Stihl MS 250 Fuel & Air Checks (Troubleshooting)
If your MS 250 will not start, stop pulling and check the basics in order: correct fresh fuel, fuel delivery, and a clean, correctly fitted air filter. Work with the engine cool and ignition off. Use the manual for your saw’s serial-number variant, and do not turn carburetor screws to guess at a fix. Stop if fuel leaks or the engine races.
A saw that has been pulled over and over can leave you tempted to buy a carburetor or ignition part. Don’t. I use a simple rule: check what you can confirm first, then stop before a risky adjustment. On the MS 250, bad fuel, a restricted filter, or an air leak can cause similar symptoms, so each check should narrow the cause.
Equipment Setup, Fuel/Air Checks & Safety Preparation
A safe check starts with a cool saw, ignition off, and the right manual for the machine. The MS 250 is a 45.4 cm³ gasoline saw, but parts and procedures can vary by serial number and market. Confirm the manual and fitted bar-and-chain combination before servicing or running the saw.
Wear chainsaw-rated leg protection, a helmet with eye or face and hearing protection, gloves, and protective boots. Work outdoors, keep bystanders clear, and set the saw on firm ground. Before starting, inspect the chain brake, throttle lockout, chain catcher, handles, bar, chain, and chain tension. The chain must stop when the brake is engaged.
Check fuel before opening the carburetor
Fuel mix is the first check because incorrect or old fuel can cause hard starting, poor running, and engine damage. For the applicable MS 250 specification, use a 50:1 gasoline-to-two-stroke-oil mix, fresh fuel, gasoline rated at least 89 AKI, and no more than 10% ethanol. Follow the fuel and oil directions in your saw’s manual.
If you do not know what is in the tank, drain and handle the fuel as directed by the manual and local rules. Mix a fresh batch in a clean, approved container. Do not add oil to an uncertain tank and assume the ratio is now correct.
| What you find | What it may mean | Next check |
|---|---|---|
| Fuel is old or its mix is unknown | Fuel quality or mixture may be wrong | Replace with fresh, correctly mixed fuel |
| Fuel cap or nearby area is wet | Possible leak or spill | Do not start; identify and correct the leak |
| Fuel lines look cracked, loose, or wet | Fuel delivery fault or leak | Stop and arrange qualified service |
| Filter is visibly dirty or damaged | Restricted fuel flow is possible | Service or replace as the manual allows |
| Fuel and air checks pass | Fault may lie elsewhere | Do not assume the carburetor is the cause |
Inspect the fuel path
With the engine cool and ignition off, check the fuel cap area and visible fuel lines for wetness, cracks, or damage. Inspect the accessible fuel filter only as the manual permits. A dirty filter can restrict fuel flow; a damaged line can leak or admit air. Do not run the saw if you find a leak.
If the filter is not accessible without special tools, or a line looks damaged, stop there. Fuel-system work can require the correct parts and checks. A qualified STIHL dealer can inspect the system without relying on guesswork.
Inspect and clean the air filter
The air filter limits dirt entering the engine. A clogged filter can reduce airflow and power, while a damaged or poorly fitted filter can let debris through. Remove and clean it only by the method in the manual for your filter type. Replace it if it is torn, misshapen, or heavily contaminated.
Refit the filter and cover before starting. Never run the saw without its air filter. Also clear loose debris around the air-intake area, taking care not to push dirt into the intake.
Follow a controlled start and read the result
Use the starting procedure in the correct manual. Start outdoors, on firm ground, with the chain brake engaged and the bar and chain clear. If it does not start, do not keep pulling indefinitely. Recheck fuel age and mix, filter condition, and visible leaks or blockages.
A flooded engine can also be mistaken for a fuel-delivery fault. Follow the manual’s procedure for a flooded saw rather than improvising with repeated choke cycles. If the saw starts but races, surges, stalls, or loses power, shut it down. A lean-running engine can overheat and seize; do not try to correct it by turning mixture screws.
A common diagnostic pattern is a saw that starts briefly, then dies: first confirm fresh fuel and a clean, correctly installed air filter. If those pass, visible fuel-line or filter problems move higher on the list. This is a troubleshooting example, not proof of a specific failed part.
Field Cutting Operation and Kickback Control
A brief run check can confirm whether the fuel and air service changed the symptom, but it is not a reason to keep using a saw that runs badly. Operate only if the saw starts and idles normally, the chain stays still at idle, and the controls and safety parts work as intended.
Keep both hands on the saw and use a stable stance. Avoid the bar nose and its upper-quadrant kickback zone. Stop the engine before refueling, adjusting the chain, or clearing debris. If the chain moves at idle, or the engine races or surges, stop using the saw and arrange service.
