Engine - Stihl MS 661 Manual

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
- Check M-Tronic with STIHL
MDG 1 and troubleshooting chart
Condition
Engine does not start easily, stalls
at idle speed, but operates normally
at full throttle
Engine does not deliver full power
or runs erratically
Engine overheating
MS 661, MS 661 C-M
Cause
Oil seals in crankcase damaged
Crankcase leaking or damaged
(cracks)
Piston rings worn or broken
Muffler / spark arresting screen
carbonized
Air filter dirty
Fuel hose kinked or torn
Decompression valve is not closed
Insufficient cylinder cooling. Air
inlets in fan housing blocked or
cooling fins on cylinder very dirty
Air inlet in fan housing dirty
Remedy
Install new oil seals
Seal or replace the crankcase
Install new piston rings
Clean the muffler (inlet and
exhaust), replace spark arresting
screen, replace muffler if necessary
Clean or replace air filter
Fit new hose or position it free from
kinks
Close and check decompression
valve, replace if necessary
Thoroughly clean all cooling air
openings and the cylinder fins
Clean air inlet on fan housing
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