Instruction Manual
Page 2
... Buffers 68 Buffers on Machines with Manual Fuel Pump 85 12.5 Carburetor 88 12.5.1 Leakage Test 89 RA_737_00_01_01 MS 231, MS 231 C, MS 251, MS 251 C q © ANDREAS STIHL AG & Co. Fuel System 81 12.1 Air Filter 81 12.2 Baffle 81 12.3 Filter Base 81 ... Ignition System Troubleshooting 55 8. Chain Lubrication 78 11.1 Pickup Body 78 11.2 Oil Suction Hose 78 11.3 Oil Pump 78 11.4 Valve 80 12. Engine 32 6.1 6.2 6.2.1 6.2.2 6.2.3 6.3 6.3.1 6.3.2 6.4 6.5 6.6 6.7 6.8 6.9 Muffler 32 Leakage Test 33 Preparations 33 Vacuum Test 34 Pressure Test 34 ...
... Buffers 68 Buffers on Machines with Manual Fuel Pump 85 12.5 Carburetor 88 12.5.1 Leakage Test 89 RA_737_00_01_01 MS 231, MS 231 C, MS 251, MS 251 C q © ANDREAS STIHL AG & Co. Fuel System 81 12.1 Air Filter 81 12.2 Baffle 81 12.3 Filter Base 81 ... Ignition System Troubleshooting 55 8. Chain Lubrication 78 11.1 Pickup Body 78 11.2 Oil Suction Hose 78 11.3 Oil Pump 78 11.4 Valve 80 12. Engine 32 6.1 6.2 6.2.1 6.2.2 6.2.3 6.3 6.3.1 6.3.2 6.4 6.5 6.6 6.7 6.8 6.9 Muffler 32 Leakage Test 33 Preparations 33 Vacuum Test 34 Pressure Test 34 ...
Instruction Manual
Page 4
...STIHL Special Tools" manual. To do this, secure the mounting plate (2) 5910 850 1650 to identify the tools in this service manual. They show the installed positions of any replacement parts. Use the part numbers to the assembly stand with the M 8 nut (arrow). 2310RA000 TG The machine is issued. MS 231, MS 231 C, MS 251, MS 251.... To help locate the fault, consult the chapter on "Troubleshooting" and the "STIHL Service Training System" for engineering changes which have several causes. You should make use of all special servicing tools currently available from...
...STIHL Special Tools" manual. To do this, secure the mounting plate (2) 5910 850 1650 to identify the tools in this service manual. They show the installed positions of any replacement parts. Use the part numbers to the assembly stand with the M 8 nut (arrow). 2310RA000 TG The machine is issued. MS 231, MS 231 C, MS 251, MS 251.... To help locate the fault, consult the chapter on "Troubleshooting" and the "STIHL Service Training System" for engineering changes which have several causes. You should make use of all special servicing tools currently available from...
Instruction Manual
Page 5
...MS 231 C, MS 251, MS 251 C Always replace damaged parts. Check disassembled parts for wear or damage before carrying out repairs or mounting the machine to the fuel hoses. The specifications must be maintained when tightening down screws, nuts and other fasteners in certain conditions. Avoid damaging the hose barb - do not use original STIHL... replacement parts. Always use sharp-edged pliers, screwdrivers, etc. Always perform leakage test after removal. Do not re-use fuel hoses after working on the fuel system and the engine. Install new fuel ...
...MS 231 C, MS 251, MS 251 C Always replace damaged parts. Check disassembled parts for wear or damage before carrying out repairs or mounting the machine to the fuel hoses. The specifications must be maintained when tightening down screws, nuts and other fasteners in certain conditions. Avoid damaging the hose barb - do not use original STIHL... replacement parts. Always use sharp-edged pliers, screwdrivers, etc. Always perform leakage test after removal. Do not re-use fuel hoses after working on the fuel system and the engine. Install new fuel ...
