9
KMR 1250 B
Unit Functions
Gasoline Engine (HONDA GX 270)
Float system
When float chamber (14) is empty, fuel passes
from fuel tank into float chamber (14) via the float
valve inlet (18). The rising fuel level causes the
float (21) to be raised.
The inflow of fuel is interrupted when the float
(21) causes the float valve to close. Drawing fuel
for engine operation from the float chamber (14)
causes the float (21) to be lowered, which in turn
opens the float valve (18).
This cyclical procedure ensures a constant level
of fuel inside the float chamber (14).
Float chamber equalization
On the carburetor side, the airspace inside the
float housing is connected with the outside at-
mosphere by means of a channel. This facili-
tates the equalization of the pressure equilibri-
um inside the float chamber as the fuel level ris-
es or falls.
Idling operation
When the engine is idling, the throttle valve (4) is
fully closed.
–
This causes negative pressure to occur be-
tween the throttle (4) and the crown of the
engine piston, with the latter moving toward
bottom dead center while the intake valve is
open.
–
The atmospheric pressure inside the float
chamber (14) then causes fuel to be con-
veyed through the main jet (15) and idle jet
(2).
–
The idle jet (2) controls the flow of fuel through
the idle bypass channel (3). The fuel then
blends with the air being drawn in through
the idle air channel (6).
–
The resulting air/fuel mixture then flows into
the engine.
–
The idle mixture adjusting screw (16) con-
trols the volume of air/fuel mixture that is al-
lowed to flow through the idle orifice (5).
Partial load operation
Engine speed rises when the throttle valve (4)
opens, and the volume of the air/fuel mixture
being drawn through the idle jet (2) increases.
Full load operation
In full load operation, the throttle is wide (4) open:
–
Air flows through the carburetor throat, or
venturi (7). Because the diameter of the ven-
turi outlet is smaller than that of the intake,
the air is accelerated as it passes through.
This increased flow velocity causes nega-
tive pressure to occur at the outlet side of the
main jet (15).
–
Pressure inside the float chamber (14) is
equalized via an atmospheric relief channel.
Because atmospheric pressure is higher than
the pressure in the carburetor throat (7), fuel
is transported from the float chamber (14),
and pressed through the main jet (15) into
the emulsion tube (12).
–
The air flowing through the main air jet (10)
is blended with the fuel that is flowing through
the orifices of the emulsion tube (12).
–
This mixture is then drawn into the carbure-
tor throat (7), where it is enriched with more
air before passing into the engine.
Choke system function
The choke valve (9) should be closed only when
starting a cold engine. This causes the following
to happen:
–
The closed choke valve (9) allows less air to
be drawn into the carburetor throat (7).
–
However, the fuel volume being drawn in
through the main jet (15) remains the same.
–
This causes the fuel component in the air/
fuel mixture to rise, with the richer mixture
providing easier engine starting.
Note:
For information on other maintenance and
adjustments of the engine, refer to the Serv-
ice manual „4-Stroke Internal Combustion
Engines“ (5.905-122).
