Stihl MS 311 Manual page 16

Condition
Engine speed drops quickly under
load – low power
Engine running extremely rich, has
no power and a very low maximum
speed
Engine running too rich, loss of
power and maximum speed too low
Erratic idle – too lean
MS 311, MS 391
Cause
Air filter dirty
Throttle shutter not opened fully
Tank vent faulty
Fuel pickup body dirty
Fuel strainer dirty
Leak on fuel hose from tank to
carburetor
Setting of high speed screw H too
rich
Main jet bores or ports blocked
Pump diaphragm damaged or
fatigued
Ignition timing wrong, flywheel out
of adjustment, key in flywheel has
sheared off
Air valve does not open
Air valve does not open fully in full
throttle position
Air valve does not close completely Check the end cover with air valve
Intake manifold faulty
Remedy
Clean air filter or replace if
necessary
Check throttle cable and rod
Replace tank vent
Install 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
Clean the carburetor
Fit new pump diaphragm
Fit key if necessary and secure
flywheel properly or install new
flywheel
Check the carburetor and service
or replace if necessary.
Check the carburetor and service
or replace if necessary.
and replace if necessary
Install new intake manifold
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