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 High Pressure Air Horn Low Pressure

System Circuits 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