Stihl MS 391 Service Manual

16 MS 311, MS 391
3.7 Engine
Always check and, if necessary,
repair the following parts before
looking for faults on the engine:
- Air filter
- Fuel system
- Carburetor
- Ignition system
Condition Cause Remedy
Engine does not start easily, stalls
at idle speed, but operates normally
at full throttle
Oil seals in engine housing
damaged
Replace the oil seals
Engine housing leaking/
damaged (cracks)
Seal or replace the engine housing
Engine does not deliver full power
or runs erratically
Piston rings worn or broken Fit new piston rings
Muffler / spark arresting screen
carbonized
Clean the muffler (inlet and
exhaust), replace spark arresting
screen, replace muffler if necessary
Air filter dirty Replace air filter
Fuel/impulse hose severely kinked
or damaged
Fit new hoses or position them free
from kinks
Decompression valve is not closed Close, check and replace
decompression valve if necessary
Engine overheating Insufficient cylinder cooling. Air
inlets in fan housing blocked or
cooling fins on cylinder very dirty
Thoroughly clean all cooling air
openings and the cylinder fins

Stihl MS 391 Specifications

General

Motor type	2-stroke motor
Chain gauge	1.6 mm
Chain pitch	3/8 \
Automatic oiling	Yes
Oil tank capacity	0.35 L
Fuel tank capacity	0.6 L
Engine displacement	64.1 cm³
Engine power	4.4 hp
Power source	Gasoline

Weight and Dimensions

Weight	6200 g