Stihl MS 170 Engine Problems (Troubleshooting)
A Stihl MS 170 that will not start usually has a problem with fuel, spark, or air flow, but compression and a safety-related fault can also stop it. Start with fresh, correctly mixed fuel and the switch and choke, then inspect the plug and filter. Test one cause at a time; stop if the saw races or runs lean.
Thirty pulls with no start can turn a quick job into a parts-buying guess. I use a simple rule: check the easy, visible causes first, then test spark, fuel delivery, and compression in order. That keeps diagnosis focused and helps avoid replacing a carburetor or ignition part without evidence.
The MS 170 is a 30.1 cm³, single-cylinder, two-stroke saw. Details can vary by production version and market, so use the operator’s manual and the saw’s nameplate for model-specific procedures and settings.
MS 170 Setup, Fuel Checks & Safety Preparation
Before troubleshooting, make the saw safe and confirm its basic setup. Correct fuel, a working chain brake, and a suitable bar and chain matter both for reliable operation and for injury prevention. Work outdoors, keep bystanders clear, and follow the manual for your exact saw.
Wear eye and hearing protection, chainsaw chaps, gloves, and suitable boots. Set the saw on stable ground, engage the chain brake, and keep the bar clear when starting. With the engine stopped, inspect the controls, chain tension, bar, air filter, visible fuel lines, and saw for leaks or damage. Do not run a saw with a damaged safety part or a fuel leak.
Confirm fuel and starting procedure
Use fresh gasoline mixed with suitable air-cooled two-stroke oil at 50:1 by volume. That equals 20 mL of oil for 1 L of gasoline. Do not use straight gasoline or guess at the mix. If fuel is old, incorrectly mixed, or of unknown age, drain it safely according to the manual and use a fresh batch.
Check that the master control is in the correct position and follow the manual’s cold-start or warm-start steps. Too much choking or repeated priming can flood the engine. If you suspect flooding, use the manual’s clearing procedure rather than continuing to pull with the choke on.
| Check | What to look for | Next step |
|---|---|---|
| Fuel | Fresh 50:1 mix; no water or debris | Replace doubtful fuel |
| Switch and choke | Correct position for cold or warm start | Reset using the manual |
| Air filter | Dirt, oil, or blockage | Clean as the manual allows, or replace if damaged |
| Plug | Wet, fouled, cracked, or badly worn | Inspect and clean or replace to the manual’s specification |
| Chain and brake | Correct tension; brake works | Correct or repair before operation |
A common bar length range is 12–16 inches, but not every bar fits every MS 170. Match the bar and chain using the bar’s stamped pitch and gauge, plus the chain’s drive-link count. Verify those details for your saw variant; do not order a chain by bar length alone.
Field Cutting Operation and Symptom-Based Troubleshooting
A symptom points toward a system, not a guaranteed failed part. A saw that will not fire needs a different first check from one that starts, then stalls under load. Record what it does, change one thing at a time, and stop running it if it shows signs of a lean condition.
If the engine will not start
Start with fuel, controls, and the starting steps. If those check out, inspect the spark plug. Remove it only with the engine off and cool, and follow the manual. A wet plug may indicate flooding; a dry plug after repeated attempts can point to fuel not reaching the cylinder, though neither clue proves the cause by itself.
Check spark with an appropriate spark tester, following the tester and saw instructions. Keep fuel away from the test area and do not test beside spilled gasoline or an open fuel container. No spark can come from the plug, ignition lead, switch circuit, or ignition module. Test those parts before replacing them.
If spark is present, fuel is fresh, and the air filter is clear, compression becomes a concern. A compression gauge can help, but the correct test method and acceptable reading depend on the service information for the exact saw. If the reading is poor or uncertain, have the cylinder, piston, and related parts checked rather than guessing from a single number.
If it starts, then stalls or bogs
First check the air filter and spark-arrestor screen for blockage, using the manual’s cleaning instructions. A clogged filter or screen can limit airflow or exhaust flow. Also inspect the fuel filter and visible fuel lines for damage, and consider whether the tank vent may be restricted. Replace damaged parts; do not keep cutting to see if the problem clears.
If the saw runs only when the choke is partly on, treat that as a warning of possible fuel starvation or an air leak. The cause could be a restricted fuel path, a damaged line, or a leak elsewhere. Do not try to hide the symptom with carburetor adjustment. An air leak or restricted fuel supply can make the engine run lean and overheat.
If it idles poorly or loses power
The specified idle speed is 2,800 rpm; confirm the figure and adjustment method in the manual for your saw. At a stable, warm idle, the chain must remain stationary. If it continues moving, stop the saw and have the idle setting, clutch, and related parts checked before using it.
A saw that races at idle, surges, or loses power abruptly may be running lean. Stop immediately. Continued use can overheat and seize the engine, and a moving chain at idle creates a serious hazard. Do not compensate by forcing the saw through a cut or by turning carburetor screws without the proper service procedure and tools.
These symptom patterns are diagnostic clues, not proof of a failed part:
| Symptom | Checks to make first | If the symptom remains |
|---|---|---|
| No start | Fuel mix, switch, choke procedure, plug, air filter | Test spark and fuel delivery; then check compression |
| Starts, then dies | Air filter, spark arrestor, fuel filter and lines, tank vent | Stop if it races, surges, or runs only with choke partly on |
| Bogs during cutting | Filter and screen, fuel supply, chain sharpness | Check for fuel-system or engine faults |
| Chain moves at idle | Stop engine; check idle and clutch through proper service steps | Do not operate until the chain stays still |
Diagnostic scenarios from the workbench
These are examples of how I sort symptoms, not claims about a specific repair. If a saw starts after fresh fuel and the correct start procedure, that supports checking fuel quality and operation before buying ignition parts. It does not rule out an intermittent fault.
