5.7
Section 5
Fuel System and Governor
5
Figure 5-5. Pre-Compliance Carburetor with Low
Idle Adjustment.
NOTE: Carburetor adjustments should be made
only after the engine has warmed up.
Adjusting Low Idle Speed and Fuel (Some Models)
To adjust the carburetor idle speed, see Figure 5-5 and
follow these steps.
1. With the engine
stopped
, turn the low idle fuel
adjusting needle in
clockwise
until it bottoms
lightly
.
NOTE: The tip of the idle fuel adjusting needle
is tapered to critical dimensions.
Damage to the needle and the seat in the
carburetor body will result if the needle
is forced.
2. Now turn the adjusting needle out
counterclockwise
1-1/2 turns.
3. Start the engine and run at half throttle for 5 to 10
minutes to warm up. The engine must be warm
before making final settings. Check that the
throttle and choke plates can fully open.
NOTE: The carburetor has a self-relieving
choke. Choke plate and shaft assembly is
spring loaded. Check to make sure plate
moves freely and is not binding and
affecting idle fuel delivery.
4. Place the throttle control into the “idle” or “slow”
position. Turn the low idle speed adjusting screw
in or out to obtain a low idle speed of 1200 RPM
(± 75 RPM). Check the speed using a tachometer.
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).
5. Turn the low idle fuel adjusting needle in
(slowly) until engine speed decreases and then
back out approximately 3/4 turn to obtain the best
low speed performance.
6. Recheck the idle speed using a tachometer and
readjust the speed as necessary.
Carburetor Adjustments (CH18-740)
General
In compliance with government emission standards,
the carburetor is calibrated to deliver the correct air-
to-fuel mixture to the engine under all operating
conditions. The high-speed mixture is preset and
cannot be adjusted. Pre-compliance carburetors
contain a low idle fuel adjusting needle, on “certified”
compliance carburetors, both the low and high speed
mixture circuits are pre-established and cannot be
adjusted. The low idle speed (RPM) is the only
adjustment available. See Figures 5-5 and 5-6.
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.
Low Idle Speed
Adjustment
Low Idle Fuel
Adjusting Needle
Main Jet
Location
Figure 5-6. “Certified” Compliance Carburetor.
Low Idle
Speed
Adjustment
Fuel Shut-Off
Solenoid (Main
Jet Location)
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