World's most tunable carburetor
Before modern electronic fuel injection was embraced by the car manufacturers, the world's most exotic cars had one thing in common: they all came with Weber carburetors. Ferrari, Porsche, Maseratti, Lamborghini...all the exotics. Why?
For starters, it's a modular carburetor design, produced in a variety of styles incorporating a number of features that enable a tuner to select exactly the right style and size for the intended use. You can even change their CFM, which begins to explain Weber's superior adaptability to any application.
Now, 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 carburetion is like an expensive musical instrument. No matter how good the instrument is, if it's not tuned properly, it isn't going to make nice music.
And therein lies the secret of making good music with Weber carburetors: it's all in the tuning. On this page, I'm going to explain to you how they work, and most importantly, how to get them to make beautiful music for YOU. And as you'll see , it's not rocket science.
You'll learn what causes them to run badly, and how to troubleshoot some of your own problems. You will see that some so-called "carburetion problems" actually have nothing to do with the carburetors at all, but they usually get the blame.
You're about to learn more about Webers in one big gulp than anything else you've read, all put together. Here we go....
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 an intake manifold, and away you go. It simply 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.
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 simplicity and a marvel of precision machining that has been around since the early 60's with no major revisions.
The number 48 tells you 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 itís a high performance downdraft carburetor. There were also 40 & 46 IDA/3C high performance 3-barrel downdrafts, available in 40 and 46 mm sizes. The 3C means this one's a "3-choke"...an in-line three barrel made for the Porsche flat 6, and a very cool carburetor. I once manufactured a manifold that mounted two of them on the Buick V6, and it really turned those engines on.
Then there are the sidedrafts. Weber sidedraft carburetors carry the suffix DCOE or DCO, their prefix numbers (sizes) ranging from 38 mm all the way to 55 mm (close to 2-1/4"). So all those numbers and letters really mean something and itís all pretty simple. Now, the next time someone mentions he's running Webers, ask him whether he's running DCOE's or IDA's and pick up a few bench-racing points. There are lots of other models, but for our purposes, these are the ones most found on V8ís.
THE VARIABLE CFM FEATURE
Everybody knows some 289 Cobras ran four 48IDAís. But somewhere along the line, you can probably recall seeing four 48 IDA's on a 427 bigblock. Did you ever wonder how the same carburetor could work on two such vastly different-sized engines?
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. It's a removable cylinder inside the carburetor...like a restrictor.
By installing a smaller choke, the carburetor is air-restricted to flow less CFM. This will make it responsive in the midrange and allow it to work on a low-compression small block engine....now it's a smaller carburetor. 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" with no restriction), now 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. It will fall flat on it's face, lack throttle response in the low and midrange, and probably be a complete nightmare in traffic ("....My buddy had a set of those Webers on his engine, and it ran real bad"). The carburetors are often blamed for poor tuning component selection.
A lot of people ask about CFM rating. It doesnít work the same, so itís 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 (4) 48IDA carburetion unit flows 1936 cfm. But, that means 242 cfm flows to each cylinder. Itís an extremely efficient system that allows1936 cfm to flow happily through the engine. Bet you didnít know a smallblock V8 could handle that kind of CFM. This is only possible with an independent runner manifold, however.
So, in order to get drivability, throttle response, and lots of torque from a Weber-carbureted V8, the choke size is the first consideration. It will determine the personality of the engine. How big is the motor, what's the compression ratio, and what is the workable rpm range? Once the correct size choke has been selected for your application, the jetting for all the circuits is calculated around that choke size. If you change the chokes (which changes the cfm), you must also change the jetting.
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.
THE IDLE CIRCUIT
The idle circuit on a downdraft 48IDA is comprised of two components, the idle jet and the idle jet holder. With these two pieces, we can meter the right proportion of fuel and air at idle ó and during low rpm operation. On a DCOE, the idle jet has an air metering hole right in it. So, the idle mixture is delivered as a pre-mixed fuel/air mixture, and the total volume of that mixture is regulated with the idle mixture screw, located on the lower part of each carburetor barrel.
