Stihl MS 661 Manual page 16

Condition
Saw chain runs while engine is
idling
Engine speed drops quickly under
load – low power
MS 661, MS 661 C-M
Cause
Engine idle speed too high
Models with M-Tronic, idle speed
too high
Air filter dirty
Throttle shutter not opened fully
Tank vent faulty
Fuel pickup body dirty
Fuel strainer dirty
Leak in fuel hose between pickup
body and carburetor
Setting of high speed screw (H) too
rich
Models with M-Tronic, carburetor
setting too rich
Main jet bores or ports blocked
Pump diaphragm damaged or
fatigued
Ignition timing wrong, flywheel out
of adjustment
– key has sheared off or slot in
flywheel is worn
Remedy
Readjust idle speed screw
LA (counterclockwise)
Check M-Tronic with STIHL MDG 1
and troubleshooting chart, b 8.7,
adjust or calibrate control unit,
b 8.1
(setting of idle speed screw must
not be changed)
Clean air filter or replace if
necessary
Check rod
Fit new tank vent
Fit new pickup body
Clean fuel strainer in carburetor,
replace if necessary
Seal connections or install new fuel
hose
Check basic carburetor setting,
correct if necessary
Check M-Tronic with STIHL MDG 1
and troubleshooting chart, b 8.7,
adjust or calibrate control unit,
b 8.1
Clean the carburetor
Fit new pump diaphragm
Install new flywheel or key
15