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5.21
Section 5
Fuel System and Governor
5
Adjustment
NOTE: Carburetor adjustments should be made only
after the engine has warmed up.
The carburetor is designed to deliver the correct fuel-
to-air mixture to the engine under all operating
conditions. The main fuel jet is calibrated at the
factory and is not adjustable*. The idle fuel adjusting
needle is also set at the factory and normally does not
need adjustment.
Depending on model and application, engines may
also be equipped with a “Governed Idle System.” If
equipped with a “Governed Idle System”, refer to
“Models with Governed Idle System” when
performing any carburetor adjustment, as an
additional step to the listed adjustment procedure(s) is
required.
*NOTE: Engines operating at altitudes above
approximately 1500 m (5000 ft.) may require
a special ‘‘high altitude’’ main jet. Refer to
‘‘High Altitude Operation’’.
If, however, the engine is hard-starting or does not
operate properly, it may be necessary to adjust or
service the carburetor.
*NOTE: The actual low idle speed depends on the
application. Refer to the equipment
manufacturer’s recommendations. The low
idle speed for basic engines is 1200 RPM. To
ensure best results when setting the low idle
fuel needle, the low idle speed should be
1200 RPM (± 75 RPM).
Low Idle Fuel Adjustment
NOTE: Engines will have fixed low idle or limiter
caps on the two idle fuel adjusting needles.
Step 3 can only be performed within the
limits allowed by the cap. Do not attempt to
remove the limiter caps.
1. Start the engine and run at half throttle for 5 to 10
minutes to warm up. The engine must be warm
before doing steps 2, 3, and 4.
2. Place the throttle control into the “idle” or
“slow” position. Adjust the low idle speed to
1200 RPM*. Follow the “Adjusting the Low Idle
Speed (RPM)” procedure.
3. Low Idle Fuel Needle(s) Setting: Place the
throttle into the “idle” or “slow” position.
a. Turn one of the low idle fuel adjusting needles
out (counterclockwise) from the preliminary
setting until the engine speed decreases
(rich). Note the position of the needle. Now
turn the adjusting needle in (clockwise). The
engine speed may increase, then it will
decrease as the needle is turned in (lean). Note
the position of the needle. Set the adjusting
needle midway between the rich and lean
settings. See Figure 5-40.
b. Repeat the procedure on the other low idle
adjustment needle.
4. Recheck/adjust the Low Idle Speed (RPM), to the
specified setting.
Figure 5-39. Keihin Two-Barrel Carburetor.
Carburetor Adjustment
Low Idle Speed (RPM) Adjustment
1. Low Idle Speed (RPM) Setting: Place the throttle
control into the “idle” or “slow” position. Set the
low idle speed to 1200 RPM* (± 75 RPM) by
turning the low idle speed adjusting screw in or
out. Check the speed using a tachometer.
Low Idle Fuel Adjusters
(with Limiters)
Low Idle Speed (RPM)
Adjustment Screw
Fuel Solenoid
Bowl Drain Screw