Troubleshoot Stihl MS 650 Chainsaw

Troubleshoot Stihl MS 650 Chainsaw

The various faults that can affect a Stihl MS 650 chainsaw are divided into seven groups. In each group, the likely symptoms are given on the left and possible causes are listed on the right. The most likely faults are given first, and so on.

Chainsaws are pretty basic engines, requiring three basic areas, along with their basic associated components to start and run smoothly as well as to deliver the power, efficiency, and reliability that is required:

Fuel – Carburettor, Fuel filter, Fuel mix, Hoses, Primer bulb, Breather

Spark – Spark Plug, Ignition unit, HT & short Lead, Stop Switch, Flywheel

Air – Air filter, Carburettor, Manifold, Muffler, Hoses, Breather

10 Basic Checks For The Stihl MS 650 Chainsaw

  • Pull the starter rope to see if the engine turns over and if compression is adequate.
  • Check fuel for quality and correct mix – recommend replacing with a known mix.
  • Check the fuel filter for blockage.
  • Check fuel supply pickup and hose for blockage or cracking from age.
  • Check the air filter for blockage.
  • Check the carburetor settings are correct (including the idle adjustment).
  • Check for spark.
  • Check the spark plug electrode for wear or carbon build-up and correct the gap.
  • Check the muffler spark arrestor screen, if fitted visually check hoses, manifold, and cylinder base gasket for leaks.
  • Remove the exhaust to check for piston seizure or ring damage if compression seems inadequate.

Troubleshooting Stihl MS 650 Chainsaw

Stihl MS 650 Clutch

ConditionCauseRemedy
The Saw chain stops under load at full throttle (1)Clutch shoes badly wornInstall new clutch
Saw chain stops under load at full throttle (2)The clutch drum is badly wornInstall new clutch drum
The Saw chain rotates at idle speed (1)Engine idle speed too highReadjust with idle speed screw LA (counterclockwise)
Saw chain rotates at idle speed (2)Clutch springs stretched or fatiguedReplace the clutch springs or install a new clutch
The Saw chain rotates at idle speed (3)Clutch spring hooks brokenReplace the clutch springs
Loud noises (1)Clutch springs stretched or fatiguedReplace all clutch springs
Loud noises (2)Needle cage damagedFit new needle cage
Loud noises (3)The clutch shoe retainer brokenFit new retainer
Loud noises (4)Clutch shoes and carrier wornInstall new clutch

Stihl MS 650 Chain Drive, Chain Brake, Chain Tensioner

ConditionCauseRemedy
Chain sprocket wears rapidly (1)The chain was not properly tensionedTension chain as specified
Chain sprocket wears rapidly (2)Wrong chain pitchFit chain of the correct pitch
Chain sprocket wears rapidly (3)Insufficient chain lubricationCheck chain lubrication
Chain sprocket wears rapidly (4)Chain sprocket wornFit new chain sprocket
The Saw chain stops under load at full throttle (1)Clutch shoes badly wornInstall new clutch
Saw chain stops under load at full throttle (2)The clutch drum is badly wornInstall new clutch drum
The Saw chain stops under load at full throttle (3)Brake band blockedCheck the freedom of movement and operation of the brake band
The Saw chain rotates at idle speed (1)Engine idle speed too highReadjust with idle speed screw LA (counterclockwise)
Saw chain rotates at idle speed (2)Clutch springs stretched or fatiguedReplace the clutch springs or install a new clutch
The Saw chain rotates at idle speed (3)Clutch spring hooks brokenReplace the clutch springs
The Saw chain does not stop activated (1)The brake spring stretched or brokenFit new brake spring
The Saw chain does not stop activated (2)
Brake band stretched or worn

Fit new brake band
The Saw chain does not stop activated (3)Clutch drum wornFit new clutch drum

Stihl MS 650 Chain Lubrication

In the event of trouble with the chain lubrication system, check and rectify other sources of faults before disassembling the oil pump.

ConditionCauseRemedy
Chain receives no oil (1)Oil tank emptyFill up with oil and check the setting of the oil pump if necessary
Chain receives no oil (2)The oil inlet hole in the guide bar is blockedClean oil inlet hole
Chain receives no oil (3)Intake hose or pickup body clogged or intake hose rupturedFit new intake hose and pickup body
Chain receives no oil (4)The valve in the oil tank blockedClean or replace the valve
Chain receives no oil (5)Teeth on worm wornInstall new worm
Chain receives no oil (6)Oil pump damaged or wornInstall new oil pump
Machine losing chain oil (1)Oil pump body damaged
Install new oil pump
Machine losing chain oil (2)Oil pump damaged or wornInstall new oil pump
Machine losing chain oil (3)Oil suction hose connection
damaged
Install new oil intake hose
Oil pump delivers insufficient oil (4)Oil pump wornInstall new oil pump
Oil pump delivers insufficient oil (5)The oil pump delivery rate is set too lowAdjust the oil pump (only on the version with an adjustable oil pump)

