The document discusses carburetion in small engines. It describes the components of a small engine fuel system including the carburetor. The carburetor atomizes fuel into a fine spray and mixes it with air. It regulates the ratio of fuel to air and the amount of fuel-air mixture entering the combustion chamber. The ratio is controlled by needle valves and affects engine performance. It then discusses the principles of carburetor operation including how vacuum pulls fuel into the air stream and how choke valves and butterfly valves control the fuel-air ratio during starting and operation. Finally, it describes different types of carburetors including float, suction-lift, and diaphragm types.

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. Natural Draft Carburetor
• This carburetor is used
where there is little
space on top of the
engine. The air enters
horizontally into the
manifold.

16. Updraft Carburetors
• This type is placed low
on the engine and use a
gravity fed-fuel supply.
In other words, the tank
is above the carburetor
and the fuel falls to it.

17. Downdraft Carburetors
• This carburetor operates
with lower air velocities
and larger passages.
This is because gravity
assists the air-fuel
mixture flow to the
cylinder.

18. 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

19. 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.

20. 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.

21. Constant vacuum carburetor