Whilst relatively old in design, Weber carburettors are precision calibrated instruments built to accurately meter fuel entering your engine. Too often do we hear of people complaining that their Weber / Webers do not work properly. In many cases, it is these customers who have purchased someone else's problem. With the rarity and associated value of certain Weber carburettors, it is important to understand and identify certain problems. Over time carburettors can wear, break, and be modified by experienced and unqualified persons alike. What appears to be a cheap deal can turn into an expensive exercise so ultimately, you get what you pay for.
Whilst this article focused mainly on the most common DCOE Weber carburettors, the principles are in fact relevant to all models of Weber carburettors.
Water is no friend to Weber carburettors. Not only is it unsightly, but makes it very difficult to seal gasket surfaces. Machining and surface grinding are ways of overcoming this, however sometimes little can be done to salvage the parts.
Progression holes are highly accurate drillings in the body of the carburettor made to release fuel into the engine from the idle jet. As the throttle plate opens past each hole, it creates a depression and draws fuel from the hole. Different model carburettors have varying sizes and numbers of progression holes. For example, a 45 DCOE 13 has 2 holes drilled, whereas a 45 DCOE 152G has 4 holes drilled. Both carburettors work well, however they will have slightly different characteristics. On the odd occasion, it is required to modify these holes for a particular application. Below is an example of a standard 45 DCOE 13, and a 45 DCOE 13 (one of three) which has been significantly modified to operate on a Fiat 6 cylinder racing engine. Whilst completely functional on this specific engine, the large holes drilled will cause significant tuning issues on most other engines.
When using multiple Weber carburettors, it is important that all carburettors are of the same model. Varying progression hole designs on multiple carburettor applications will make it impossible to tune the idle/progression circuit and result in sub-standard operation. This is why when purchasing carburettors as a "set" it is important to check that the progression holes have not been modified, and if they have, that it has been done properly and accurately.
Broken Castings / Parts
Some breakages are obvious, and some are not so. It is important to check over the carburettor, inside and out, to ensure the functionality of the unit. Whilst some breakages will have little or no effect on the operation of the carburettor, others will render them useless. A very common breakage is that of the float pin support bracket. Inexperienced hands can break this bracket quite easily and make the carburettor useless. This can be repaired however is a costly exercise. Over-tightening DCOE carburettors can also cause cracks to form on the base flange. Once again these can be repaired however are unsightly and expensive to fix.
Another common fault we find with Weber carburettors is that previous hands have over-tightened screws on the carburettors. Areas to pay attention to are;
- Idle mixture screw seats - over-tightening will cause the seat of the idle mixture screw to distort, cause very inaccurate idle mixtures, and eventually break away.
- Venturi & auxiliary venturi retaining screws - will distort the auxiliary venturi and puts added pressure on the carburettor throat (potential for cracks to form)
- Base flange nuts/studs - over-tightening here can cause the flange to break, and on certain downdraft models, distort into a convex shape.
Modified Calibrated Parts
Probably the most common mistake we see made is with venturi design. Instead of purchasing a larger size main venturi, people will machine out existing venturi on a lathe. There is no problem in doing this, as some will experiment with odd sizing, however it is extremely important to machine the correct taper into the choke tube (venturi). For example, we often see a 36mm venturi machined straight out to 40mm, however the part has simply been bored straight through. This does not work as the venturi effect has been lost and low down response will significantly be effected. Many original cast venturi will also begin to show porosities once machined.
Jets are also commonly drilled, soldered, and drilled. Again, this is not a problem if it is done accurately with the appropriate tools, the change recorded, and measured accordingly. However if you have purchased a used carburettor, you need to be sure that the size of the jets is what is actually stamped on the side of them. As accurate as most might claim to be, jet sizes are typically measured in .05mm increments and it is very hard to drill a set of jets identically by hand.
Over time, and typically in conjunction with moisture, parts will seize within the carburettor. This can lead to other parts failing also. Screws, jets, venturi and plungers can all become stuck, and in some cases can only be removed on a mill or with specialised tools. A common area to check is the operation of the cold start device. In many applications they are not used and over time the plungers will seize. The next time someone then goes to operate the cold start, the little tabs on the cover break off rendering the part useless.
Venturi can also be very difficult to remove once corrosion has set in. Soaking carburettors for several days can help in the removal of seized parts.
Weber Models & Imitation Webers to stay away from
Whilst the DCOE series of Weber carburettors has been renowned and fitted to almost every engine every produced prior to 1990, there were a couple of model we would advise to steer clear of.
40 DCOM - Some online sellers claim that these are the "latest" design from Weber. Quite simply, they will not work properly on any engine apart from the Alfa Romeo they were designed for. They are an emissions style carburettor and also use a completely different pump jet design. In fact they use a diaphragm for the pump circuit similar to that used on the Dellorto carburettor. Having attempted and spoken with numerous customers, you may obtain the 40 DCOM carburettor cheap, but you will spend three times more trying to get it to even resemble working.
Late model 40 DCOE carburettor - Late model Alfa Romeo 40 DCOE carburettors are similar to the 40 DCOM in that they are an emissions style carburettor. There are many large progression holes and the emulsion tubes used are very specifc in that they are covered in holes. Many have attempted to tune these to vehicles other than Alfa Romeos with little success.
** Please note: All Weber carburettors now manufactured in Spain are performance carburettors. They are NOT emissions style carburettors **
Weber Copies - As with anything good, someone is bound to make a copy. For several years, copy Weber carburettors have been made in South America, EMPI have been manufacturing their own range of copy Weber carburettors in China, and more recently, a company by the name of FAJS has been making extremely poor copies of IDF and DCOE carburettors in China also. We are unable to help with any technical issues on these carburettors. It is well advised to steer clear of all of these counterfeit products.