The various faults that can affect a Stihl MS 780 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
Table of Contents
10 Basic Checks For The Stihl MS 780 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 780 Chainsaw
Stihl MS 780 Clutch
| Condition | Cause | Remedy |
|---|---|---|
| The Saw chain stops under load at full throttle (1) | Clutch shoes badly worn | Install new clutch |
| Saw chain stops under load at full throttle (2) | The clutch drum is badly worn | Install a new clutch drum |
| The Saw chain rotates at idle speed (1) | Engine idle speed too high | Readjust with idle speed screw LA (counterclockwise) |
| Saw chain rotates at idle speed (2) | Clutch springs stretched or fatigued | Replace the clutch springs or install a new clutch |
| The Saw chain rotates at idle speed (3) | Clutch spring hooks broken | Replace the clutch springs |
| Loud noises (1) | Clutch springs stretched or fatigued | Replace all clutch springs |
| Loud noises (2) | Needle cage damaged | Fit new needle cage |
| Loud noises (3) | The clutch shoe retainer broken | Fit new retainer |
| Loud noises (4) | Clutch shoes and carrier worn | Install new clutch |
Stihl MS 780 Chain Drive, Chain Brake, Chain Tensioner
| Condition | Cause | Remedy |
|---|---|---|
| Chain sprocket wears rapidly (1) | The chain was not properly tensioned | Tension chain as specified |
| Chain sprocket wears rapidly (2) | Wrong chain pitch | Fit chain of the correct pitch |
| Chain sprocket wears rapidly (3) | Insufficient chain lubrication | Check chain lubrication |
| Chain sprocket wears rapidly (4) | Chain sprocket worn | Fit new chain sprocket |
| The Saw chain stops under load at full throttle (1) | Clutch shoes badly worn | Install new clutch |
| Saw chain stops under load at full throttle (2) | The clutch drum is badly worn | Install a new clutch drum |
| The Saw chain stops under load at full throttle (3) | Brake band blocked | Check the freedom of movement and operation of the brake band |
| The Saw chain rotates at idle speed (1) | Engine idle speed too high | Readjust with idle speed screw LA (counterclockwise) |
| Saw chain rotates at idle speed (2) | Clutch springs stretched or fatigued | Replace the clutch springs or install a new clutch |
| The Saw chain rotates at idle speed (3) | Clutch spring hooks broken | Replace the clutch springs |
| The Saw chain does not stop activated (1) | The brake spring stretched or broken | Fit 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 worn | Fit new clutch drum |
Stihl MS 780 Chain Lubrication
In the event of trouble with the chain lubrication system, check and rectify other sources of faults before disassembling the oil pump.
| Condition | Cause | Remedy |
|---|---|---|
| Chain receives no oil (1) | Oil tank empty | Fill 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 blocked | Clean oil inlet hole |
| Chain receives no oil (3) | Intake hose or pickup body clogged or intake hose ruptured | Fit new intake hose and pickup body |
| Chain receives no oil (4) | The valve in the oil tank blocked | Clean or replace the valve |
| Chain receives no oil (5) | Teeth on worm worn | Install new worm |
| Chain receives no oil (6) | Oil pump damaged or worn | Install 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 worn | Install 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 worn | Install new oil pump |
| Oil pump delivers insufficient oil (5) | The oil pump delivery rate is set too low | Adjust the oil pump (only on the version with an adjustable oil pump) |
Stihl MS 780 Rewind Starter
| Condition | Cause | Remedy |
|---|---|---|
| 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 wear | Fit new starter rope |
| The starter rope does not rewind (3) | Very dirty or corroded | Clean or replace the rewind spring |
| The starter rope does not rewind (4) | Insufficient spring tension | Check the rewind spring and increase the tension |
| The starter rope does not rewind (5) | Rewind spring broken | Fit new rewind spring |
| The starter rope cannot be pulled out far enough | Spring over tensioned | Check 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 worn | Fit new pawl |
| The starter rope can be pulled out almost without resistance (the crankshaft does not turn) (2) | Spring clip-on pawl fatigued | Fit new spring clip |
| The starter rope is difficult to pull or rewinds very slowly (1) | The starter mechanism is very dirty | 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 open | Open, check, and replace the decompression valve if necessary |
Stihl MS 780 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.