If the idle circuit is correctly jetted, the mixture screws on an independent runner Weber system are usually never more than 1 turn out from closed (in the old days it was 3/4 turn, but nowadays, the oxygenates and ethanol in our gas may require extra richness). I find 7/8 of a turn is very common in most situations. So if you have to open the mixture screws more than that, itís time to richen the idle circuit. So even though it idles okay, going more than 1 full turn out tells you the jetting is basically lean (and you're usually going to have a drivability problem), which brings us to the next part of the idle jet's functionÖ
The idle jets in the Weber arenít just for idling. The idle circuit is actually the low-speed circuit and must carry the engine all the way up to 2200-2,500 rpm, where the transition to the main circuit takes place. That means if you don't drive over 2500 rpm, you may be still running on the idle jets. After 2500 rpm or so, the main circuit tips in, and 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 with a Weber setup is pretty simple.
As mentioned above, one the most common "gremlins" with Weber carburetors is a seemingly incurable and very annoying flat spot that typically 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 due to an under-or-over emulsified mixture at that particular rpm range, or the idle circuit is falling off too early and 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 stumble, and then you pass through it.
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 can cause a big problem. If the flat spot is still there even with the correct emulsion tube, then you'll need to adjust 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. A little experience comes in handy here. Seeking some 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 work with the symptoms. Any Weber setup can be tuned perfectly.
ACCELERATOR PUMP CIRCUIT
The accelerator pump circuit, just like on any carburetor, squirts raw gas into the carburetor to provide instant enrichment when you crack the throttle. This circuit 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 for drag racing, 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 (this is the"shooter", or "squirter"). 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
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. You probably thought the main jet was all there is, and in a conventional carburetor, youíd be right. But these arenít conventional carburetors. They offer the ultimate in fine tunability.
The main jet is stuck into the bottom of the emulsion tube and sits in the fuel on a tapered seat. 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 at the bottom. The air and fuel are mixed, and this blows out of the emulsion tube through a series of holes along its length, and this aerated mixture is sucked up the emulsion tube "well" and into the delivery nozzle as the velocity (and suction) in the carburetor increases. Main fuel delivery usually happens by 2500 rpm or so, after which you are all the way into the main circuit of the carburetor.
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 is, if you want to implement 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 job 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, Iíd drop the air corrector about three sizes, and that may be all it needs to buzz the engine right up to 6500 rpm. If it feels sour all the way up, then Iíd go one or two sizes heavier on the mains only. So whatís so hard about jetting Webers? Obviously, nothing...if you understand how they work.
Most people don't realize that the IDA and DCOE/DCO carburetors are extremely simple with very few moving parts. There are no metering rods, power valves, rubber seals, diaphragms, or plastic parts. The accelerator pump is a brass piston. The throttle shafts ride in precision bearings. Itís a superior example of excellent machining and beautifully-fitting components. They are incredibly reliable.
With the infinite tunability of Weber carburetors, there is no need to compromise the drivability or road manners of your car. If you know someone who suffers from drivability problems with such a nice carburetion system, he is suffering unnecessarily. Weber carburetion should be crisp, responsive and smooth. If it isnít, somethingís wrong. Or, let's just say he's not done tuning yet. I was in that ďnot doneĒ mode for about a year-and-a-half with my first Cobra. At times, that little Weber carbureted beast absolutely defied meóthen I gained an understanding of how the carburetors worked. After that, I got it tuned right and it was a complete pleasure to drive.
A WHOLE DIFFERENT ENGINE
The first thing most people notice when they go to Webers is an increased flexibility from the engine. There is a natural tendency for a Weber-carbureted engine to idle smoother, particularly with a hot camshaft that idled roughly beforehand. This is because of the perfect fuel distribution.
The reason Webers are so well-suited to road racing is that they make so much more power over the entire rpm range, which is a great characteristic for street-driven cars. When you think about it, you spend very little time at the upper end, near the redline, like drag cars do. And while making all that midrange power, what you also have is an engine that revs much faster.
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 ("I.R.") 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. Now youíre talking about real efficiency, and this is what causes the dramatic increase in horsepower and torque throughout the midrange, when the rpm's are coming up...we're talking fast revs. And because street engines spend 90% of their time in the midrange, itís an ideal carburetion system for street use, where the increased flexibility provides more seat-of-the-pants enjoyment. Any time you put you foot to the floor, the engine delivers "right now".
The throttle response with the Weber carburetion system is just like a fuel injection unit, and thatís because it incorporates short, isolated intake runners. This means only a small fuel/air mass has to move when the throttle plate is opened. Throttle response is all about the velocity of the incoming charge....how fast you can get it moving.
