Weber Carburetors

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We explode the myths to help you understand how Weber carburetors work and, more importantly, why they sometimes don’t!

Originally designed by Eduardo Weber in the 1920s in Italy, the Weber carburetor was unique in using two ‘venturis’ (or chokes): a small one for low-speed running and a larger one for high-speed use.

By the 1930s, Weber had added units with equal-size twin chokes This meant each carb had two separate air passages, so fitting a pair to a four-pot engine would give each cylinder an independent air/fuel supply. Webers have been made for side-, down- and even up-draft layouts, and are still available as performance conversions as well as having been standard fit on many production cars.

Fake new Webers from China can be found cheaply, but beware – experts agree they aren’t as well made and will not give the ultimate performance and efficiency.

The official company is owned by Fiat subsidiary Magnetti Marelli, and Webers were manufactured in Bologna, Italy, until the late 1980s, when the tooling and production moved to Guadalajara, Spain. The real thing will have the Weber logo (a stylised thick underlined ‘W’), the WEBER name and MADE IN SPAIN (or ITALY) cast into the body of the carburetor.

Once set up correctly, a Weber cannot really be adjusted. Unfortunately, setting it up is a skilled and time-consuming process, best carried out with plenty of rolling road or on-road testing time. This is because there are so many variables.

First, there is the carburetor’s physical size, measured by the barrel dimensions. In the DCOE range, you can get 40, 42, 45, 48, 48/50SP and 55SP. Within each, you can fit different main venturi sizes. These control the airflow and hence peak power – and they overlap considerably, 40DCOEs taking 24-36mm venturis and 45s taking 28-40mm venturis (carbs above 45DCOE have a much narrower range). Don’t go too big, though: good performance relies on good fuel atomization, which is best at high airflow. Oversize venturis may give optimum performance at very high revs, but the car will run like a pig lower down.

Inside the carburetor is an emulsion tube, with a variety of holes to let the fuel out. At the top is an air-corrector jet and at the bottom is the main jet. All three come in a wide range, the emulsion tube being most confusing due to the number and size of holes in the top, middle and lower ends all being variable, with no logical progression in numbering. The air-corrector jet makes the most difference at high revs, the main jet in the mid-range.

The separate ‘idle’ jet (which controls the progression from idling to full throttle, not the idle mixture/speed itself) also comes in various sizes – and with a range of sizes for the air bleed hole in the side. Also critical to good performance is the float level, as the petrol runs through the carburetor and around the base of the emulsion tube. At the top of the carburetor is an auxiliary jet (accelerator or pump jet) which sprays fuel down into the carb when the throttle is floored. This helps with cold starting, as not all Webers are fitted with a mixture-richening device – pumping the throttle a few times will squirt extra petrol into the manifold, encouraging the engine to burst into life.

The pump jet also assists with sudden acceleration demands: when you floor the throttle, air accelerates faster than fuel and can cause a flat spot, but the pump jet provides the extra gas on demand to bridge the gap. It also bleeds slightly more petrol in at high revs, when quicker airflow emulsifies the fuel better and can cause the mixture to go a little lean (although on some Webers you will find a bleed valve in the float chamber that, for economy reasons, prevents this high-rev bleed).

If you’re setting up a car from scratch, a Weber tuning manual or the help of a Weber expert is essential, as there are formulae to assist you and charts of best settings for numerous applications that have been developed by far more extensive testing than you’re likely to be able to afford. You will need access to a range of parts so that you can refine the set-up.

With age, carburetors do suffer from a build-up of deposits – dirt suspended in the fuel and additives that remain when it evaporates. These will foul up jets and passages, causing poor running. The only area that normally suffers from wear is the throttle spindles; particularly on cheaper versions where these run directly in the aluminum rather than in bronze bushes.

Venturis can work loose, especially on a rough-running engine, then let air in, exacerbating the problem. On original applications the air box often clamps the venturis tight: aftermarket filters don’t, and may cause issues.

Damage can occur through careless handling and electrolytic corrosion between the aluminum body of the carb and steel inserts, which effectively seizes them in place. However, all this can be made good by carburetor restoration specialists, often at much less than the cost of a new unit.

If buying secondhand, look for wear and damage, but also check the existing set-up as – if it is close to what you need – it may save you purchasing a lot of parts. And make sure you know what you are getting: Webers manufactured for production applications, especially for Ford, were built down to a price and are not so adaptable as aftermarket units – on FoMoCo carburetors the venturis are cast in and cannot be changed.

Webers are not just performance-oriented: that’s the province of the twin-sidedraft DCOEs and the downdraft IDAs for Vees, which offer great response but aren’t that fuel efficient. Smaller downdraft Webers can give an excellent range of economy and performance, as just one barrel operates up to two-thirds of the throttle opening, the second, higher-geared one coming in at two-thirds and fully opening by full throttle. You get the economy of a single carb driving gently, but the performance of a twin carb driving hard.

If buying Webers, make sure they are matched (serial numbers identical or close) and that they are directionally correct. The float chambers should be positioned ahead of the main jets, so that under hard acceleration fuel surges to the main jet and cornering has little effect: get this wrong and the car will never perform well on the track.

And do not fit twin Webers to a worn-out engine: SUs or Strombergs compensate for wear because they rely on motor vacuum and will not flood the powerplant when the vacuum is weak; Webers don’t compensate. Rebuild the engine and fit a better cam and performance manifolds first: if it breathes better, the motor can draw in more air and fuel.


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