idle circuit, the tuner can meter the right proportion of each during low rpm operation. The idle mixture is delivered in a ratio of fuel and air that the tuner decides on, and the total volume is regulated with the idle mixture screw, located on the lower part of each carburetor barrel. On the IDA's, the air is metered in the idle jet holder; on the others, it's through a cross-drilled hole in the idle jet itself. The size of the fuel orifice and the size of the air bleed hole will determine the ratio ...and the strength of the mixture.

If the idle circuit is correctly jetted, the mixture screws on a downdraft Weber system are usually never more than 3/4-7/8 of a turn out from seated, and 1-1/2 to-2-1/2 turns for the sidedrafts. This will hold true just about 100% of the time. So if you have to open the mixture screws more than that, it’s time to heavy-up the idle circuit because chances are, you're compensating for a small jet size. So, even though it idles okay, if the mixtures are open more than 3/4-7/8 on your downdraft unit, that tells you the jet is lean (and you're usually going to have a drivability problem), which brings us to the next part of the idle jet's function.

The idle jets in the Weber aren’t just for idling. The idle circuit is actually the low-speed circuit and must carry the engine all the way up to about 2,300 rpm, where the transition to the main circuit takes place. That means if you don't drive over 2300 rpm, you're only running on the idle jets. After 2500 rpm or so, the idle circuit is entirely bypassed and no longer has anything to say. So, if you have a drivability issue, like a stumble, or rough spot that "goes away" after about 2500 rpm, that tells you to spend time tweaking the idle circuit. Or maybe the opposite is true. Either way, the two circuits are completely separate, so isolating the problem to a specific circuit is fairly simple.

As mentioned above, one the most common gremlins with Weber carburetors is a seemingly incurable and very annoying flat spot shows up anywhere between 2,200-2,500 rpm. This condition is generally caused by one of two things. Either the wrong emulsion tube is in the carburetor, which is causing a rich stumble perhaps due to an under-emulsified mixture at that particular rpm range, or the idle circuit is too weak and falling off too early, not carrying the engine up to the point where the main circuit takes over. When that happens, it leaves a "lean hole" that feels like a blubbering stumble.

In the case of the wrong emulsion tube, there are really only a few that work really well for V8 applications. If you aren't using one of them, it’s going to cause a problem. If the flat spot is still there even with the correct emulsion tube, then you'll need to richen or lean-out 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, choke sizes, and timing can accomplish the same thing while sticking with the original jet size. I will admit, a little experience comes in handy here.

Sound advice can save a lot of time and aggravation. Drivability 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. Remember, these carburetors will do just about anything you want them to, as long as you know how to interpret the symptoms.

The accelerator pump circuit, just like on any carburetor, is responsible for eliminating "bog" when you floor the pedal. This circuit also has two basic calibration elements: the pump exhaust valve and the pump jet. The pump exhaust is a bypass valve 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 in, for when you need all the gas you can get. 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. 

The main circuit is the fun part to tune because it's where you make the power. This circuit has three primary elements you should concern yourself with: the main jet itself, the emulsion tube, and the air corrector. In conventional carburetors, the main jet was all there is to work with. But remember these aren’t conventional carburetors.

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 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 coming in from the main jet. The air blows out of the emulsion tube through a series of holes along its length and aerates the fuel as it rises up the well around the tube. This emulsified mixture is then sucked out of the main delivery nozzle as the velocity in the carburetor increases. This usually occurs by 2300-2500 rpm, and once that main circuit comes in, hang on --- you’re off and running.

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 effect a change over the entire rpm range, you play with the main jet. If you want to change the way the car feels at the high end, that's more the domain of the air corrector. Also, you should keep in mind that the air corrector is a finer adjustment than the main jet. 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 if the engine pulls strong to 5,000 rpm and then lays down before the redline. In that case, I’d drop the air corrector about three sizes to start with, and see if it will buzz right up to 7,000 rpm. If it feels sour all the way up, then I’d play with the main jets only. So what’s so hard about jetting Webers? Obviously, nothing... if you understand how they work.

