In the tapered-needle type Carburetor , the choke size is the diameter of the section immediately upstream or downstream of the throttle valve and its size is cast on the nameplate together with the model type of Carburetor eg PHBE 36BS signifies a 36 mm venturi Carburetor .
An initial selection of the optimum choke size can be made with the help of the graph in figure 8, where a range of possible Carburetor sizes in relation to the anticipated power output per cylinder of the engine is suggested.
For example, for a two-cylinder 60 HP engine ie. 60/2=30 HP per cylinder, the suggested size range is between 32 and 38mm.
a larger-size Carburetor generally allows more power at high rpm ie. a higher maximum speed. However, simply fitting just a larger Carburetor may not bring about the desired increase in power output as this often only follows from several additional engine modifications, each designed to improve some other aspect of the engine's performance.
a smaller Carburetor will give better pickup and therefore in selecting a choke size, you should always balance your power and acceleration requirements.
usually in conversions an increase in the Carburetor size also requires an increase in the main jet size of about 10 % for each 1 mm increase in the choke size, without changing the other setting parts.
on a modified engine, whenever you require a Carburetor larger than the original, it is preferable to use one which has already been set up for a similar engine ie. an engine having the same operation (two or four stroke), a similar power output and similar cylinder displacement, in order to have a good comparable base for subsequent tuning.
tuning of racing engines is best carried out on the racing circuit with well run-in engines which are thoroughly warmed up.