The diagram below shows a schematic of a typical float type carburetor with
the engine running at high speed. The choke plate is fully open and the
throttle plate is opened the proper amount be feedback from the governor
to maintain the speed set by the user throttle control.
|NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Basic operation of a float type carburet..
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<< Crankshaft anatomy |
| The four strokes of a fou.. >>
- Air is sucked through the venturi by the intact stroke of the piston. The
lower pressure created by the faster moving air draws gasoline up through
the main nozzle and into the air stream. This aerosol of gasoline vaporized
- The air bleed allows the pressure inside the float chamber to equalize
as fuel it drawn off
- The float maintains a more or less constant level of liquid gasoline in
the float chamber by opening the inlet needle valve when the level drops.
The fuel supply is either above the level of the carburetor or there is
a separate fuel pump (usually operated by crankcase pressure pulsations
or a direct mechanical linkage).
- The main nozzle either has precisely machined orifices to set the ratio
of air to gasoline in the mixture or has an adjustable needle valve for
this purpose (not shown).
- Closing the choke forces more gasoline into the mixture and makes it
- The throttle plate is used to vary the amount of the air-fuel mixture to
the engine but maintains nearly the same ratio of air to fuel over a wide
range of engine speeds.
- The user throttle (speed) control adjusts the spring force against which
the governor must act to close the throttle plate. Increasing the spring
force increases the engine speed.
- Operation at idle speed depends on a separate system of an idle nozzle
and idle speed adjustment (not shown) and does not depend on governor
feedback to control the throttle - which is kept open just a small amount.
Air filter (Air: -->, Air+fuel: ==>)
/ \ Choke plate Throttle plate
| ||||||||| | (Fully open) Venturi (Partially open)
| ||||||||| |______________ v _______________________________
| ||||||||| \______/
| ||||||||| -> -> -> -> -> ==> ==> / ==> Intake
| ||||||||| -> ----O---- ---> ===> ==> ==> O ==> pipe to
| -> -> -> -> -> __||__ ==> ==> / | ==> cylinder
\____________________ ___/ || \_____________|__________________
|| || |
Fuel __________ Air bleed ->|| ||<- Main nozzle o---------+
Inlet ______ |____________||______||__ |--> Pull
| |_ _| || \ Speed <--/\/\/--+ from
Needle ->| __A_______________ || | control Spring governor
Valve | |o__/ \ || | closes
|-----| Float |-----||---| throttle
| . . | | Gas || . |<- Float bowl plate
| . . .\_____________/. . . || . |
<<Crankshaft anatomy |
| The four strokes of a fou..>>