Adjusting The Carburetor - Stihl SR 420 Instruction Manual

English
► Remove the main filter (3) from the cover and
inspect it – if it is dirty or damaged, fit a new
one.
Always install a new prefilter together with the
new main filter.
► Take the prefilter (4) out of the filter cover.
► If prefilter is wet, dry it – then knock it out on
the palm of your hand or blow it out with com‐
pressed air.
Always replace a damaged prefilter.
► Clean loose dirt from the filter cover and filter
chamber.
► Install the main filter (3) and prefilter (4) in the
filter cover.
► Fit the cover on the filter base (5) and tighten it
down firmly.

14 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 and low speed screws within fine
limits.
14.1 Standard Setting
► Shut off the engine.
► Check the air filter and clean or replace if nec‐
essary.
18
3
5
4
► Check the spark arresting screen (not in all
models, country-specific) in the muffler and
clean or replace if necessary.
► Carefully turn both adjusting screws counter‐
clockwise as far as stop.
► The high speed screw (H) is 1/4 turn open.
► The low speed screw (L) is 1/4 turn open.
14.2 Adjusting Idle Speed
► Carry out the standard setting.
► Start and warm up the engine.
14.2.1 Engine stops while idling
► Turn the idle speed screw (LA) slowly clock‐
wise until the engine runs smoothly.
14.2.2 Erratic idling behavior, engine stops
even though setting of LA screw has
been corrected, poor acceleration
Idle setting is too lean
► Turn the low speed screw (L) counterclock‐
wise, no further than stop, until the engine
runs and accelerates smoothly.
14.2.3 Erratic idling behavior
Idle setting is too rich
► Turn the low speed screw (L) clockwise, no
further than stop, until the engine runs and
accelerates smoothly.
It is usually necessary to change the setting of
the idle speed screw (LA) after every correction
to the low speed screw (L).
14 Adjusting the Carburetor
H L
L
LA
0458-372-0121-C