| Condition | Cause | Remedy |
|---|---|---|
| Engine runs roughly, misfires, temporary loss of power (1) | The spark plug boot is loose | Press the boot firmly onto the spark plug and fit a new spring if necessary |
| Engine runs roughly, misfires, temporary loss of power (2) | Spark plug sooted, smeared with oil | Clean 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 module | Secure ignition lead properly |
| Engine runs roughly, misfires, temporary loss of power (4) | Fuel/oil mixture – too much oil | Use the correct mixture of fuel and oil |
| Engine runs roughly, misfires, temporary loss of power (5) | An incorrect air gap between the ignition module and the flywheel | Set 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 blue | Install new flywheel |
| Engine runs roughly, misfires, temporary loss of power (7) | Ignition timing wrong, flywheel out of adjustment, key in flywheel has sheared off | Fit key if necessary and secure flywheel properly or install new flywheel |
| Engine runs roughly, misfires, temporary loss of power (8) | Weak magnetization in the flywheel | Install new flywheel |
| Engine runs roughly, misfires, temporary loss of power (9) | Irregular spark | Check 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 faulty | Install a new spark plug |
| No spark (2) | Faulty insulation or short in short circuit wire | Check short circuit wire for a short circuit to ground |
| No spark (3) | Break in ignition lead or insulation damaged | Check ignition lead and replace it if necessary |
| No spark (4) | Ignition module faulty | Install a new ignition module |
Stihl MS 780 Carburetor
| Condition | Cause | Remedy |
|---|---|---|
| Carburetor floods; engine stalls (1) | Inlet needle not sealing – foreign matter in valve seat or cone | Remove and clean the inlet needle, clean the carburetor |
| Carburetor floods; engine stalls (2) | The inlet control lever sticking on the spindle | Check 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 lever | Remove 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 lever | Fit a new metering diaphragm |
| Carburetor floods; engine stalls (5) | Metered diaphragm deformed | Fit a new metering diaphragm |
| Poor acceleration (1) | The setting of the low-speed screw too lean | Check basic carburetor setting, correct if necessary |
| Poor acceleration (2) | The setting of the high-speed screw is too lean | Check basic carburetor setting, correct if necessary |
| Poor acceleration (3) | The inlet needle sticking to the valve seat | Remove the inlet needle, clean, and refit |
| Poor acceleration (4) | Diaphragm gasket leaking | Fit new diaphragm gasket |
| Poor acceleration (5) | Metering diaphragm damaged or shrunk | Fit a new metering diaphragm |
| Poor acceleration (6) | Impulse hose damaged or kinked | Install new impulse hose |
| Poor acceleration (7) | Tank vent faulty | Replace tank vent |
| Poor acceleration (8) | Leak on fuel hose from tank to carburetor | Seal 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 LA | Reset idle speed screw LA correctly |
| The engine will not idle, idle speed is too high (3) | Oil seals/crankcase leaking | Seal or replace oil seals/crankcase |
| Engine stalls at idle speed (1) | Idle jet bores or ports blocked | Clean the carburetor |
| Engine stalls at idle speed (2) | The setting of the low-speed screw is too rich or too lean | Reset low-speed screw L correctly |
| Engine stalls at idle speed (3) | The setting of idle speed screw LA is incorrect – the throttle shutter is completely closed | Reset idle speed screw LA correctly |
| Engine stalls at idle speed (4) | Tank vent faulty | Replace tank vent |
| Engine stalls at idle speed (5) | Leak on fuel hose from tank to carburetor | Seal connections or install a new fuel hose |
| Engine speed drops quickly under load – low power (1) | Air filter dirty | Clean the air filter or replace it if necessary |
| Engine speed drops quickly under load – low power (2) | The throttle shutter is not opened fully | Check the throttle cable and rod |
| Engine speed drops quickly under load – low power (3) | Tank vent faulty | Replace tank vent |
| Engine speed drops quickly under load – low power (4) | Fuel pickup body dirty | Install new pickup body |
| Engine speed drops quickly under load – low power (5) | Fuel strainer dirty | Clean fuel strainer in carburetor, replace if necessary |
| Engine speed drops quickly under load – low power (6) | Leak on fuel hose from tank to carburetor | Seal 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 rich | Check basic carburetor setting, correct if necessary |
| Engine speed drops quickly under load – low power (8) | Main jet bores or ports blocked | Clean the carburetor |
| Engine speed drops quickly under load – low power (9) | Pump diaphragm damaged or fatigued | Fit new pump diaphragm |
| Engine speed drops quickly under load – low power (10) | Impulse hose damaged or kinked | Install 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 off | Fit the key if necessary and secure the flywheel properly or install a new flywheel |
Stihl MS 780 Engine
Always check and, if necessary, repair the following parts before looking for faults in the engine:
- Air filter
- Fuel system
- Carburetor
- Ignition system
| Condition | Cause | Remedy |
|---|---|---|
| The engine does not start easily, stalls at idle speed, but operates normally at full throttle (1) | Oil seals in crankcase damaged | Replace 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 broken | Fit new piston rings |
| The engine does not deliver full power or runs erratically (2) | Muffler / spark arresting screen carbonized | Clean 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 dirty | Replace air filter |
| The engine does not deliver full power or runs erratically (4) | Fuel/impulse hose severely kinked or damaged | Fit new hoses or position them free from kinks |
| The engine does not deliver full power or runs erratically (5) | The decompression valve is not closed | Close, check, and replace decompression valve if necessary |
| Engine overheating | Insufficient cylinder cooling. Air inlets in fan housing blocked or cooling fins on cylinder very dirty | Thoroughly 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 780 Chainsaw workshop Manual and parts list available