Stihl FC 90 - Accelerator Pump; Adjusting Idle Speed

25Series 4180 Powerhead
The carburetor is equipped with an
accelerator pump.
: The piston (1) is located in the
bore (a) behind the throttle shaft
(2) and is held in the idle position
by the spring (3).
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a
: Filter (4) is fitted in the cylinder
bore (a) in front of the throttle
shaft (2) to help prevent the
ingress of dirt.
Perform the following operations to
strip down and clean the accelerator
pump:
: Take out the screw (1).
: Remove the throttle shutter (2).
: Remove the E-clip (3).
: Take out the strainer (5).
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a
: Cover the bore (a) with one
finger.
: Push back the throttle shaft with
lever (4) at least 15 mm until the
end of the shaft is visible in the
choke tube.
: Slowly expose the bore (a), take
out the piston (6) with sealing ring
(7) and spring (8).
: Remove the filter (9), check it for
damage and contamination,
clean it or replace if necessary.
: Inspect piston (6) and spring (8)
for damage and replace if
necessary.
: Install a new sealing ring (7) in
the piston’s groove.
Reassemble all other parts in the
reverse sequence.
Standard Setting
– Shut off the engine
– Check the air filter and clean or
replace if necessary.
– Inspect the spark arresting
screen (if fitted) and clean or
replace if necessary.
The limiter caps must not be
removed for the standard setting.
: Corrections to the high speed
screw H (1) and low speed screw
L (2) are only possible within fine
limits.
– Unscrew the high speed screw
(H) counterclockwise as far as
stop (no more than 3/4 turn).
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– Unscrew the low speed screw (L)
counterclockwise as far as stop
(no more than 3/4 turn).
– Start and warm up the engine.
: Use the idle speed screw LA (3)
to adjust engine idle speed:
Turn clockwise to increase
engine speed;
turn counterclockwise to reduce
engine speed.
6.3 Accelerator Pump 6.4 Adjusting Idle Speed