The Weber carburetor is extremely simple with very few moving parts. There are no metering rods or power valves. The accelerator pump on the 48 IDA and DCOE is a brass piston that goes down into its cylinder.. The throttle shaft rides in sealed bearings. It’s a superior example of precision machining and beautifully-fitting components, and these carburetors won't "fail" and cause you to be stranded somewhere. That's another reason why they're so well-suited to street use and long-distance cruising. They are incredibly reliable.

With the infinite tunability of Weber carburetors, there's no need to compromise the drivability or road manners of your car. If you know someone who suffers from drivability problems with such a tunable carburetion system as this, he's suffering unnecessarily. Weber carburetion should be responsive and smooth. If it isn’t, something’s wrong. So, in that case, the tuner isn't done tuning it yet. I was in that “not done tuning” mode for about a year-and-a-half with my first Cobra. That little Weber carbureted car defied me—until I understood how the carburetors worked. After that, it became a lot of fun.

The first thing most people notice when they go to Webers is an increased flexibility from the engine. The Weber-carbureted engine is going to idle smoother, so if the camshaft gave the engine a rough idle beforehand, it will not have the rough idle with Webers. This is because of the perfect fuel distribution of the 8-stack independent runner system.

Webers are so well-suited to road racing because that they make so much more power over the entire rpm range...also a wonderful characteristic for street-driven cars. When you think about it, you spend very little time at the upper end near redline, like the drag cars do. With all that midrange power, what you also have is an engine that revs really fast. You will enjoy maneuvers that rely on instant midrange response, like passing a car as quickly as possible on a two lane road.

Again, this is attributed to the independent-runner manifold, which does not incorporate a plenum. In a typical Weber 4-carburetor layout, one barrel directly feeds each cylinder without any intercommunication between barrels or cylinders. This "isolated runner" design ensures that each cylinder is fed exactly the same as the next, without any chance of charge-robbing or over-feeding. What you are doing, in effect, is separately tuning each cylinder. We're talking about real efficiency.

This is what causes the dramatic increase in horsepower and torque in midrange. And because street engines spend 90% of their time there, it’s an ideal carburetion system for street use, where flexibility means more driving enjoyment. 
The throttle response with an independent runner induction system is second to none. A Weber carburetion system will respond to the gas pedal just like a fuel injection unit, and that’s because both have short, isolated intake runners with a small mass of fuel/air to move when you crack the throttle. Throttle response is all about air speed, or "Velocity".

Remember, you're not asking each cylinder to gulp the mixture from a big plenum area; that's a lot of air mass, by comparison. The only air mass that moves on the intake stroke is what's in that intake runner. The main difference between fuel injection and Weber carburetion is that one relies on fuel being injected under very high pressure, while the other responds to the needs of the engine via the depression principle (air velocity in the carburetor based on the venturi size). 

In the mileage department, it really depends on the rest of the engine and your driving habits, but the engine is fed by all eight barrels constantly. There’s no such thing as a progressive system here. MPG can be 8-10 on a big engine and low to mid teens on a smallblock. But if MPG is what you're after, you're in the wrong church here to begin with.

