Low-speed fuel system (Fig. 2 and Fig. 3)
Since the engine is operated with the throttle valve almost closed at idling or in the low
speed range, the velocity of air flowing through the needle jet (2) is slow. Consequently, a
vacuum strong enough to draw fuel from the needle jet in the main fuel system is not
created. The fuel supply during this low speed operation is controlled by means of the
pilot outlet (3) and the bypass (4) that are situated near the intake port. At idle, when the
throttle valve is slightly opened, fuel metered by the pilot jet (5) is mixed with air adjusted
in a proper amount by the air screw (6) and is broken into fine particle (vapor).
The mixture is again mixed with fuel coming from the bypass and is drawn into the pilot
outlet to mix with air flowing through the main bore (7). The fuel mixed at this stage then
goes into the engine. When the throttle valve is opened slightly during low speed
operation, the pilot outlet alone cannot supply the required fuel and the shortage has to
be made up with fuel injected from the bypass. the adjustment of the mixture ratio during
this stage is made by the pilot jet and the air screw, in the case of a two-hole type fuel
system (Fig. 3). While at low speed operation if full throttle is initiated a similar shortage of
fuel exists and during this transition from low to medium or low to high the fuel again has
to be injected from the bypass until enough (vacuum) can be created to draw fuel from
the main fuel system.
B - Main Fuel System
On Mikuni VM-type carburetors, the pilot system and the main system are of independent
construction.
The fuel flow in these two systems is shown in Fig. 5.
