2.FEATURES

2.1 Carburettor diagram and principal parts









fig. 1




2.2 Operating ranges. Scheme of phases while running



fig. 2


Figure 2 shows the section of a venturi according to the operating periods regulated by the throttle valve opening. In every phase of operation, it is possible to vary and select the optimum setting.

In the idle stage, the idle circuit and idle adjustment is set with the mixture screw and idle-speed screw.

In the "B" progression phase, fuel mixture delivery from the idle hole is steadily replaced by mixture delivery from the progression hole, drawing emulsion mixture from the idle circuit, and in this range, choosing the correct idle jet and throttleslide cutaway is necessary. The throttle valve cutaway slightly affects the carburation up to about half throttle.

In the "C" high-speed period, mixture delivery from the idle circuit and from the progression hole is replaced by mixture from the main circuit and selection of both the atomiser and the tapered needle should then be made.

In the "D" period of full throttle and, with all the circuits of the earlier periods operating correctly, the size of the main jet is now finally selected.

2.3 Installation angles


The tapered-needle-type carburettors with concentric, central float chambers have a horizontal main barrel and can be mounted up to a maximum inclination of 40 degrees from the horizontal (figure 3).
For applications on motocross and trials engines, etc, this inclination should be 30 degrees or less.


fig. 3

2.4 Engine connections


The carburettor is usually connected to the engine with one of the following:

A-male clamp fixing (figure 4)

B-female clamp fixing (figure 5)

C-flange fixing (figure 6)

— the male clamp connection used for the flexible fixing of the carburettor to the engine is usually recommended on motorcycles for motocross, trials, etc or fitted to engines which run to high rpm or those which produce strong vibrations.

— the female clip connection and the flange connection, with a rigid fitting to the engine, are usable on road motorcycles or fitted to engines which do not generate very strong vibrations.

Note that with the female clamp fixing and the flange connection, as you can see in figure 5 and 6, there is also the need to provide both effective heat insulation and a perfect airtight seal.

fig. 4

fig. 5

fig. 6


2.5 Air intakes
Different air intake arrangements are possible for each type of carburettor:

Open air intakes; Trumpets of various shapes and lengths; Aircleaners and filter-silencers
As far as the lengths of the trumpets is concerned, remember that short trumpets are usually used on carburettors for two-stroke engines and longer ones on carburettors for four-stroke engines.

For particular requirements, such as on some racing engines, carburettors with air intakes having a special shape are available eg PHBE H and PHM H models.
On motorcycles with simple aircleaners or air filter-silencers, it is extremely important to check on the efficiency of the filter and for perfect sealing of the filter box to prevent damage to the engine and to the carburettor.
Any change in the filter-silencer may produce a change in the carburation and consequently fresh adjustment and tuning of the carburettor may then become necessary.
Remember also that replacing the filter or silencer with a trumpet usually results in an increase in the amount of air drawn into the engine and consequently there should also be a suitable increase in the size of the main jet fitted.



2.6 Construction materials
The carburettor bodies are diecast in aluminium or zamak alloys.

For special weight-conscious requirements, there are some small-volume carburettors in elektron magnesium alloy.

All the setting parts such as the jets, atomisers, needle-valve seats, etc are made of brass.

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