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 to move is what's in one short 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 its venture size).
Weber carburetion only needs a low-pressure, constant-volume fuel supply, so a stock block-mounted mecahnical fuel pump works fine, which usually puts out 7-12 psi. The key is to regulate the fuel pressure at 2.5 psi to 3 psi. For this reason, I supply a special 0-4 psi fuel regulator with every system, because a standard pressure regulator will not lower the pressure enough; they typically start at 4 psi and go up to 9 psi. So, the right regulator is a key component. I use, and recommend the Holley #12-804.
In the mileage department, it really depends on the rest of the engine and your driving habits, but low-to-mid-teens mpg is not unusual on the highway, especially if you have an overdrive and youíre cruising on the idle circuit Ėand if you can keep your foot out of it (not always easy). Just realize it can also deliver less, as thereís no such thing as a progressive system here. But you didn't build a hot rod to sip fuel; you built it to suck gas and go fast.
If you can run regular fuel now, you can continue doing so after installing the Webers. This is purely a function of compression ratio and ignition timing, not induction. You donít need a high compression engine to run these carburetors. As long as the cfm is calculated right, they can work fine with 8.5:1 pistons.
Let's 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. Generally speaking, 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. Webers make the engine more tolerant.
THE IMPORTANCE OF IGNITION TIMING
This is one of those things I can't stress enough. I wish I had a dollar for every time somebody had a Weber setup that ran terrible, that was corrected by timing. This is the single most common issue with a badly-running Weber system, bar none. Very few people have an understanding of, nor pay enough attention to ignition timing. So I will say it plainly: WEBERS NEED A LOT OF IGNTION TIMING! IT IS ABSOLUTELY CRITICAL. Set the total advance at 38 degrees BTDC and forget about "intial" timing; whatever it is, that's what it is. Initial timing means nothing; total advance is EVERYTHING. If it's too retarded, you'll have a lazy, rich, engine with farting and popping carburetors, and way too much heat in the heads, which can make the carburetors boil the fuel in the bowls...and drip. It will cause all sorts of issues...none to do with the carburetors...but they'll get the blame most of the time.
TUNING AND MAINTENANCE
A Weber carburetion system must be synchronized so each carburetor is doing exactly the same as the rest. The synchronization procedure is done with a Unisyn or a Synchromer (this tool should be supplied with any new V8 Weber system). It can either be a breeze or a nightmare, depending on whether or not you have a well-designed and correctly-installed linkage system. 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 idle speed screw" to raise or lower the rpm of the engine without disturbing any of the settings of the individual carburetors. The master idle speed screw acts on the entire linkage system as a whole. So, if someone tells you Webers are absolutely impossible to synchronize, there's something wrong with the linkage. Chances are it's a mess...and fighting itself.
The final idle mixture adjustment for each barrel is a simple adjustment 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 listen to each cylinder, one at a time. So, while it may take you four times longer, it's not four times harder. When you're done, you can forget it.
Each mixture screw, as it is turned, will have a noticeable effect on engine rpm; you'll hear the cylinder "go away", almost like 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 between 3/4 and 1 turn. Thatís the correct starting point. From there, turn the screw 1/8-1/4 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 more than 1 turn, the idle circuit is jetted too lean.
Once the unit is synchronized and the idle mixtures are dialed-in to give you the smoothest possible idle, you can hang up your synchronizing tool and screwdriver, because now it's set, and when it's set, it's SET. They will not suddenly "go out" on you and become unsynchronized, unless your linkage pieces are not tight on the throttle shafts. This is another good argument for quality linkage pieces that fit right, and this is why I have my own linkage parts manufactured. When somebody says "Webers constantly go out of adjustment" it's absolutely not true; they do not, if the linkage is right. I donít think I touched mine for 3 years on my 427 Cobra, and I drove that car A LOT.
Another BIG reason that Webers operate poorly is a wrong float setting, which is a key to correct fuel delivery of the main circuit. Webers don't come with their floats set from the factory; it has to be done on the bench before the carburetors are installed Rather than try to explain how to do it, it's easier to give you the instructions visually. Below is a diagram of the float setting instructions for the 48IDA downdraft. I have enhanced the diagram to make them as easy as possible to follow. Click on the diagram to bring up a larger version.
Copyright 2010, Jim Inglese