Stihl MS 650 Rewind Starter

ConditionCauseRemedy
The starter rope broke (3)The rope pulled out too vigorously as far as stop or over the edge, i.e. not
vertically
Fit new starter rope
The starter rope broke (2)Normal wearFit new starter rope
The starter rope does not rewind (3)Very dirty or corrodedClean or replace the rewind spring
The starter rope does not rewind (4)Insufficient spring tensionCheck the rewind spring and increase the tension
The starter rope does not rewind (5)Rewind spring brokenFit new rewind spring
The starter rope cannot be pulled out far enoughSpring over tensionedCheck rewind spring and reduce
tension
The starter rope can be pulled out almost without resistance (the crankshaft does not turn) (1)Guide peg on pawl or pawl itself is wornFit new pawl
The starter rope can be pulled out almost without resistance (the crankshaft does not turn) (2)Spring clip-on pawl fatiguedFit new spring clip
The starter rope is difficult to pull or rewinds very slowly (1)The starter mechanism is very dirtyThe thoroughly clean complete starter mechanism
The starter rope is difficult to pull or rewinds very slowly (2)Lubricating oil on rewind spring becomes viscous at very low outside temperatures (spring windings stick together)Coat the rewind spring with a little standard solvent-based degreasant (containing no chlorinated or halogenated hydrocarbons), then pull the rope carefully several times until normal action is restored
The starter rope is difficult to pull or rewinds very slowly (3)The decompression valve is not openOpen, check, and replace the decompression valve if necessary

Stihl MS 650 Ignition System

Exercise extreme caution while carrying out maintenance and repair work on the ignition system. The high voltages which occur can cause serious or fatal accidents.

ConditionCauseRemedy
Engine runs roughly, misfires, temporary loss of power (1)The spark plug boot is loosePress the boot firmly onto the spark plug and fit the new spring if necessary
Engine runs roughly, misfires, temporary loss of power (2)Spark plug sooted, smeared with oilClean the spark plug or replace it if
necessary.
If sooting keeps recurring, check the air
filter
Engine runs roughly, misfires, temporary loss of power (3)Ignition lead loose in the ignition moduleSecure ignition lead properly
Engine runs roughly, misfires, temporary loss of power (4)Fuel/oil mixture – too much oilUse the correct mixture of fuel and oil
Engine runs roughly, misfires, temporary loss of power (5)The incorrect air gap between the ignition module and the flywheelSet the air gap correctly
Engine runs roughly, misfires, temporary loss of power (6)Flywheel cracked or has other damages or the pole shoes have turned blueInstall new flywheel
Engine runs roughly, misfires, temporary loss of power (7)Ignition timing wrong, flywheel out of adjustment, key in flywheel has sheared offFit key if necessary and secure
flywheel properly or install new
flywheel
Engine runs roughly, misfires, temporary loss of power (8)Weak magnetization in the flywheelInstall new flywheel
Engine runs roughly, misfires, temporary loss of power (9)Irregular sparkCheck the operation of the switch shaft/contact springs and ignition module. Faulty insulation or break in ignition lead or short circuit wire. Check the ignition lead/ignition module and replace the ignition module if necessary. Check the operation of the spark plug. Clean the spark plug or replace it if necessary.
Engine runs roughly, misfires, temporary loss of power (10)Crankcase damaged (cracks)Install new crankcase
No spark (1)Spark plug faultyInstall a new spark plug
No spark (2)Faulty insulation or short in short circuit wireCheck short circuit wire for a short circuit to ground
No spark (3)Break in ignition lead or insulation damagedCheck ignition lead and replace it if necessary
No spark (4)Ignition module faultyInstall a new ignition module