Instruction Manual
Page 6
... engine speed with bar and chain: Idle speed: Clutch: Clutch engages at: Crankcase leakage test at gauge pressure: under vacuum: 2.2 Fuel System 2.3 Ignition System 2.4 Chain Lubrication MS 231 40.6 cm3 41.5 mm 30.0 mm 2.0 kW (2.7 bhp) at 9,500 rpm 13,000 rpm 2,800 rpm Centrifugal clutch without linings 3,500 rpm 0.5 bar 0.5 bar MS 251... (resistor type): Electrode gap: 0.30 (+ 0.05/- 0.10) mm NGK CMR 6 H 0.5 mm Speed-controlled Ematic oil pump Oil delivery rate: 8.0 (+/3.0) cm3/min at 10,000 rpm MS 231, MS 231 C, MS 251, MS 251 C 5 2.
... engine speed with bar and chain: Idle speed: Clutch: Clutch engages at: Crankcase leakage test at gauge pressure: under vacuum: 2.2 Fuel System 2.3 Ignition System 2.4 Chain Lubrication MS 231 40.6 cm3 41.5 mm 30.0 mm 2.0 kW (2.7 bhp) at 9,500 rpm 13,000 rpm 2,800 rpm Centrifugal clutch without linings 3,500 rpm 0.5 bar 0.5 bar MS 251... (resistor type): Electrode gap: 0.30 (+ 0.05/- 0.10) mm NGK CMR 6 H 0.5 mm Speed-controlled Ematic oil pump Oil delivery rate: 8.0 (+/3.0) cm3/min at 10,000 rpm MS 231, MS 231 C, MS 251, MS 251 C 5 2.
Instruction Manual
Page 7
... Hand guard / fan housing / engine housing 6.0 M 5x14 Shroud / engine housing 5.0 P 5x16 Spiked bumper / engine housing 4.0 D 4x12 Manifold / cylinder 4.0 D 5x18 Bearing plug / cylinder 8.0 P 5x18 Fan housing / engine housing 4.0 P 4x12 Air baffle / engine housing 2.0 M 12x1 L Carrier / crankshaft 50.0 P 6x28 Engine housing / bearing plug 6.0 D 4x16 Oil pump 4.0 M 5x16 Muffler / cylinder 10.0 M 8x1 Flywheel / crankshaft 28.0 P 4x10 Retainer 1.0 Q B 2), 3) 2) 1), 3) 1), 3) 1), 3) 2), 3) 4) 6 MS 231, MS 231 C, MS 251, MS 251 C
... Hand guard / fan housing / engine housing 6.0 M 5x14 Shroud / engine housing 5.0 P 5x16 Spiked bumper / engine housing 4.0 D 4x12 Manifold / cylinder 4.0 D 5x18 Bearing plug / cylinder 8.0 P 5x18 Fan housing / engine housing 4.0 P 4x12 Air baffle / engine housing 2.0 M 12x1 L Carrier / crankshaft 50.0 P 6x28 Engine housing / bearing plug 6.0 D 4x16 Oil pump 4.0 M 5x16 Muffler / cylinder 10.0 M 8x1 Flywheel / crankshaft 28.0 P 4x10 Retainer 1.0 Q B 2), 3) 2) 1), 3) 1), 3) 1), 3) 2), 3) 4) 6 MS 231, MS 231 C, MS 251, MS 251 C
Instruction Manual
Page 8
... down slightly. Do not mix up screws with medium strength Loctite 242 or 243 before reinstalling. MS 231, MS 231 C, MS 251, MS 251 C 7 Fastener Thread size For component Torque Nm Remarks Screw Screw Screw Screw P 4x12 P 4x10 M 10x1 D 4x20 D 5.3x41 Pre-separator / engine housing Elbow connector / air guide shroud Spark plug / cylinder Ignition module/cylinder Cylinder...
