Carburetion Systems
Mr. Thaden
3rd Term 2000-2001
Adv. Small Gas Engines
Overview
 Purpose
 Types
 Gas,
of Carburetor
of Carburetors
Oil, & Air
The “BIG” Picture
 One
of the “Big 3” systems
–
Ignition
–
Compression
–
Carburetion
Purpose of the Carburetor
 One
main purpose...
 Secondary
Purposes & Requirements
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
 River currents
– always faster
wide pools
 Definition
2 buildings
in a narrower, shalllower place than deep,
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
–
called a “Flo-Jet” by Briggs & Stratton
 Vacuum
–
Feed
called “Vacu-Jet”
 Pulse
–
Feed
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.
THE END