Stihl MS 660 & 066 Problems (Common Fixes)
The 066 and MS 660 are large, roughly 91.6 cc saws, but age, serial-number differences, and setup affect the right fix. Start with safety, fresh 50:1 fuel, chain and oil checks, then diagnose symptoms before adjusting the carburetor. A saw that races, surges, overheats, or loses power may have an air leak and should stop work.
Cold weather, dry summer air, high elevation, and hard or frozen wood can all change how a saw behaves in the field. But a new symptom is not proof that the carburetor needs adjustment. A blocked filter, worn part, poor fuel, or air leak can produce similar signs, and guessing at settings can make damage worse.
I use a symptom-first approach: check the simple items, identify what changes the behavior, and stop if there is a risk of engine damage or loss of control. The 066 name covers saws and versions whose parts and settings can differ. Check the serial number and the correct STIHL manual before ordering parts or using a torque or tuning value.
Equipment Setup and Safety Preparation
This stage helps separate setup faults from engine faults. Before starting, inspect the saw and work area, then confirm the fuel, chain, bar, and oil match the specific machine. A sound setup will not fix a damaged engine, but it can prevent avoidable starting, cutting, and lubrication problems.
Identify the saw and inspect the setup
The MS 660 has a listed displacement of 91.6 cc. An 066 is also a large-displacement saw, but do not assume every 066 shares the same parts or settings. Use its serial number to check the manual, parts diagrams, and service information.
Many setups use 3/8-inch standard-pitch chain, not 3/8-inch low-profile chain. A .063-inch gauge is common, while .050-inch setups also exist. Pitch must match the sprocket and bar; gauge must match the bar groove. Bar length alone does not establish compatibility. Confirm the bar mount, oiling arrangement, sprocket, and approved combination for your exact saw.
| Check | What to look for | What to do |
|---|---|---|
| Chain and bar | Matching pitch and gauge; no cracked links, bent drive links, or damaged rails | Fit a compatible set; replace damaged parts |
| Chain tension | Chain sits in the bar groove and can move by hand with gloves when the brake is released | Adjust by the manual; avoid running a loose chain |
| Sprocket | Deep wear grooves or damage where the chain runs | Replace if worn, using the correct part |
| Controls and handles | Brake, throttle lockout, stop switch, catcher, handles, and anti-vibration mounts intact | Do not use the saw if a safety part fails |
| Leaks and oiling | Fuel or oil leaks; blocked oil holes; no visible oil delivery | Repair leaks or diagnose oiling before cutting |
Prepare the work area and start safely
Wear a helmet with eye and face protection, hearing protection, chainsaw chaps, gloves, and protective boots. Keep both hands on the saw, establish stable footing, and plan escape routes and the drop zone before cutting. Logging work is subject to OSHA 1910.266; arboricultural work should follow applicable ANSI Z133 practices. Do not attempt felling or aerial work without suitable training.
Apply the chain brake for starting and follow the cold- or warm-start instructions in the manual. Start on firm ground, not by dropping or “bumping” the saw. After it starts, check for leaks and verify that the chain receives oil before cutting. Never run the saw without bar-and-chain oil. If the brake, stop switch, throttle lockout, or chain catcher is faulty, take the saw out of service.
Common 066 and MS 660 Problems and Fixes
A symptom narrows the search but does not prove which part has failed. Begin with checks that do not require changing factory settings, then move to testing or repair. If the saw shows signs of a lean-running engine, stop rather than trying to tune around the fault.
Saw will not start or stay running
Check the stop switch position, starting procedure, fuel freshness, and air filter. STIHL specifies a 50:1 mix when using its two-stroke oil: 20 mL of oil per 1 L of gasoline. If using another approved oil, follow that oil maker’s directions. Do not use straight gasoline or guess at the mix.
If fuel and procedure are correct, inspect the spark plug and fuel filter, and look for cracked or loose fuel lines. A wet plug may point to flooding; follow the manual’s clearing procedure rather than repeatedly pulling the starter. A dry plug after attempts may indicate fuel is not reaching the engine. Carburetor diaphragms can harden with age, but confirm the fault before replacing parts.
Engine races, surges, or hangs at idle
A hanging idle means engine speed stays high after releasing the throttle. Surging or an unstable idle can also signal an air leak or fuel-delivery fault. Cracked intake or fuel lines, hardened carburetor parts, blocked filters, and leaking crankshaft seals are possible causes.
Stop using the saw. An air leak can make the engine run lean, overheat, and score the piston or cylinder, potentially causing seizure. Do not casually turn the carburetor screws to mask the symptom. Have the fuel system and crankcase checked with pressure and vacuum tests, then repair the source before restarting.
Loss of power, overheating, or seizure
A dirty air filter, dull chain, blocked exhaust, poor fuel, or incorrect setup can reduce cutting performance. A long bar, hardwood, frozen wood, and dirty wood also increase cutting load. Start with a clean filter, sharp chain, correct tension, and a clear bar groove and oil holes. Do not force the saw through a cut.
If power loss comes with abnormal heat, surging, or a sudden stop, do not keep testing it under load. Those signs can indicate engine damage or a lean condition. A technician may need to inspect the piston and cylinder and test the intake, fuel system, and crankcase. Replacing a plug or adjusting the carburetor will not repair scoring or an air leak.
Chain does not oil or cuts poorly
Check the oil level, bar oil holes, bar groove, and oil passage area for packed debris. Confirm the bar and chain are suitable for the saw and that oil delivery is present. If the chain remains dry, stop cutting and diagnose the oiling system. Lack of lubrication can damage the bar and chain quickly.
