The choke size of carburettors is used by sanctioning organizations to limit the HP of various engines in order to make poorly designed cars competitive with well designed cars. This went on for years until most cars went on to fuel injection. In fuel injected engines there is no need for a choke to create a low pressure area to make a carburettor work, so every throttle body is just an air valve that has little effect on performance. So except for very large engines the throttle body needs not be very large at all as it is not designed to offer any restriction to flow. As many have noticed, the last little bit of throttle opening has little to no affect on output, as the full flow capability of the engine has been satisfied, by the often over sized TB.

 

The choke or venturi in the carburettor is there in order to constrict the flow and produce a lower than ambiant,  pressure. This is used to force fuel in the float bowl (at ambiant pressure) to flow into this low pressure area. The fuel flows through a number of valves used to meter the fuel quantity and maintain a constant mixture ratio.

 

In the ordinary production carburettor all of the valves are just drilled holes. The choke size is cast into the body of the carb and not changeable.

 

Go to the Bernulli web page and play with flow restrictions and obverve the pressure profiles at the bottm of the page as you make changes.

 

 

The Weber carburettor is a production carb used on many cars from the past. All across the pond where people play with their cars all of their lives. The biggest were the 48MM size in downdraft or side draft versions. Since these were used on many sizes of engines a huge number of sizes of every part are available. Those folks are puzzled by our lack of tuning skills.

 

One casting may be used to produce many carb sizes. So a 48MM casting can be used to make a 32MM carb or a 44MM carb by just changing the choke size. Nothing to fear at all. The choke size represents the actual size (breathing capability) of the carb. The 48MM number is the size of the bore below the choke area where the butterfly is stationed.

 

American built carbs wear out along with the cars. Webers use real ball bearings on the throttle shafts and real lip seals outboard of the bearings. My first Weber was purchased used in 1980 and still works just fine.

 

All jetting information must be associated with a specific choke size. The choke size is the actual size of the carburettor.

 

For Weber replica carbs above 44MM and jets for Weber, Dellortor, Solex and Berg carbs and anything VW see Gene Berg.

 

 

Lynn E. Hanover

 

Pictures are a 38MM choke for a 48 Weber and an air horn for a 48 Weber.