Adjusting The Carburetor - Stihl HT 75 Instruction Manual

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Adjusting the Carburetor

The carburetor comes from the factory
with a standard setting.
This setting provides an optimum fuel-air
mixture under most operating
conditions.
With this carburetor it is only possible to
adjust the high speed screw within fine
limits.
Standard Setting
Shut off the engine.
N
Check chain tension.
N
Check the air filter and clean or
N
replace as necessary.
Check that the throttle cable is
N
properly adjusted – readjust if
necessary – see chapter on
"Adjusting the Throttle Cable".
24
H
3 / 4
Turn high speed screw (H)
N
counterclockwise as far as stop (no
more than 3/4 turn).
Turn the low speed screw (L)
N
carefully clockwise as far as stop,
then turn it back 1 turn
Start and warm up the engine.
N
Adjust idle speed with the idle speed
N
screw (LA) so that the chain does
not run.
Fine Tuning
A slight correction of the setting of the
high speed screw (H) may be necessary
if engine power is not satisfactory when
operating at high altitude or at sea level.
Rule of thumb:
Turn the high speed screw (H) about
one quarter turn for every 1000 m (3300
ft) change in altitude.
Conditions for adjustment
Carry out the standard setting.
N
Warm up the engine for about 5
N
minutes.
L
1
Open the throttle wide.
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At high altitude
Turn the high speed screw (H)
N
clockwise (leaner), no further than
stop, until there is no noticeable
increase in engine speed.
LA
At sea level
Turn the high speed screw (H)
N
counterclockwise (richer), no further
than stop, until there is no
noticeable increase in engine
speed.
It is possible that maximum engine
speed may be reached with the standard
setting in each case.
Adjusting Idle Speed
It is usually necessary to change the
setting of the idle speed screw (LA) after
every correction to the low speed
screw (L).
Warm up the engine for about 5
N
minutes.
Engine stops while idling
Turn the idle speed screw (LA)
N
slowly clockwise until the engine
runs smoothly – the chain must not
rotate.
HT 75