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Troubleshooting Stihl MS 261 CM Chainsaw

Troubleshoot Stihl MS 261 CM Chainsaw

Troubleshooting Stihl MS 261 CM
The various faults that can affect a Stihl MS 261 CM 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 in order 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 261 CM 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 carburettor settings are correct (including the idle adjustment).
  • Check for spark.
  • Check spark plug electrode for wear or carbon build up and correct 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.

Stihl MS 261 CM Clutch

ConditionCauseRemedy
Saw chain stops under load at full throttle (1)Clutch shoes badly wornInstall new clutch
Saw chain stops under load at full throttle (2)Clutch drum badly wornInstall new clutch drum
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 new clutch
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)Clutch shoe retainer brokenFit new retainer
Loud noises (4)Clutch shoes and carrier wornInstall new clutch

Stihl MS 261 CM Chain Drive, Chain Brake, Chain Tensioner

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

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

Stihl MS 261 CM 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 setting of oil pump if necessary
Chain receives no oil (2)Oil inlet hole in 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)Valve in oil tank blockedClean or replace 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)Oil pump delivery rate set too lowAdjust oil pump (only on version with adjustable oil pump)

Stihl MS 261 CM Rewind Starter

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

Stihl MS 261 CM 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)Spark plug boot is loosePress boot firmly onto spark plug and fit new spring if necessary
Engine runs roughly, misfires, temporary loss of power (2)Spark plug sooted, smeared with oilClean the spark plug or replace if
necessary.
If sooting keeps recurring, check air
filter
Engine runs roughly, misfires, temporary loss of power (3)Ignition lead loose in ignition moduleSecure ignition lead properly
Engine runs roughly, misfires, temporary loss of power (4)Fuel/oil mixture – too much oilUse correct mixture of fuel and oil
Engine runs roughly, misfires, temporary loss of power (5)Incorrect air gap between ignition module and flywheelSet air gap correctly
Engine runs roughly, misfires, temporary loss of power (6)Flywheel cracked or has other damage or 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 flywheelInstall new flywheel
Engine runs roughly, misfires, temporary loss of power (9)Irregular sparkCheck operation of switch shaft/contact springs and ignition module. Faulty insulation or break in ignition lead or short circuit wire. Check ignition lead/ignition module and replace ignition module if necessary. Check operation of spark plug. Clean the spark plug or replace if necessary.
Engine runs roughly, misfires, temporary loss of power (10)Crankcase damaged (cracks)Install new crankcase
No spark (1)Spark plug faultyInstall new spark plug
No spark (2)Faulty insulation or short in short circuit wireCheck short circuit wire for short circuit to ground
No spark (3)Break in ignition lead or insulation damagedCheck ignition lead and replace if necessary
No spark (4)Ignition module faultyInstall new ignition module

Stihl MS 261 CM 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)Inlet control lever sticking on spindleCheck inlet control lever, replace if necessary
Carburetor floods; engine stalls (3)Helical spring not located on nipple of inlet control leverRemove the inlet control lever and refit it correctly
Carburetor floods; engine stalls (4)Perforated disc on 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)Setting of low speed screw too leanCheck basic carburetor setting, correct if necessary
Poor acceleration (2)Setting of high speed screw too leanCheck basic carburetor setting, correct if necessary
Poor acceleration (3)Inlet needle sticking to valve seatRemove 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 new fuel hose
Engine will not idle, idle speed too high (3)Throttle shutter opened too wide by idle speed screw LAReset idle speed screw LA correctly
Engine will not idle, idle speed 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)Setting of low speed screw too rich or too leanReset low speed screw L correctly
Engine stalls at idle speed (3)Setting of idle speed screw LA incorrect – throttle shutter 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 new fuel hose
Engine speed drops quickly under load – low power (1)Air filter dirtyClean air filter or replace if necessary
Engine speed drops quickly under load – low power (2)Throttle shutter not opened fullyCheck 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 new fuel hose
Engine speed drops quickly under load – low power (7)Setting of high speed screw H 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 flywheel is missing or has sheared offFit key if necessary and secure flywheel properly or install new flywheel

Stihl MS 261 CM Engine

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

  • Air filter
  • Fuel system
  • Carburetor
  • Ignition system
ConditionCauseRemedy
Engine does not start easily, stalls at idle speed, but operates normally at full throttle (1)Oil seals in crankcase damagedReplace the oil seals
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
Engine does not deliver full power or runs erratically (1)Piston rings worn or brokenFit new piston rings
Engine does not deliver full power or runs erratically (2)Muffler / spark arresting screen carbonizedClean the muffler (inlet and exhaust), replace spark arresting screen, replace muffler if necessary
Engine does not deliver full power or runs erratically (3)Air filter dirtyReplace air filter
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
Engine does not deliver full power or runs erratically (5)Decompression valve is not closedClose, check and replace 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, trouble shooting 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 carburettor
  • Pressure testing the decompression valve
  • Pressure testing the cylinder

Stihl MS 261 CM Chainsaw workshop Manual and parts list available

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