Carburetion Systems

The “BIG” Picture One of the “Big 3” systems Ignition Compression Carburetion

Carburetor Theory Atmospheric Pressure a constant downward force of air on the Earth usually varies between 13 and 15 lbs per sq. in. areas of low pressure must be created in the carburetor to create air flow

Carburetor Theory (con’t) Venturi What is it? Wind blowing in downtown Chicago always stronger in the smaller areas between 2 buildings River currents always faster in a narrower, shalllower place than deep, wide pools Definition

Carburetor Theory (con’t) Airfoil low pressure airfoil high pressure

Carburetor Theory (con’t) All carburetors work basically the same: As piston moves from TDC to BDC on the intake stroke, the intake valve opens, air is “sucked” through the air cleaner into the airhorn and . . . Venturi creates an area of low pressure that draws fuel from the fuel supply through the main discharge nozzle. Venturi also causes the air flow to rapidly accelerate--thus vaporizing and mixing air and fuel together very efficiently.

Carburetor Theory (con’t) Because the main discharge nozzle is extended into the airstream, an airfoil is also created, further lowering the pressure “above” the nozzle. If an engine ran at only one speed under ideal conditions, the story of carburetion would end here.

Carburetor Theory (con’t) The carburetor must be able to adjust the air/fuel mixture to the conditions presented. It must run smoothly and economically at widely varying speeds. Richer mixture--more gas added to mixture. Leaner mixture--less gas added to mixture. To make these adjustments, a threaded needle valve is added at the bottom of the main discharge nozzle. This regulates the amount of fuel that gets to the venturi

Carburetor Theory (con’t) To further regulate the mixture, two “air regulators” or butterfly valves are also added: These restrict the amount of air flow through the carburetor--either manually or automatically. This action decreases the power and speed and the richness of the mixture within the engine. Throttle valves restrict air movement at all speeds and are generally manually controlled. Choke valves restrict air movement at start-up to allow for a richer mixture and can be manually or automatically engaged.

Carburetor Theory (con’t) To further regulate the air/fuel mixture at lower speeds, another threaded needle valve is added. This allows the engine to run smoothly and economically at very low speeds by allowing a slight bit of fuel to slip past the throttle valve. Note: At low speeds, the throttle valve is fully closed. Idle speed should be 1750 RPM on all B&S engines.

Types of Carburetors Gravity Feed Vacuum Feed Pulse Feed called a “Flo-Jet” by Briggs & Stratton Vacuum Feed called “Vacu-Jet” Pulse Feed called “Pulsa-Jet”

Flo-Jet Carburetors Gravity feed system fuel flows by gravity to the carburetor gas tank must have a vent hole to provide atmospheric pressure to “push” fuel to carburetor The float is found in the bowl. As fuel flows into the bowl, it raises and lowers the float.

Flo-Jet Carburetors (con’t) The float is attached to a needle valve. The needle, along with its seat, work together to turn on and off fuel flow to the main discharge nozzle. Float level should be high enough to allow an ample supply of fuel at full throttle, yet remain low enough to prevent flooding and/or leaking. should be set at 5/16 of an inch on our engines.

Vacu-Jet Carburetors Fuel tank is below the carburetor Again, atmospheric pressure is employed to help get the fuel to the carburetor. Air pushes down on the fuel in the tank, when the piston starts downward on the intake stroke, it creates an area of pressure that is lower than that of the atmospheric pressure. This causes the fuel to rise through the pickup tube and travel toward the main discharge nozzle.

Vacu-Jet Carburetors (con’t) Vacu-Jet carburetors require a richer mixture setting because the fuel system “lags” behind the fuel requirements of the engine at the high speeds that most small engines run at. As the throttle closes to idle, the leading edge takes a position between 2 different sized discharge holes that are found on the main discharge nozzle. The larger of the holes now becomes an area of high air pressure and the flow of fuel ceases.

Vacu-Jet Carburetors (con’t) The smaller hole now becomes an area of lower pressure and fuel continues to flow--just enough for the engine to idle. Choke valve is also different: There are many problems with this choke system including sticking open and closed, especially when remotely controlled.

Pulsa-Jet Carburetors Only “true” fuel system contains a diaphragm type fuel pump and a “constant level” fuel chamber Newest design of carburetors for small engines can obtain just as much (or more) horsepower as older, more complicated float-type carburetors This is due to the fact that it provides a constant fuel level directly below the venturi very little “lift” is needed to get the fuel to the carburetor

Pulsa-Jet Carburetors (con’t) The venturi can be made larger, allowing more air/fuel mixture into the engine allowing an increase in horsepower within the same sized engine. See diagram for operating principals

Carburetor Adjustments Adjustment of the needle valve for maximum power Causes overheating, early or late detonation, and short valve life No accelerator pump in Flo-Jet and Vacu-Jet models so engine will “kill” if throttle is opened suddenly. “Floods” the engine causing “raw” fuel into the combustion chamber (cylinder). dilutes the crankcase oil washing away the film on the cylinder wall causing “scuffing”and lost compression.

Carburetor Adjustments (con’t) Throttle Adjustment To adjust for maximum power and efficiency: This indicates a lean mixture. This indicates a rich mixture.

Carburetor Adjustments (con’t) Slowly turn the main needle valve back clockwise to a point midway between the 2 readings This method can also be used to set the maximum speed the engine will run by first opening the throttle all the way.

Carburetor Adjustments (con’t) Idle Speed Adjustment Very similar to Throttle Adjustment IS NOT necessarily the slowest speed at which the engine will run. Can be any speed you choose

Carburetor Adjustments (con’t) Again, turn the screw in (clockwise) until the engine speed decreases. Lean mixture Turn the screw out (counter-clockwise) until the speed increases and again decreases Rich mixture Usually, the idle adjustment needs to be reset with each new application of the engine.

Gas, Oil, & Air Gas-- use fresh, clean, unleaded gasoline with a minimum of 77 octane In Minnesota, the minimum octane allowed by law is 87. Purchase an amount that can be used up within 30 days. Ethanol an/or methanol is fine to use in today’s small engines.

Gas, Oil, & Air (con’t) Oil-- Detergent oils keep the engine free of gum and varnish deposits and generally keep the engine cleaner. No other additives should be used in the oil.

Gas, Oil, & Air (con’t) Air-- Clean air is an extremely important part of the carburetion system. New technologies in foam materials make these air cleaners somewhat obsolete.

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