... down slightly. Do not mix up screws with medium strength Loctite 242 or 243 before reinstalling. MS 231, MS 231 C, MS 251, MS 251 C 7 Fastener Thread size For component Torque Nm Remarks Screw Screw Screw Screw P 4x12 P 4x10 M 10x1 D 4x20 D 5.3x41 Pre-separator / engine housing Elbow connector / air guide shroud Spark plug / cylinder Ignition module/cylinder Cylinder...
Instruction Manual
Page 9
... full load Cause Clutch shoes badly worn Clutch drum badly worn Remedy Install new clutch Install new clutch drum Saw chain rotates at idle speed Engine idle speed too high Clutch springs stretched Clutch springs broken Readjust idle speed screw LA Replace the clutch springs or install new clutch Replace the... Clutch shoe retainer broken Clutch shoes and carrier worn Replace all clutch springs Fit new needle cage Install new retainer or clutch Install new clutch 8 MS 231, MS 231 C, MS 251, MS 251 C
... full load Cause Clutch shoes badly worn Clutch drum badly worn Remedy Install new clutch Install new clutch drum Saw chain rotates at idle speed Engine idle speed too high Clutch springs stretched Clutch springs broken Readjust idle speed screw LA Replace the clutch springs or install new clutch Replace the... Clutch shoe retainer broken Clutch shoes and carrier worn Replace all clutch springs Fit new needle cage Install new retainer or clutch Install new clutch 8 MS 231, MS 231 C, MS 251, MS 251 C
Instruction Manual
Page 10
... Brake band blocked Install new clutch Install new clutch drum Check freedom of movement and operation of brake band Saw chain rotates at idle speed Engine idle speed too high Clutch springs stretched Clutch springs broken Readjust idle speed screw LA Replace the clutch springs or install new clutch Replace the... Brake spring stretched or broken Fit new brake spring Brake band stretched or worn Clutch drum worn Fit new brake band Install new clutch drum MS 231, MS 231 C, MS 251, MS 251 C 9
... Brake band blocked Install new clutch Install new clutch drum Check freedom of movement and operation of brake band Saw chain rotates at idle speed Engine idle speed too high Clutch springs stretched Clutch springs broken Readjust idle speed screw LA Replace the clutch springs or install new clutch Replace the... Brake spring stretched or broken Fit new brake spring Brake band stretched or worn Clutch drum worn Fit new brake band Install new clutch drum MS 231, MS 231 C, MS 251, MS 251 C 9
Instruction Manual
Page 12
... chain oil Oil pump damaged or worn Oil suction hose connection damaged Engine housing cracked Install new oil pump Install new oil intake hose Install new engine housing Oil pump delivers insufficient oil Oil pump damaged or worn Worm driver is loose Install new oil pump Install new worm MS 231, MS 231 C, MS 251, MS 251 C 11
... chain oil Oil pump damaged or worn Oil suction hose connection damaged Engine housing cracked Install new oil pump Install new oil intake hose Install new engine housing Oil pump delivers insufficient oil Oil pump damaged or worn Worm driver is loose Install new oil pump Install new worm MS 231, MS 231 C, MS 251, MS 251 C 11
Instruction Manual
Page 15
... fit new spring if necessary Spark plug sooted, smeared with oil Clean the spark plug or replace if necessary. 3.5 Ignition System Condition Engine runs roughly, misfires, temporary loss of the spark plug, clean or replace spark plug if necessary. 14 MS 231, MS 231 C, MS 251, MS 251 C Check ignition lead/module, replace ignition module if necessary.
... fit new spring if necessary Spark plug sooted, smeared with oil Clean the spark plug or replace if necessary. 3.5 Ignition System Condition Engine runs roughly, misfires, temporary loss of the spark plug, clean or replace spark plug if necessary. 14 MS 231, MS 231 C, MS 251, MS 251 C Check ignition lead/module, replace ignition module if necessary.
