Main System - TOHATSU NB-1000 Service Manual

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Under idling conditions, the mixture of fuel and air is mainly output
from the pilot outlet. As throttle is increased, the negative pressure
applied to the pilot system increases and allows more of the
mixture to flow from the bypass. When the throttle is opened
further, the increase of air causes a pressure drop which allows the
main system to assume full control of fuel delivery.

Main System

The main system in the carburetors is a high speed system
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consisting of the main jet, main nozzle and main air jet. As
the throttle valve opens, mixed fuel (14) enters the carburetor and
flows from the carburetor float chamber (15) to main jet (16). The
fuel is then measured and directed to the main nozzle (17) bleed
hole where it combines with air (18) from the main air jet (19). The
increased air quantity and flow rate at venturi (22) allows pressure
drop at the tip of main nozzle (20), causing increased output of fuel/
air mixture (21). The fuel mixture combines with main air at the
venturi (22) before entering the engine (23).
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1-/ 2-Cylinder
FUEL SYSTEM
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