If it starts only with partial choke, I move toward fuel delivery and air-leak checks, not a blind carburetor adjustment. If spark and fuel checks do not explain a no-start, compression testing is the next useful step. Each result narrows the search; none alone confirms a repair.
Maintenance, Chain Sharpening & Carburetor Checks
Regular cleaning and a sharp chain reduce avoidable strain, but they do not fix a fuel or ignition fault. Inspect the saw after it cools, and use the manual for cleaning and service limits. Carburetor work requires the correct procedure; changing settings by guess can create a lean-running engine.
A dull chain can make cutting slow and may tempt you to push harder. Stop, inspect, and sharpen or replace it as needed. Check the bar groove, oil passages, chain tension, and sprocket cover for packed debris. Keep the air filter and spark-arrestor screen clean as directed by the manual.
The spark-plug electrode gap is 0.5 mm. Check it with a suitable gauge and adjust or replace the plug only as the manual permits. A damaged plug, cracked insulator, or persistent fouling needs attention, but a new plug will not solve a blocked fuel path or low compression.
Do not tune the carburetor by ear or turn adjustment screws to mask stalling. If the saw races, surges, or runs only with choke partly on, stop and have the fuel system and crankcase seals checked. Building on this, use a tachometer and the model-specific service procedure when carburetor adjustment is required.
Shutdown, Storage & Seasonal Care
Good shutdown and storage habits help prevent fresh fuel and debris from becoming the next no-start problem. Let the engine cool before cleaning or storage, and follow the manual for fuel handling and long-term storage. Do not store fuel or operate the saw near flames, sparks, or other ignition sources.
After cutting, stop the engine and allow it to cool. Clean debris from the air intake, sprocket cover, bar groove, and oil passages. Check chain tension and inspect for damage before the next use. Use clean bar oil and keep it separate from the fuel mix.
For storage, follow the manual for your model and the fuel product’s instructions. If the saw has been sitting, do not assume old fuel is usable. Inspect it, replace it if doubtful, and start with fresh 50:1 mix. If the same fault returns with fresh fuel and correct starting steps, continue diagnosis rather than repeating the same pulls.
A practical order of checks
Use this sequence to keep the work focused:
- Confirm there is no visible fuel leak and the saw is safe to handle.
- Check fresh 50:1 fuel, switch position, choke steps, and possible flooding.
- Inspect the air filter and spark plug.
- Test spark safely with an appropriate tester.
- Check the fuel filter, lines, and tank vent if it starts then stalls or runs poorly.
- Check compression using the correct service method if earlier checks do not explain the fault.
- Stop and seek qualified service for lean-running symptoms, damaged safety parts, or uncertain internal repairs.
Bottom line: Start with fuel, controls, and the filter before buying parts. Then test spark, fuel delivery, and compression in order. If the engine races, surges, loses power abruptly, or the chain moves at idle, shut it down and do not keep cutting.
Frequently Asked Questions
What fuel mix does a Stihl MS 170 use?
Use gasoline and suitable air-cooled two-stroke oil at 50:1 by volume. That is 20 mL of oil for each 1 L of gasoline. Follow the manual and use fresh fuel.
Why will my MS 170 not start after many pulls?
Check fuel freshness and mix, switch position, choke procedure, and whether the engine is flooded. Then inspect the plug and air filter. If it still will not start, test spark and fuel delivery before checking compression.
How can I tell if my MS 170 is flooded?
A wet spark plug after repeated starting attempts can be a clue, but it does not prove flooding. Follow the manual’s clearing procedure. Do not keep pulling with the choke on or repeatedly prime the saw.
Why does my MS 170 start and then die?
A dirty air filter or spark-arrestor screen, restricted fuel supply, damaged fuel line, or tank-vent problem can cause stalling. Inspect these items using the manual. Stop if the saw races or runs only with partial choke.
What does it mean if the saw runs only with the choke partly on?
It may point to fuel starvation or an air leak. Do not use partial choke to keep cutting or adjust the carburetor by guess. Stop and have the fuel system and crankcase seals checked.
What is the correct idle speed?
The stated idle speed is 2,800 rpm, but confirm the specification and procedure in the manual for your exact version. The chain must stay stationary at idle. If it moves, stop using the saw until the fault is corrected.
What is the spark-plug gap on an MS 170?
The stated electrode gap is 0.5 mm. Check the manual for your exact saw and inspect the plug for damage or fouling. A correct gap does not rule out a fault elsewhere in the ignition system.
Which bar and chain fit an MS 170?
Fit depends on the saw version and bar setup. Match the bar’s stamped pitch and gauge with the chain, and confirm the chain’s drive-link count. Do not choose a chain by bar length alone.
Can I keep using the saw if it surges or races at idle?
No. Shut it down. Surging or racing can indicate a lean-running condition, which may overheat and damage the engine. A chain that moves at idle is also a safety hazard.
When should I take the saw to a technician?
Get qualified service if it has a fuel leak, damaged safety parts, persistent lean-running symptoms, uncertain compression results, or a fault that remains after basic checks. Avoid replacing parts or adjusting the carburetor without test evidence.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