Instruction Manual
Page 16
engine stalls Cause Remedy Inlet needle not sealing - Helical spring not located on nipple Remove the inlet control lever ...screw too lean Check basic carburetor setting, correct if necessary Inlet needle sticking to carburetor Seal connections or install new fuel hose Engine loses power during acceleration Sealing ring or spring in valve seat or cone Remove and clean the inlet needle, clean the ... replace if necessary. 3.6 Carburetor Condition Carburetor floods; foreign matter in accelerator Install new carburetor pump worn or damaged MS 231, MS 231 C, MS 251, MS 251 C 15
engine stalls Cause Remedy Inlet needle not sealing - Helical spring not located on nipple Remove the inlet control lever ...screw too lean Check basic carburetor setting, correct if necessary Inlet needle sticking to carburetor Seal connections or install new fuel hose Engine loses power during acceleration Sealing ring or spring in valve seat or cone Remove and clean the inlet needle, clean the ... replace if necessary. 3.6 Carburetor Condition Carburetor floods; foreign matter in accelerator Install new carburetor pump worn or damaged MS 231, MS 231 C, MS 251, MS 251 C 15
Instruction Manual
Page 17
...Reset idle speed screw (LA) high idle speed screw (LA) correctly Oil seals/engine pan leaking Seal or replace oil seals/engine pan Throttle shutter does not close Install new carburetor Engine stops while idling Idle jet bores or ports blocked Low speed screw too rich ...to carburetor Seal connections or install new fuel hose Engine idle speed too high Readjust with idle speed screw LA (counterclockwise) Clutch springs stretched or fatigued Replace the clutch springs or install new clutch Clutch spring hooks broken Replace the clutch springs 16 MS 231, MS 231 C, MS 251, MS 251 C
...Reset idle speed screw (LA) high idle speed screw (LA) correctly Oil seals/engine pan leaking Seal or replace oil seals/engine pan Throttle shutter does not close Install new carburetor Engine stops while idling Idle jet bores or ports blocked Low speed screw too rich ...to carburetor Seal connections or install new fuel hose Engine idle speed too high Readjust with idle speed screw LA (counterclockwise) Clutch springs stretched or fatigued Replace the clutch springs or install new clutch Clutch spring hooks broken Replace the clutch springs 16 MS 231, MS 231 C, MS 251, MS 251 C
Instruction Manual
Page 18
...Clean the carburetor Pump diaphragm damaged or fatigued Fit new pump diaphragm Ignition timing wrong, flywheel out of adjustment - Condition Cause Engine speed drops quickly under Air filter dirty load - low power Remedy Clean air filter or replace if necessary Throttle shutter not ...Fuel strainer dirty Clean fuel strainer in flywheel has sheared off Install new flywheel Engine running extremely rich, has Choke shutter does not open no power and a very low maximum speed Check carburetor and choke shaft, service or replace if necessary MS 231, MS 231 C, MS 251, MS 251 C 17
...Clean the carburetor Pump diaphragm damaged or fatigued Fit new pump diaphragm Ignition timing wrong, flywheel out of adjustment - Condition Cause Engine speed drops quickly under Air filter dirty load - low power Remedy Clean air filter or replace if necessary Throttle shutter not ...Fuel strainer dirty Clean fuel strainer in flywheel has sheared off Install new flywheel Engine running extremely rich, has Choke shutter does not open no power and a very low maximum speed Check carburetor and choke shaft, service or replace if necessary MS 231, MS 231 C, MS 251, MS 251 C 17
Instruction Manual
Page 19
... stalls Oil seals in fan housing dirty Clean air inlet on the engine: - Fuel system - Engine overheating Insufficient cylinder cooling. Carburetor - Engine does not deliver full power Piston rings worn or broken or runs ...Engine pan leaking or damaged (cracks) Intake manifold damaged / bore blocked Remedy Replace the oil seals Inspect engine pan, re-seal or replace if necessary. Air filter - Clean bore or install new manifold. 3.7 Engine Always check and, if necessary, repair the following parts before looking for faults on fan housing 18 MS 231, MS 231 C, MS 251, MS 251...
