Carburetion Systems

1
Carburetion Systems

• Mr. Thaden
• 3rd Term 2000-2001
• Adv. Small Gas Engines

2
Overview

• Purpose of Carburetor
• Types of Carburetors
• Gas, Oil, Air

3
The BIG Picture

• One of the Big 3 systems
• Ignition
• Compression
• Carburetion

4
Purpose of the Carburetor

• One main purpose...
• Secondary Purposes Requirements

5
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

6
Carburetor Theory (cont)

• 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

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Carburetor Theory (cont)

• Airfoil

low pressure
airfoil
high pressure
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Carburetor Theory (cont)

• 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.

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Carburetor Theory (cont)

• 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.

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Carburetor Theory (cont)

• 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

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Carburetor Theory (cont)

• 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.

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Carburetor Theory (cont)

• 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 BS engines.

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Types of Carburetors

• Gravity Feed
• called a Flo-Jet by Briggs Stratton
• Vacuum Feed
• called Vacu-Jet
• Pulse Feed
• called Pulsa-Jet

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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.

15
Flo-Jet Carburetors (cont)

• 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.

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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.

17
Vacu-Jet Carburetors (cont)

• 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.

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Vacu-Jet Carburetors (cont)

• 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.

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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

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Pulsa-Jet Carburetors (cont)

• 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

21
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 scuffingand lost
  compression.

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Carburetor Adjustments (cont)

• Throttle Adjustment
• To adjust for maximum power and efficiency
• This indicates a lean mixture.
• This indicates a rich mixture.

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Carburetor Adjustments (cont)

• 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.

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Carburetor Adjustments (cont)

• 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

25
Carburetor Adjustments (cont)

• 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.

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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 todays
  small engines.

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Gas, Oil, Air (cont)

• 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.

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Gas, Oil, Air (cont)

• 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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THE END