Carburetion Systems

1
Carburetion Systems

• Small Engines

2
Carburetion

• Functions of the carburetion system are
• To mix the fuel with the proper proportion
  of air.
• To vaporize the fuel
• To deliver the correct amount of the air-fuel
  mixture to the cylinder.
• A carburetor may be defined as
• A device for automatically mixing fuel in the
  proper proportion with air to produce a
  combustible gas mixture.

3
Carburetion

• Additional engine parts closely related to the
  carburetor include
• The fuel tank, fuel lines, air cleaner, choke,
  and speed control devices (governors).

4
Venturi and Air Flow

• A venturi in the the carburetor increases the
  velocity of the incoming air.
• When the air is forced through a restricted area,
  it must accelerate in order to maintain the
  volume of flow.
• This is a lot like a narrow space in between to
  buildings.

5
Venturi and Air Flow

• A venturi is defined as
• An area in an air flow tube of a carburetor that
  restricts the flow of air through the tube
  resulting in a high velocity and low pressure at
  the restricted area.
• Venturies may differ in their general
  configuration from one carburetor to another.

6
Venturi and Air Flow

• Air flow through the venturi area of the
  carburetor is due to the movement of the piston.
• The pressure in the intake passage is decreased.
• Thus creating a low pressure condition in the
  carburetor venturi area.

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Throttle

• To regulate engine operating speed a throttle
  valve is employed to restrict air flow.
• This is mounted just beyond or above the venturi.
• When the throttle valve is fully opened the air
  flow is affected very little.
• When as the throttle is closed the flow of air is
  restricted to the intake passage.

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Throttle

• This decreases the speed and power of the engine.
• At the same time it allows the pressure in the
  venturi area to increase.
• The difference between the air pressure in the
  fuel chamber and the venturi is decreased.
• Therefore the movement of fuel through the nozzle
  is reduced.

9
Carburetor Types

• All carburetors used on small engines are
  basically the same.
• Carburetors can be classified into three kinds
• Float Feed
• Suction Feed
• Diaphram
• The difference between these is the way the fuel
  is supplied to the fuel chamber.

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Carburetor Types

• Float Feed carburetors locate their fuel tanks
  some distance from the carburetor.
• The fuel flows either be gravity, or due to the
  force of a fuel pump, through fuel lines to the
  lower part of the carburetor.
• The position of the float controls the fuel level
  in the bowl.

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Carburetor Types

• Suction Feed Carburetor is very similar to the
  float type.
• The one exception is there is no float to meter
  and control the level of fuel in the fuel
  chamber.
• The difference in pressure between the tank and
  the carburetor throat lifts the fuel up the fuel
  pipe past the main needle valve and through the
  discharge holes.

12
Carburetor Types

• The diaphram carburetor has a rubber diaphram
  exposed to the cylinder intake stroke vacuum on
  one side and to atmospheric pressure on the
  other.
• The diaphram moves against the inlet needle
  allowing it to move from its seat.
• A spring returns the needle to its seat when the
  vacuum stops.

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Carburetor Adjustments

• Carburetors need adjustment to meet climatic
  conditions of both sea level and temperature
  variations.
• Carburetors should be adjusted so it will not
  exhaust an unnecessary amount of unburned
  hydrocarbons or it should not be so lean that it
  will give unsatisfactory power performance.

14
Idle and Main Needle Valve Adjustments

• The main needle valve adjustment is has many
  different names.
• Power load adjustment, high speed adjustment
  screw, main jet, etc
• The valve is usually turned by hand or by a
  screwdriver.
• The turning moves the needles tapered point in
  and out of a seat in the lower part of the
  nozzle, thus varying the amount of fuel that can
  be metered.

15
Idle and Main Needle Valve Adjustments

• Adjustments of the main needle valve should be
  done before adjusting the idle valve.
• Turn the main needle until the engine starts to
  flood.
• Then quickly turn the valve in the opposite
  direction until until the engine starts to miss
  from the lack of fuel.

16
Idle and Main Needle Valve Adjustments

• Somewhere between these points will be the
  correct main needle valve setting.
• The idle valve should be adjusted next.

17
Chokes

• Starting an engine when it is cold requires a
  richer air-fuel mixture than when it is warm.
• The choke regulates the pressure in the venturi,
  thereby increasing the flow of fuel from the
  discharge nozzles.

18
Governors

• The function of the governor on an engine is to
  maintain a desired speed regardless of load.
• It is a fixed throttle position, the engine will
  speed up or slow down depending on the load
  applied.

19
Governors

• The two most common governor systems are the air
  vane type and the mechanical type.
• The air vane governor is operated by the force of
  air currents away from the flywheel fins.
• This force and movement of the air on the air
  vane opens and closes the throttle valve.

20
Governors

• Mechanical governors work in a manner similar to
  the air vane type except centrifugal weights
  oppose the governor spring rather than an air
  vane.

21
Air Cleaners

• It is important to prevent dirt and dust from
  being cared into the engine through the
  carburetor.
• If the air cleaner is operating improperly dirt
  will enter the combustion chamber, causing
  excessive wear of the rings, cylinder walls and
  other moving parts.

22
Air Cleaners

• There are several different types of air cleaners
  used.
• The oil bath type is the oldest type.
• All air passes through the oil and oil soaked
  mesh before entering the carburetor.
• Dust and dirt are removed from the air and
  accumulates as sediment in the bottom of the oil
  cup.

23
Air Cleaners

• Oil saturated type of air cleaner, contains foam
  or fine wire mesh which is saturated in oil.
• A third type of air cleaner is the dry element
  type.
• Commonly found on larger engines.