... stalls Oil seals in fan housing dirty Clean air inlet on the engine: - Fuel system - Engine overheating Insufficient cylinder cooling. Carburetor - Engine does not deliver full power Piston rings worn or broken or runs ...Engine pan leaking or damaged (cracks) Intake manifold damaged / bore blocked Remedy Replace the oil seals Inspect engine pan, re-seal or replace if necessary. Air filter - Clean bore or install new manifold. 3.7 Engine Always check and, if necessary, repair the following parts before looking for faults on fan housing 18 MS 231, MS 231 C, MS 251, MS 251...
Instruction Manual
Page 21
...the rear handle - Its efficiency is measured in order if deceleration of its remaining thickness is less 2 than a second) is depressed. Starting the engine 2 1 - With the chain brake activated (locked), open the throttle wide and activate the brake manually - Remove the brake band without overstretching it..., etc.) and smoothing of the friction surfaces of the brake band and clutch drum impair the coefficient of the machine. 20 MS 231, MS 231 C, MS 251, MS 251 C The clutch drum must come areas on inside diameter and/or parts of the lockout lever on the chain saw chain coming...
...the rear handle - Its efficiency is measured in order if deceleration of its remaining thickness is less 2 than a second) is depressed. Starting the engine 2 1 - With the chain brake activated (locked), open the throttle wide and activate the brake manually - Remove the brake band without overstretching it..., etc.) and smoothing of the friction surfaces of the brake band and clutch drum impair the coefficient of the machine. 20 MS 231, MS 231 C, MS 251, MS 251 C The clutch drum must come areas on inside diameter and/or parts of the lockout lever on the chain saw chain coming...
Instruction Manual
Page 23
...necessary. 3 2 : Fit the strap (1) on the pivot pins (2) and (3) so that the brake spring attachment point (arrow) is worn, install a new engine housing. - 1 2 1 1 TOP 2310RA021 TG 2310RA019 TG 2310RA016 TG : Remove the retaining ring (1). 2 1 : Take the brake lever (2) out of the...as far as stop. : Hold the brake lever (2) so that "TOP" faces outwards and the curve (arrow) faces up the holes. 2310RA023 TG 22 MS 231, MS 231 C, MS 251, MS 251 C Remove the hand guard and brake lever. - Lubricate the pivot pins, b 14 1 2 1 : Slip the spacer washer (1) onto the pin (2)....
...necessary. 3 2 : Fit the strap (1) on the pivot pins (2) and (3) so that the brake spring attachment point (arrow) is worn, install a new engine housing. - 1 2 1 1 TOP 2310RA021 TG 2310RA019 TG 2310RA016 TG : Remove the retaining ring (1). 2 1 : Take the brake lever (2) out of the...as far as stop. : Hold the brake lever (2) so that "TOP" faces outwards and the curve (arrow) faces up the holes. 2310RA023 TG 22 MS 231, MS 231 C, MS 251, MS 251 C Remove the hand guard and brake lever. - Lubricate the pivot pins, b 14 1 2 1 : Slip the spacer washer (1) onto the pin (2)....
