Service Manual
3-6
Table of Contents
Twin Cylinder Engine Service Manual
February 2017 - Digital Edition
TWO BARREL CARBURETOR AND FUEL SYSTEM
Operation (Cont.)
Fast Throttle: The fuel-air mix is metered by the main circuit. The throttle discs open, engine is under medium
to full load, air is drawn through each barrel and the venturi creates a low pressure. Air at atmospheric pressure
in the oat bowl pushes on the fuel. Fuel is metered through the main jets and enters through the nozzles. The
nozzle has a series of holes for air to mix with the fuel. Fuel-air mixture ows up the nozzle and into the venturi.
NOTE: High altitude applications: Toro has a high altitude jet and nozzle kit available. Use Toro Parts
Lookup in the fuel system drawing, reference 99.
Vacuum Override Choke: A unique feature of the carburetor is the vacuum override choke. The “override” allows
the choke to partially open when the engine starts and the choke cable is positioned at full choke. The intake
vacuum of the operating engine “overrides” the choke spring and partially opens the choke buttery, allowing the
engine to run smoothly until the choke cable is returned to the “no choke” position.
NOTE: Once the engine starts, always return the choke control to the “no choke” position.
The override feature is possible due to the two springs on the choke shaft. The outer spring (closest to the
carburetor body) opens the choke when the choke control is in the “no choke” position.
The inner spring (under the cap) (W) (Figure 3) allows the intake vacuum from the running engine to partially open
the choke, slightly overriding the choke spring.
Figure 5
Choke Open
Vacuum Override Choke Cap Off
Vacuum Override Choke Cap On
Choke Closed