Stihl MS 650 Carburetor

ConditionCauseRemedy
Carburetor floods; engine stalls (1)Inlet needle not sealing – foreign matter in valve seat or coneRemove and clean the inlet needle, clean the carburetor
Carburetor floods; engine stalls (2)The inlet control lever sticking on the spindleCheck the inlet control lever, and replace it if necessary
Carburetor floods; engine stalls (3)The helical spring not located on the nipple of the inlet control leverRemove the inlet control lever and refit it correctly
Carburetor floods; engine stalls (4)The perforated disc on the diaphragm is deformed and presses constantly against the inlet control leverFit a new metering diaphragm
Carburetor floods; engine stalls (5)Metered diaphragm deformedFit a new metering diaphragm
Poor acceleration (1)The setting of the low-speed screw is too leanCheck basic carburetor setting, correct if necessary
Poor acceleration (2)The setting of the high-speed screw is too leanCheck basic carburetor setting, correct if necessary
Poor acceleration (3)The inlet needle sticking to the valve seatRemove the inlet needle, clean, and refit
Poor acceleration (4)Diaphragm gasket leakingFit new diaphragm gasket
Poor acceleration (5)Metering diaphragm damaged or shrunkFit a new metering diaphragm
Poor acceleration (6)Impulse hose damaged or kinkedInstall new impulse hose
Poor acceleration (7)Tank vent faultyReplace tank vent
Poor acceleration (8)Leak on fuel hose from tank to carburetorSeal connections or install a new fuel hose
The engine will not idle, idle speed is too high (3)The throttle shutter opened too wide by the idle speed screw LAReset idle speed screw LA correctly
The engine will not idle, idle speed is too high (3)Oil seals/crankcase leakingSeal or replace oil seals/crankcase
Engine stalls at idle speed (1)Idle jet bores or ports blockedClean the carburetor
Engine stalls at idle speed (2)The setting of the low speed screw is too rich or too leanReset low-speed screw L correctly
Engine stalls at idle speed (3)The setting of idle speed screw LA is incorrect – throttle shutter is completely closedReset idle speed screw LA correctly
Engine stalls at idle speed (4)Tank vent faultyReplace tank vent
Engine stalls at idle speed (5)Leak on fuel hose from tank to carburetorSeal connections or install a new fuel hose
Engine speed drops quickly under load – low power (1)Air filter dirtyClean the air filter or replace it if necessary
Engine speed drops quickly under load – low power (2)The throttle shutter is not opened fullyCheck the throttle cable and rod
Engine speed drops quickly under load – low power (3)Tank vent faultyReplace tank vent
Engine speed drops quickly under load – low power (4)Fuel pickup body dirtyInstall new pickup body
Engine speed drops quickly under load – low power (5)Fuel strainer dirtyClean fuel strainer in carburetor,
replace if necessary
Engine speed drops quickly under load – low power (6)Leak on fuel hose from tank to carburetorSeal connections or install a new fuel hose
Engine speed drops quickly under load – low power (7)The setting of high-speed screw H is too richCheck basic carburetor setting, correct if necessary
Engine speed drops quickly under load – low power (8)Main jet bores or ports blockedClean the carburetor
Engine speed drops quickly under load – low power (9)Pump diaphragm damaged or fatiguedFit new pump diaphragm
Engine speed drops quickly under load – low power (10)Impulse hose damaged or kinkedInstall new impulse hose
Engine speed drops quickly under load – low power (11)Ignition timing wrong, flywheel out of adjustment, key in the flywheel is missing or has sheared offFit the key if necessary and secure the flywheel properly or install a new flywheel

Stihl MS 650 Engine

Always check and, if necessary, repair the following parts before looking for faults in the engine:

  • Air filter
  • Fuel system
  • Carburetor
  • Ignition system
ConditionCauseRemedy
The engine does not start easily, stalls at idle speed, but operates normally at full throttle (1)Oil seals in crankcase damagedReplace the oil seals
The engine does not start easily, stalls at idle speed, but operates normally at full throttle (2)Crankcase leaking or damaged (cracks)Seal or replace the crankcase
The engine does not deliver full power or runs erratically (1)Piston rings worn or brokenFit new piston rings
The engine does not deliver full power or runs erratically (2)Muffler / spark arresting screen carbonizedClean the muffler (inlet and exhaust), replace the spark arresting screen, and replace the muffler if necessary
The engine does not deliver full power or runs erratically (3)Air filter dirtyReplace air filter
The engine does not deliver full power or runs erratically (4)Fuel/impulse hose severely kinked or damagedFit new hoses or position them free from kinks
The engine does not deliver full power or runs erratically (5)The decompression valve is not closedClose, check, and replace the decompression valve if necessary
Engine overheatingInsufficient cylinder cooling. Air inlets in fan housing blocked or cooling fins on cylinder very dirtyThoroughly clean all cooling air openings and the cylinder fins

Troubleshooting methods

In addition to the faults described in the above tables, troubleshooting can be carried out on specific components or sub-systems of the chainsaw. The various procedures are described in the relevant chapters (see the full manual on this website), see the contents page, as follows:

  • Checking the operation of the chain brake
  • Measuring the resistance of the stop plate
  • Pressure testing the carburetor
  • Pressure testing the decompression valve
  • Pressure testing the cylinder

Stihl MS 650 Chainsaw workshop Manual and parts list available

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