Instruction Manual
Page 24
... guard (1) with brake lever (2) over the pivot pins (arrows). 1 The turns of the hand guard (arrow) slips passed it. If this is worn, install a new engine housing, b 6.6 2310RA028 TG : Use the assembly tool (2) 1117 890 0900 to attach the brake spring (1) to the brake lever (arrow). 1 2 2310RA030 TG 1 TOP 2310RA025 ...until they are positioned against the pivot pins (arrows). : Fit the retaining ring (1). 2 2310RA027 TG TOP 1 : Hook the brake spring (1) to the anchor pin (arrow). - MS 231, MS 231 C, MS 251, MS 251 C 23 Reassemble all other parts in hand guard, b 14 - -
... guard (1) with brake lever (2) over the pivot pins (arrows). 1 The turns of the hand guard (arrow) slips passed it. If this is worn, install a new engine housing, b 6.6 2310RA028 TG : Use the assembly tool (2) 1117 890 0900 to attach the brake spring (1) to the brake lever (arrow). 1 2 2310RA030 TG 1 TOP 2310RA025 ...until they are positioned against the pivot pins (arrows). : Fit the retaining ring (1). 2 2310RA027 TG TOP 1 : Hook the brake spring (1) to the anchor pin (arrow). - MS 231, MS 231 C, MS 251, MS 251 C 23 Reassemble all other parts in hand guard, b 14 - -
Instruction Manual
Page 25
... 1 : Take the brake lever (2) out of the flat spring (1) into its seat (arrow) as far as stop. 2310RA020 TG 24 MS 231, MS 231 C, MS 251, MS 251 C The brake spring is worn, install a new engine housing. - Check the anchor pin - 2310RA035 TG 5.4 Brake Lever on the hand guard and replace the hand 1 guard if necessary. Check...
... 1 : Take the brake lever (2) out of the flat spring (1) into its seat (arrow) as far as stop. 2310RA020 TG 24 MS 231, MS 231 C, MS 251, MS 251 C The brake spring is worn, install a new engine housing. - Check the anchor pin - 2310RA035 TG 5.4 Brake Lever on the hand guard and replace the hand 1 guard if necessary. Check...
Instruction Manual
Page 27
... b 6.6 1 If problems occur on the coasting brake even though the brake band is in order, the reason may be tightly against engine housing (arrows) and the clutch drum must rotate freely. - Checking condition and free travel is necessary to check free travel must locate ...hook (arrow) is visible. : Attach the spring (1) to the anchor pin (2). : Use the assembly tool 1117 890 0900 to the brake lever (arrow). 26 MS 231, MS 231 C, MS 251, MS 251 C The brake band (1) must be within the mark (a). 2 1 1 TOP 1 2310RA042 TG 2310RA031 TG 2310RA044 TG TOP : Attach the spring (1) to the ...
... b 6.6 1 If problems occur on the coasting brake even though the brake band is in order, the reason may be tightly against engine housing (arrows) and the clutch drum must rotate freely. - Checking condition and free travel is necessary to check free travel must locate ...hook (arrow) is visible. : Attach the spring (1) to the anchor pin (2). : Use the assembly tool 1117 890 0900 to the brake lever (arrow). 26 MS 231, MS 231 C, MS 251, MS 251 C The brake band (1) must be within the mark (a). 2 1 1 TOP 1 2310RA042 TG 2310RA031 TG 2310RA044 TG TOP : Attach the spring (1) to the ...
Instruction Manual
Page 28
...and lower the tank TOP housing, b 12.11.2 1 - Reassemble all other parts in that position. The brake band (1) must locate snugly against engine housing (arrows) and the clutch drum must rotate freely. 2 1 - Turn adjusting screw counter- Disconnect the brake spring, b 5.4 1 - ...set free travel . 1 1 - Remove the carburetor, b 12.5 - Check the brake cable (1) and retainer (2), replace if necessary MS 231, MS 231 C, MS 251, MS 251 C 27 clockwise to increase free travel on the adjusting screw (1). 2310RA051 TG 2310RA050 TG : Pull out the brake cable (1) a little...
...and lower the tank TOP housing, b 12.11.2 1 - Reassemble all other parts in that position. The brake band (1) must locate snugly against engine housing (arrows) and the clutch drum must rotate freely. 2 1 - Turn adjusting screw counter- Disconnect the brake spring, b 5.4 1 - ...set free travel . 1 1 - Remove the carburetor, b 12.5 - Check the brake cable (1) and retainer (2), replace if necessary MS 231, MS 231 C, MS 251, MS 251 C 27 clockwise to increase free travel on the adjusting screw (1). 2310RA051 TG 2310RA050 TG : Pull out the brake cable (1) a little...