Stihl MS 172 Chainsaw Problems (Causes & Fixes)

Most MS 172 problems start with fuel, airflow, chain condition, tension, or oil delivery. Check those items before assuming the carburetor or engine has failed. Use fresh 50:1 fuel mix, the correct bar-and-chain parts, and the model-specific manual. Stop using the saw if its chain moves at idle, its brake fails, or the bar gets no oil.

If your saw will not start, stalls under load, cuts poorly, or leaves the bar hot and dry, work through one symptom at a time. A simple check can often identify the cause, but do not keep running a saw with a safety fault or suspected engine damage.

I use a basic rule when diagnosing a small chainsaw: check the items that are easy to inspect first, then move to faults that need tools or dealer service. This guide follows that order. Before troubleshooting, confirm the exact MS 172 version and parts listed for its serial number and market. Specifications and approved setups can vary by region.

Check the MS 172 setup before troubleshooting

The MS 172 is a 31.8 cm³ homeowner saw with a rated power of 1.4 kW. A correct setup matters: a mismatched chain, bar, or sprocket can cause poor cutting, excess wear, or a chain that does not fit safely. Check the saw’s manual and component markings before changing parts.

Common approved bar lengths are 30–40 cm (12–16 in), but length alone does not prove compatibility. The bar, chain, and drive sprocket must match in pitch, gauge, and drive-link count. The chain pitch is commonly 3/8 in Picco (3/8 P), and the gauge is often 1.1 mm (.043 in), but confirm both on your own saw.

Those engine-speed numbers are references, not instructions to adjust the carburetor. If the saw races, stalls after basic checks, or seems to run lean, have a STIHL dealer test it with the right tools. Do not make undocumented carburetor changes.

Before starting, check the chain brake, throttle trigger and interlock, stop control, chain catcher, handles, bar, chain tension, and visible fuel or oil leaks. Wear chainsaw-rated leg protection, protective boots, gloves, a helmet, and eye, face, and hearing protection. Keep people clear and choose stable footing and an escape route.

Diagnose starting and running problems

Starting trouble means the engine will not fire, will not stay running, or responds poorly to the throttle. Begin with fuel and the start procedure, then inspect the air filter and spark plug. If the basic checks do not solve the problem, avoid repeated adjustments and ask a dealer to check the fuel and ignition systems.

If the engine will not start

First, check that the fuel is fresh and mixed at 50:1 with STIHL two-stroke oil. Do not use straight gasoline. If the fuel is old or its mix is uncertain, drain it and refill as the manual directs. Then repeat the cold- or warm-start procedure for your saw; incorrect use of the start control can flood the engine.

A flooded engine has too much fuel in the cylinder to start normally. Follow the manual’s flooded-start steps rather than guessing or repeatedly pulling the starter. If it still will not start, inspect the spark plug as instructed. Replace a fouled or damaged plug with the specified type, or have a dealer check for a spark or fuel-system fault.

If it starts, stalls, or runs poorly

A dirty air filter can restrict airflow, while a blocked fuel filter can limit fuel flow. Check and clean the air filter using the method in the manual. Inspect the fuel condition and filter; replace parts only with the correct ones. If the saw still stalls or accelerates poorly, a carburetor, ignition, or air-leak problem may need professional diagnosis.

A chain that runs while the engine idles is a stop-use problem, not a minor tuning issue. The cause may be a high idle setting, sticking throttle linkage, or carburetor fault. Shut the saw off and do not use it until the chain stays still at idle and the fault is fixed.

Fix cutting, chain, and lubrication problems

Poor cutting often comes from a dull or uneven chain, incorrect tension, or worn bar rails. A dry or hot bar points to an oil supply problem. Stop the engine before inspecting or adjusting the cutting parts, and keep hands clear of the chain. If a chain derails or a safety control fails, do not restart until the cause is corrected.

Chain cuts slowly or pulls to one side

A dull chain makes cutting slow and can raise heat, engine load, and kickback risk. If the saw pulls to one side, the cutters may be sharpened unevenly, or the bar rails may be damaged. Sharpen to the chain maker’s specified file size and angles, using the correct depth-gauge tool. Do not use a generic depth-gauge setting for every chain.

If sharpening does not restore a straight cut, inspect the bar rails and chain for damage or uneven wear. Replace cracked or damaged chain links. A damaged bar or chain should not be put back into service; use replacement parts that match the saw’s approved setup.

Chain is loose, tight, or derails

Check tension with the engine stopped and cool. The chain should sit in the bar groove and move by hand with a gloved hand, without sagging. Follow the manual’s adjustment steps. Recheck tension as the chain warms, since heat changes how it fits. Do not overtighten a hot chain; it can become too tight as it cools.

If the chain loosens repeatedly or derails, stop and inspect the chain, bar, sprocket, and side-cover clamping parts. Wear, damage, or incorrect assembly may be the cause. Do not simply tighten the chain and continue without checking why it came loose.

