12 MS 440, MS 440 C
Condition Cause Remedy
Engine will not idle,
idle speed too high
Throttle shutter opened too wide by
idle speed screw (LA)
Reset idle speed screw (LA)
correctly
Oil seals/crankcase leaking Seal or replace oil seals/
crankcase
Engine stalls at idle speed Idle jet bores or ports blocked Clean jet bores and ports, and blow
through with compressed air
Idle jet too rich or too lean Set low speed screw (L)
correctly
Setting of idle speed (LA)
incorrect – throttle shutter
completely closed
Set idle speed screw (LA)
correctly
Small plastic plate in valve jet
does not close
Clean or renew valve jet
Engine speed drops quickly under
load – low power
Air filter dirty Clean the air filter
Throttle shutter not opened fully Check actuation
Tank vent faulty Clean tank vent or replace if
necessary
Fuel pickup body dirty Clean the pickup body, fit a new
filter
Fuel strainers dirty Replace the fuel strainers
Leak in fuel line between tank and
fuel pump
Seal connections or install a new
fuel line
Pump diaphragm damaged or
fatigued
Fit new pump diaphragm
Main jet bores or ports blocked Clean bores and ports
Setting of high speed screw (H)
too rich
Rotate high speed screw (H)
clockwise (leaner), no further than
stop
Impulse hose damaged or kinked Fit new impulse hose
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