1. Carburetion
The fuel system on a small engine consists of several components;
Carburetor
Fuel Lines
Fuel Filter
Fuel tank

2. Carburetion The function of the carburetor is threefold;
It breaks up or atomizes the fuel into a fine spray and it mixes with air to make a mixture that will burn readily
It regulates the ratio of fuel to air
It regulates the amount of the fuel-air-mixture going into the combustion chamber

3. Carburetion
The ratio of fuel air is controlled by adjusting needle valves and this allows you to select the proper mixture for the conditions under which you operate your engine
Too lean a fuel-air mixture may result in hard starting, overheating, pre-ignition & valve burning
Too rich a fuel-air mixture may result in excessive fuel consumption, carbon build up in the cylinder and cause pre-ignition

4. Carburetion: Principles of Operation
As the piston moves down a partial vacuum is created in the cylinder
Atmospheric pressure pushes air through the carburetor air intake to equalize this pressure
The air speed increases in the venturi (the narrow passage in the air intake passage)

5. Carburetion: Principles of Operation
As the air speed increases the pressure is lowered.
Because the pressure is lowered atmospheric pressure in the fuel bowl pushes fuel through the pipe to the venturi and into the air stream

6. Carburetion: Principles of Operation
The speed of the air in the venturi and the turbulence past the venturi atomize the fuel and mix the tiny droplets with air.
Once the air & fuel are mixed, the next job of the carburetor is to provide a means whereby the amount of mixture that enters the cylinder can be controlled

7. Carburetion: Principles of Operation
A butterfly (throttle) valve in the manifold controls the fuel air mixture
If you want your engine to run fast open your butterfly (throttle) valve, the more fuel and air that gets delivered the faster it will run

8. Carburetion: Principles of Operation
A “Choke” valve is used to aid in cold starts
It helps provide a better vaporization of the fuel–air and it provides more fuel-air (a richer mixture)
The choke valve is similar to the butterfly except it is placed on the air-intake side of the carburetor.

9. Carburetion: Principles of Operation
When the choke is closed air entering is restricted.
The pressure inside the carb. & cylinder is further reduced and this increases the vaporization of the fuel.
Some engines use a primer setup instead of a choke valve

10. Carburetion: Principles of Operation

11. Carburetor Types
Briggs and Stratton uses many different types of carburetors for their many different types of small gas engines
The principal of operation of each is the same but there are many variations

12. Carburetor Types
There are basically three different types of carburetors used in small gas engines depending on how fuel is supplied from the tank to the fuel chamber in the carburetor.
The Float Type
The Suction – Lift Type
The Diaphragm Type

13. The Float Type
These are so called because the fuel level in the fuel chamber is maintained by a float – controlled valve.
There are several different styles of float-type carbs such as Updraft and, Side-draft types

14. The Float Type
The float type carburetor has features which provide for adjustment and regulation of the fuel air to meet different operating conditions.
When a sudden load or acceleration is demanded , a richer mixture (more fuel-air) is required
These carburetors have what is called an accelerating well that surrounds the lower part of the fuel discharge nozzle and remains full of fuel while the engine is operating under normal load.
When there is a sudden demand for power a governor opens the throttle valve and air moves past the nozzle much faster thus picking up more fuel

15. Suction-Lift Carburetor
This type is usually mounted on the top of the fuel tank.
Vacuum from the engines intake stroke causes a low pressure in the venturi.
Atmospheric pressure forces fuel up through the tube into the low pressure area of the venturi and then into the engine.
This type of carburetor will not work with larger engines and tanks

16. Diaphragm – Type Carburetor
This type uses a spring-loaded diaphragm for regulating the fuel flow into the carburetor fuel chamber
The diaphragm serves the same purpose as a float in the float type carb.

17. Diaphragm – Type Carburetor
The main difference between this and the float type is that the use of the diaphragm carburetor will allow the engine to work on any angle. For this reason the diaphragm type is used a lot in multi-positional engines.