Bar is dry or overheats

A dry bar may mean the oil tank is empty, the bar’s oil passage is blocked, or the oiling system has a fault. Refill with chainsaw bar-and-chain oil. With the engine off, clean the bar groove and oil inlet, following the manual. Then use the manual’s check for oil delivery.

If oil still does not reach the chain, stop using the saw and arrange service. Do not keep cutting to see if the problem clears. There is no single universal oil-flow rate or viscosity for every MS 172 setup, so use the oil and approved parts specified for your saw.

Work through common diagnostic examples

These examples show how to narrow down symptoms; they are not claims about a specific repair or a promise that one fix will work. Start with the safest, simplest checks, and stop if you find damage or a safety fault.

Example: starts, then dies during a cut

A saw that idles but stalls when cutting may have restricted airflow, limited fuel supply, stale fuel, or a deeper engine fault. I would check fuel condition and the air filter first, then inspect the fuel filter as the manual allows. If those checks do not help, a dealer can assess the carburetor, ignition, and air leaks.

Example: cuts slowly and leaves a hot bar

A slow cut with excess heat can point to a blunt chain, incorrect tension, or poor oil delivery. Stop the engine. Check chain sharpness and tension, then confirm the oil tank has oil and the bar passage is clear. Do not force a dull chain through wood; let a sharp, correctly fitted chain feed without pushing hard.

Example: chain loosens after use

A chain can change tension as it heats, but repeated loosening or derailment calls for a full inspection. Check that the chain fits the bar and sprocket, then inspect the drive links, sprocket, and clamping assembly for wear or damage. Refit and tension it according to the manual before restarting.

Maintain the saw and know when to stop

Routine care can prevent some common running and cutting problems. Clean the air filter and cooling-air inlets at the intervals and by the method in the manual. Keep the bar groove and oil holes clear, and turn the bar periodically if its design allows. Inspect the chain, bar, and saw for wear or damage before and after use.

For extended storage, follow the manual’s fuel-storage procedure. Use STIHL-approved storage steps or drain and run the fuel system as directed. Clean the saw and store it in a cool, dry place, out of children’s reach and away from ignition sources. Protect the bar and chain; empty the oil tank for storage or transport if the manual requires it.

The guide-bar nose can cause kickback if its upper quadrant contacts wood. Keep the nose clear, hold the saw with both hands, and avoid cutting with the bar tip. A working chain brake is essential, but it does not make unsafe technique safe.

Stop and get qualified service if the chain moves at idle, the chain brake fails, oil does not reach the chain, the engine races, or fuel or oil leaks. Also stop if you find a damaged chain, bar, or control. Do not fell trees beyond your training, the saw’s capacity, or your control of the site.

Frequently asked questions

These short answers cover common MS 172 troubleshooting questions. Use them as a starting point, not as a substitute for the manual for your serial number and market. If a check requires a repair you are not trained to do, contact a STIHL dealer.

Why will my MS 172 not start?
Common causes include stale or incorrect fuel, flooding, a fouled spark plug, or an incorrect start procedure. Begin with fresh 50:1 fuel mix and the manual’s starting steps.

What fuel mix does the MS 172 use?
The reference mix is 50:1 gasoline to STIHL two-stroke oil. Measure it correctly and confirm fuel guidance in your regional manual.

Why does my saw start and then stall?
A dirty air filter, blocked fuel filter, stale fuel, or a carburetor, ignition, or air-leak fault may cause stalling. Check fuel and filters first; seek service if the fault persists.

Why does the chain move while the saw idles?
The idle may be too high, the throttle linkage may stick, or the carburetor may need repair. Stop using the saw until the chain remains still at idle.

Why does my chain cut slowly or pull sideways?
A dull or unevenly sharpened chain is a common cause. Incorrect tension or damaged bar rails can also affect the cut. Sharpen with the specified file and gauge, then inspect the bar.

Why is the bar getting hot?
A dry oil tank, blocked oil passage, incorrect oil, or an oiling-system fault may be responsible. Refill with chainsaw bar-and-chain oil and check the oil inlet and groove. Stop if delivery remains absent.

How tight should the chain be?
With the engine stopped and cool, it should sit in the bar groove and move by hand with a gloved hand without sagging. Follow the manual’s adjustment steps.

Which bar and chain fit the MS 172?
Use only combinations approved for your serial number and market. Match the bar, chain, and sprocket markings for pitch, gauge, and drive-link count; bar length alone does not confirm fit.

Can I tune the carburetor by ear?
No. Do not make undocumented adjustments or exceed the manual’s engine-speed limit. Persistent racing, stalling, or suspected lean running needs proper diagnosis.

When should I stop using the saw and get service?
Stop for a chain that moves at idle, a failed brake, absent oil delivery, fuel or oil leaks, a damaged cutting part, or an engine that races. Have the fault repaired before cutting again.

(This article was written by one of our staff writers, Marcus Thorne. Visit our Meet the Team page.)