Stihl MS 251 Manual

95MS 231, MS 231 C, MS 251, MS 251 C
12.7 Adjusting the Carburetor
12.7.1 Basic Setting
The basic setting is necessary only
if the high speed screw (H) or low
speed screw (L) has to be replaced
or after cleaning and adjusting the
carburetor from scratch.
It is necessary to carry out the basic
setting after removing the limiter
cap.
The carburetor and air filter are
installed, the adjusting screws fitted
and the new limiter cap pre-
installed.
– Check chain tension and adjust if
necessary.
– Inspect the spark arresting
screen (if fitted) and clean or
replace if necessary, b 3.7 or
b 6.1
– Check the air filter and clean or
replace if necessary, b 12.1
For the sake of clarity the adjusting
screws are shown on the exposed
carburetor.
: Starting with the high speed
screw H (1) against its seat,
open it 1 1/2 turns counter-
clockwise – this is the basic
setting.
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1
2
: Starting with the low speed screw
L (2) against its seat, open it
1 full turn counterclockwise
– this is the basic setting.
– Warm up the engine.
The setting disc 5910 893 6600
may be fitted on the screwdriver
5910 890 2306 to aid adjustment.
: Insert the screwdriver (1)
5910 890 2306 through the
opening (arrow) and into the low
speed screw L and high speed
screw H
– push the screwdriver through
the pre-installed limiter cap on
the high speed screw H.
Adjust idle speed with a tachometer.
Adjust specified engine speeds
within a tolerance of ± 200 rpm.
1. Adjust engine speed with
idle speed screw (LA),
to 3,300 rpm.
2. Turn the low speed screw (L)
counterclockwise or clockwise
to obtain maximum engine
speed.
0
14
12
10
8
6
4
2
0
14
12
10
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1
If this speed is higher than
3,700 rpm, abort the procedure and
start again with step 1.
3. Use the idle speed screw (LA)
to set the engine speed again to
3,300 rpm.
4. Use the low speed screw (L)
to set engine speed to
2,800 rpm.
5. Use the high speed screw (H)
to set the engine's maximum
speed to 13,000 rpm.
(starting with H = 1 1/2 turns
open).
As soon as a maximum engine
speed of 13,000 rpm is obtained, do
not attempt to make the mixture any
leaner – the ignition module limits
maximum engine speed to 13,000
rpm. This speed cannot be
increased by making the setting
leaner. Making the mixture over-
lean increases the risk of engine
damage.
: Insert a drift through the
opening (arrow) and then push
home the limiter cap until it
engages.
2310RA284 TG
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Stihl MS 251 Specifications

General

Idle speed	2800 RPM
Motor type	2-stroke motor
Chain pitch	1/308 \
Blade length	450 mm
Product color	Black, Grey, Orange
Motor speed (max)	10000 RPM
Oil tank capacity	0.2 L
Fuel tank capacity	0.39 L
Engine displacement	45.6 cm³
Kickback protection	Yes
Engine power	3 hp
Power source	Gasoline
Sound power level	114 dB
Vibration emission	3.9 m/s²
Sound pressure level	103 dB

Weight and Dimensions

Weight	4900 g