If the saw runs but cuts slowly, inspect sharpness and depth gauges, also called rakers. For common 3/8-inch chain, STIHL’s typical depth-gauge setting is 0.65 mm (0.025 inch), but verify the chain maker’s specification and use the right gauge. Do not lower rakers by eye or remove too much material; an overly aggressive chain can increase kickback and vibration.
Field Cutting Operation
Correct operation helps distinguish a saw fault from excessive cutting load. A sharp, compatible chain and sound technique matter more than forcing a long-bar setup through difficult wood. Stop if the saw behaves abnormally, oil delivery fails, or the work area becomes unsafe.
Match the cut to the setup
Bars of about 20 to 36 inches are commonly fitted to these saws, but that range does not mean every length suits every 066 or MS 660. Confirm the exact model’s approved bar, chain, sprocket, mount, and oiling combination. Long bars and dense, dirty, or frozen wood place more load on the saw.
Keep the chain sharp and let it cut without forcing the handles. Avoid the bar nose and its upper quadrant, where kickback can occur, and do not cut above shoulder height. Maintain stable footing and keep others outside the drop zone. Felling requires assessment of lean, wind, defects, and escape routes; use a planned notch and adequate hinge, not improvised dimensions. If the tree or site is beyond your training, do not fell it.
Maintenance, Sharpening, and Carburetor Diagnosis
Routine cleaning and inspection can expose common problems before they damage the saw. Carburetor settings require more care: use the exact service information and a tachometer, not sound alone. If symptoms suggest an air leak or fuel fault, correct that first rather than tuning around it.
Clean and inspect after use
Shut down, engage the brake, let the saw cool, and then clean the air filter, covers, cooling fins, bar groove, oil holes, and sprocket area. Inspect the chain, tension, bar rails, and visible lines for wear or damage. Replace or repair parts that affect safe control, chain retention, or lubrication.
Keep the chain sharp and check depth gauges with the proper tool. A dull chain can make a healthy saw seem weak and increase heat and effort. Avoid running a damaged chain or forcing a cut. If the saw idles unpredictably, races, fails to oil, or loses power, take it out of service until the cause is known.
Tune only to the correct specification
Common MS 660 reference values are about 2,500 rpm at idle and 13,500 rpm maximum. These are not universal settings for every 066 version, modified saw, or condition. Check the model- and serial-specific manual or service data before adjusting anything.
Use a tachometer to check maximum speed; do not tune by ear or exceed the specified limit. Carburetor adjustment cannot fix a blocked filter, fuel leak, damaged seal, or scored cylinder. When the saw’s history is unknown, or its speed is unstable, have a qualified saw technician inspect it and test the crankcase and fuel system.
Illustrative repair scenarios
These examples show a diagnostic sequence, not a guarantee that the same symptom always has the same cause. Several faults can look alike, so verify each finding before replacing parts.
- Starts, then races after a brief cut: Stop the saw. Check for loose or cracked intake and fuel lines, then have the crankcase pressure- and vacuum-tested. Do not lower idle or alter mixture screws as a substitute for finding a leak.
- Runs but cuts slowly in hardwood: First check chain sharpness, tension, depth gauges, bar condition, and oil delivery. If those are correct, assess whether the bar and chain combination is suitable for the exact saw and workload.
- Chain stays dry despite oil in the tank: Stop cutting. Clean the bar groove and oil holes, then check delivery according to the manual. Continued use without oil can damage the cutting equipment.
Storage and Seasonal Care
Storage problems often begin with stale fuel, debris, or an overlooked leak. Before putting the saw away, clean and inspect it, and follow the manual’s fuel-storage instructions. When returning to work, recheck controls, chain condition, tension, and oil delivery rather than assuming the saw is ready.
Use fresh fuel mixed at the correct ratio, and do not leave an unapproved mix in the saw. Clean the air filter and cooling areas, inspect the bar and chain, and check for fuel or oil leaks. Before the next cut, verify the brake and stop switch, start the saw as directed, and confirm oil delivery. If it races, surges, overheats, or loses power, stop and diagnose it before use.
Frequently Asked Questions
What is the correct fuel mix for an MS 660?
STIHL specifies 50:1 when using STIHL two-stroke oil: 20 mL of oil per 1 L of gasoline. Follow the oil maker’s directions for another approved oil.
Can I use 3/8-inch low-profile chain?
Do not assume it fits. Many setups use standard 3/8-inch pitch. Match chain pitch to the sprocket and bar, and chain gauge to the bar groove.
Why does my saw idle high after cutting?
A hanging idle can indicate an air leak or fuel-delivery fault. Stop using the saw and have it checked; do not try to hide the symptom with casual carburetor adjustment.
Why does my 066 or MS 660 lack cutting power?
Check the chain, air filter, bar, oil delivery, and fuel first. If power loss comes with excess heat, surging, or sudden stopping, stop and seek engine diagnosis.
What maximum RPM should an MS 660 reach?
A common reference is about 13,500 rpm, but verify the exact manual or service data. Check with a tachometer and do not exceed the specified limit.
What depth-gauge setting should I use?
A typical setting for common 3/8-inch chain is 0.65 mm (0.025 inch). Confirm the chain manufacturer’s specification and use its gauge.
Can I fit a 36-inch bar?
Some setups use bars up to that length, but suitability depends on the exact saw, bar mount, chain, sprocket, and oiling system. Check serial-specific STIHL information before fitting one.
What should I do if the chain will not oil?
Stop cutting. Check the oil level and clean the bar groove and oil holes. If delivery still cannot be confirmed, have the oiling system diagnosed before using the saw.
(This article was written by one of our staff writers, Marcus Thorne. Visit our Meet the Team page.)
