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Idle Valve
To supply fuel for the idle, the nozzle is extended up into the idle valve chamber. It fits snugly in the
upper body to prevent leaks. Because of this tight fit, the nozzle must be removed before upper and
lower bodies are separated, or the nozzle will be bent.
The idle valve chamber leads into the carburetor throat above the throttle. Here the pressure is low,
and the fuel rises in the nozzle past the idle valve and into the carburetor throat through the discharge
slot.
The amount of fuel is metered by turning the idle valve in or out until the proper mixture is
obtained. Here again we see what happens if the needle is screwed in too far. A damaged idle value
can result.
Adjustment of the idle valve is similar to that of the needle valve but should be made after the needle
valve has been adjusted. The idle speed is not the slowest speed at which the engine will run. On
small engines it is 1750 RPM. On larger engines the idle speed may be as low as 1200 RPM. Use
a tachometer to set the speed.
Turn the idle speed adjusting screw (located on throttle shaft) until the desired idle speed is obtained
and hold throttle closed. Turn the idle valve in until speed decreases, then out until speed increases
and again decreases. Then turn the idle valve to a point midway between these two settings. Usually
the idle speed adjusting screw will have to be reset to the desired idle speed.
The next problem is starting the engine in different temperatures and with different fuels. A butterfly,
mounted on a shaft, is placed in the air horn. With this choke we can close, or almost close, the air
horn and get a low pressure area in the venturi and throat.
Thus, a rush of fuel is obtained from the nozzle with a relatively small amount of air. Even with low
vaporization this extra rich mixture will give easy starting. Only a portion of the fuel will be
consumed while choking, and a large portion will remain in the cylinder. This raw gasoline will
dilute the crankcase oil and may even cause scuffing due to washing away of the oil film from between
the piston rings and the cylinder wall. For this reason, prolonged choking should be avoided.
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