Transcript SMALL ENGINES CARBURETION - Faribault Public Schools

SMALL ENGINES
CARBURETION
• Principles
• 1. Air Pressure
• 2. Air Flow
Air Pressure: Fuel is forced in the intake by a
low pressure (vacuum) created by the
downward movement of the piston.
• Air Flow: As air speed increases (throttle
opens) air flow through the carburetor also
increases. This causes the venturi effect in
the carburetor. Atmospheric pressure forces
the fuel in the fuel bowl up the main
discharge nozzle,and into the air horn.
• Venturi effect: As air flows across an object
the bottom of that object a high pressure is
formed. On the top a low pressure is
formed by small eddy currents.
VENTURI PARTS
Restriction
Low
Pressure
High
Pressure
Air Horn
System Circuits
•
•
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•
•
Cold Start Enrichment: Choke, Primer Bulb
Float
Idle
Low Speed
High Speed
Idle/Low Speed Circuit
• Work on Manifold vacuum
• Throttle plate is closed or open partially not
enough air flow to create the venturi effect.
• Gas enters from the Idle low speed gas jet.
• Air enters from the Idle Air Bleed.
• Air and fuel mix and pass through an Idle
Mixture Screw*, The gas and air then exit
the Idle port and or the Low Speed Ports.
High Speed Circuit
• Throttle open at least ¼ throttle.
• Enough air flows to create the Venturi
Effect.
• Fuel Passes the Main gas jet into the main
discharge nozzle.
• Air enters the Main Air Bleed
• Fuel and Air mix in the Emulsion tube.
• Fuel and air exit the Main discharge nozzle.
Float Circuit
• Used to maintain the proper fuel level, so
the carburetor does not present a lean or
rich mixture.
• Components:(FLO-JET) Float, Bowl,
Needle Valve, Seat, Bowl Vent
• Components:(PULSA-JET) Pick-up Tube,
Fuel Pump, Gas Well (with spill way)
Choke Circuit
• Used to Enriched the Fuel and Air mixture
under cold start conditions.
• Components: Choke shaft, Choke Plate
• Operation: Creates Manifold vacuum
Behind the choke plate so that all orifices
have fuel exiting from them