For fitment, the MS 250 is commonly used with 16–18 inch bars, but bar length alone does not confirm compatibility. Use only a bar and chain combination listed for your saw. Check pitch, gauge, and drive-link count against the bar markings; 3/8 inch Picco low-profile pitch and .050 inch (1.3 mm) gauge are common, but other variants exist. Do not infer gauge from pitch.
If the saw runs properly after the checks, make only a cautious test cut in suitable material. Stop if it loses power, surges, or behaves differently under load. Do not force a dull, damaged, or incorrectly tensioned chain through wood.
Maintenance, Chain Sharpening & Carburetor Service
Routine care helps keep fuel and air faults from returning. Clean the filter as directed, check the bar and chain, and keep service records if you are unsure when fuel or parts were last changed. Carburetor mixture settings are not a safe trial-and-error adjustment for a home diagnosis.
After the engine cools, clean the bar groove and oil hole, inspect the chain for damage, and check tension using the manual’s procedure. Set chain depth gauges to the chain maker’s specification. A 0.65 mm (0.025 inch) setting is common on compatible chains, but it is not universal; use the correct gauge tool and chain instructions.
The MS 250 speed reference is approximately 2,800 rpm at idle and 13,000 rpm maximum for the applicable specification. These are not targets to set by ear. A qualified technician should check engine speed with a tachometer and use the correct manual for the saw’s variant.
| Symptom after fuel and air checks | Sensible next step |
|---|---|
| Still will not start | Stop repeated starting attempts; further diagnosis may involve spark or compression |
| Starts, then stalls or surges | Shut down; fuel delivery or an intake leak needs qualified diagnosis |
| Lacks power under load | Recheck fuel and filter; stop if the symptom continues |
| Chain moves at idle | Stop use; have idle and clutch operation checked |
| Fuel leak is visible | Do not start or run the saw; repair the leak first |
A second practical scenario is a saw that idles but races or surges after a filter check. That is not a cue to adjust screws until the sound changes. A fuel restriction or intake leak can make an engine run lean, and continued operation may cause severe damage. Have a qualified technician inspect it.
Storage, Fuel Handling & Seasonal Care
Fuel condition matters after the immediate repair, too. Use and store fuel according to the saw and fuel-maker instructions, and do not leave an unknown or questionable mix in the tank. Before storage, clean the air-intake area and follow the manual’s directions for fuel handling and storage.
After use, let the saw cool before cleaning or checking the bar and chain. Keep the bar groove and oil hole clear, inspect chain tension and damage, and use suitable STIHL bar-and-chain oil. Check oil delivery at the bar tip only by following the manual’s test procedure; do not assume a fixed flow rate.
If the saw will not start after these checks, the fault may be outside the fuel-and-air items you can safely confirm. Spark, compression, carburetor condition, and intake sealing require proper tests. A dealer visit is more useful when you can report the fuel used, filter condition, visible leaks, and the exact symptom.
Frequently Asked Questions
These short answers cover the most common fuel and air checks on an MS 250. Use the manual for your serial-number variant whenever a procedure or part differs, and stop operation if the saw leaks fuel, races, surges, or moves its chain at idle.
What fuel mix does a STIHL MS 250 use?
The applicable specification calls for a 50:1 gasoline-to-two-stroke-oil mix. Use fresh gasoline rated at least 89 AKI, with no more than 10% ethanol, and follow the manual for your market.
Can stale fuel stop an MS 250 from starting?
Yes. Old or incorrectly mixed fuel can cause starting and running problems. If the fuel’s age or mix is uncertain, replace it with fresh fuel mixed to the manual’s specification.
Can I run the MS 250 without its air filter for testing?
No. Refit the air filter and cover before running. Operating without the filter can let dirt enter the engine.
What should I check if the saw starts, then stalls?
Confirm fresh, correctly mixed fuel and a clean, correctly installed air filter. Look for visible fuel leaks or damaged lines. If the problem continues, stop and have the fuel system and intake checked.
Should I adjust the carburetor screws if the saw surges?
No. Do not compensate by turning carburetor screws. Surging can point to a fuel-delivery or intake problem, and a lean-running engine may overheat. Have a qualified technician diagnose it.
What does a moving chain at idle mean?
Stop using the saw. The chain should not move at idle. Have the idle and clutch operation checked before using it again.
Does a 16-inch bar fit every MS 250?
No. Bar length alone does not confirm fit. Use a combination listed for your saw and match the bar and chain markings for pitch, gauge, and drive-link count.
When should I stop troubleshooting at home?
Stop if you find a fuel leak, damaged line, racing or surging engine, chain movement at idle, or a fault that remains after fuel and air checks. Spark, compression, carburetor, and intake-leak diagnosis may need a qualified technician and proper test tools.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