Webers can run on regular gas, so if you can run regular now, you can continue doing so after installing the Webers. This is purely a function of compression ratio and ignition timing, not induction. You don’t need a high compression engine to run these carburetors. As long as the cfm is calculated right, they can be made to work fine even with 8.5:1 pistons. 
Say you're running a 10.5:1 engine and it's a little bit fussy about which brand of fuel it wants, giving you detonation (pinging) at times. Weber carburetion changes that, suppressing the tendency to "ping". The reason for this is that the fuel distribution is now fully controlled, eliminating the "lean spots" that are characteristic of conventional manifolds that distribute fuel from a central plenum. Lean cylinders run hot, and excessive cylinder heat means detonation. 
A Weber carburetion system won't run right unless it's synchronized; each carburetor needs to be doing exactly the same thing. The synchronization procedure is done with a Unisyn or a Synchromer (one should ALWAYS be supplied with a new V8 Weber system). It can either be a breeze or a nightmare, depending on whether you have a good linkage system or not. The secret to a good linkage setup is that it must allow independent adjustment of each carburetor without affecting all the rest as you go through the procedure, and there should be a Master Speed Screw to raise or lower the rpm of the engine without disturbing any of the settings of the individual carburetors. It acts on the entire linkage system as a whole, or there may be a screw for each bank. Also, the levers on the carburetors have to be tight with no slop. So, if someone tells you Webers are absolutely impossible to keep synchronized, his linkage isn't right. Mine stay synchronized just fine.

The final idle mixture adjustment for each barrel is performed by ear, but for some reason, a lot of people feel intimidated because there are four carburetors. It's done exactly the same way you adjust the mixture on a four barrel, except with Webers, you can hear how each cylinder effects the idle. So, while it may take you four times longer, it's not four times harder.

Each mixture screw, as it is turned, will have a noticeable effect on engine rpm; the wrong setting will cause the cylinder to "go away", almost like if you pulled a plug wire. No matter how hard you try, you can't mess this up if you remember one thing: always start from scratch with the screw seated, and then come out 3/4 to 7/8 of a turn (or 1-1/2 turns on the DCOE's). That’s the correct starting point. From there, you only go 1/8 of a turn either way and listen carefully. This method will always get you out of the woods if you get lost. If the mixture screws want to be out way too far, the jetting should be made richer. The mixture screws should always be set at "lean best", meaning the leanest (least opening) on each one with the fastest idle it can give you. No more than that. The leaner the better, without losing idle quality.

Once the unit is synchronized and the idle mixtures are dialed-in to give you the best possible idle, you can hang up your synchronizing tool and screwdriver til' at least next spring, because now it's set, and when it's set, it's SET. They will not suddenly "go out" on you and become unsynchronized. For some reason, it’s one of the stories we’ve all heard a hundred times…that they constantly go out of adjustment. That is absolutely not true; THEY DO NOT. I don’t think I touched mine for 3 years on my 427 Cobra, and back in those days, I drove it to work every day all summer, year after year.

When it comes to the Price of Admission to "Weberdom", they aren’t for everyone. This type of induction unit represents a sizeable investment. Webers usually fall into the dollar category of a supercharger with carburetors, or an EFI with no software, so that makes them a bargain. The price of opening up a box and pulling out a science-out Weber unit with all the right pieces and associated hardware will run you between $4400 and $4700. Sound expensive? Maybe not, if you consider that all things are relative. When you figure the price of a top quality paint job at $8,000.00 to $10,000.00, a nice interior at $3,000.00 to $5,000.00, a set of trick wheels and tires at maybe $3,500.00, and your basic "nice street engine" costing $5-10,000.00, another 4-5 grand for something that takes your car to the next level and raises its value by the same amount is a pretty good investment. In the end, it all boils down to what you really want. One thing is guaranteed: when you open the hood, nobody's going to be yawning.

And as they say, "the fun's in the driving". Weber carburetion offers much more than something that's exciting to look at. Every time you take that car down the road, the throttle response, torque, quick acceleration, and overall flexibility are constant reminders that you've spent your money on the ultimate carburetion system.

Copyright 2015, Jim Inglese

Weber 48IDA float setting procedure

Weber Float setting procedure

The yellow fixture is a gauge that can be made out of any thin-gauge metal you have laying around; it has a 5.5mm notch in it with a relief for the float seam. The orange fixture can be made from a piece of flat metal and a 10-32 screw with a jam nut on each side. It sits exactly where the needle and seat would be, and the extended part is 24.2mm below the plate. It should be mounted firmly to the carburetor using the two screw holes in the carb body at that location, which you will see when you take the top off the carburetor. It's so easy a caveman could do it.