A situation where it is reliable Stihl MS 250 refuses to start, often takes you by surprise, especially if the work is at a standstill. Instead of the usual roar of the engine, the user hears only silence or fruitless jerks of the starter. This can be caused by a variety of factors, from a simple lack of spark to more complex fuel system or compression problems.
Before taking the tool to a service center, it is worth conducting independent diagnostics. In most cases, the problem can be fixed on your own if you know where to look for the cause. Systematic approach will help you quickly return the saw to service, saving time and money.
In this article we will analyze the main components that affect engine starting and provide a step-by-step algorithm of actions. You'll learn how to check mixture quality, spark plug condition, and crankcase leaks to pinpoint the source of the problem.
Checking the fuel system and mixture quality
The first thing you need to pay attention to if it won't start chainsaw Shtil 250, is the state of the fuel mixture. Two-stroke engines are extremely sensitive to the quality of gasoline and the correct proportions of oil. If the tool was left idle for a long time, the mixture could separate or oxidize, making it unsuitable for ignition.
Old gasoline loses its octane properties, and oil precipitates, clogging the carburetor channels. Fuel freshness — critical parameter: it is recommended to store the finished mixture for no more than two weeks in a tightly closed canister in a cool place. Using old fuel often makes it impossible to start the saw even with a working spark plug.
It is also worth checking the fuel filter in the tank. If it is clogged with dirt or fuel breakdown products, gasoline simply does not flow into the carburetor in the required volume. Visually inspect the hoses for cracks and make sure the filter moves freely throughout the tank.
Use only high-quality gasoline AI-92 or AI-95 and special two-stroke oil for garden equipment. Mix the components immediately before work or in a transparent container to see the homogeneity of the solution.
It is important to ensure that there are no air pockets in the fuel system. Sometimes, after a long period of parking or working in the cold, stagnation occurs in the highways. A few presses on the primer (primer bulb) will help push fuel to the carburetor.
Diagnostics of the ignition system and spark plugs
The second key element that ensures engine starting is the spark generation system. If Stihl MS 250 If it won't start, first unscrew the spark plug and inspect its condition. The color of soot can tell a lot about the processes occurring inside the cylinder.
The normal color of the electrode is light brown or brick. If the candle is black and wet, this indicates over-enrichment of the mixture or excess oil. A whitish coating or melting of the electrode indicates operation with a mixture that is too lean, which can lead to overheating of the piston group.
Check the gap between the electrodes. It should be approximately 0.5 mm. Too much or too little clearance may prevent the spark from producing sufficient power. Wipe the candle with a dry rag, heat it over a fire (if it is wet) and check for a spark by touching it to the metal.
If there is no spark or it is very weak, the problem may lie not only in the spark plug itself, but also in the high-voltage wire or ignition module. Make sure the wire is tightly seated in the cap and has no visible damage to the insulation. Sometimes the contact inside the cap oxidizes, which disrupts impulse transmission.
Condition of the air filter and intake tract
For normal combustion of the fuel-air mixture, a constant supply of clean air is required. If chainsaw If it stalls immediately after starting or does not start at all, the air supply may be blocked. The air filter is the first barrier that absorbs dust and sawdust.
A clogged filter creates a vacuum in the carburetor, causing the mixture to become too rich (too much gas, not enough air). In this state, the engine “chokes.” Regular cleaning of the filter (rinsing in soapy water and drying) or replacing it is a mandatory maintenance procedure.
Also inspect the intake tract for foreign objects. Sometimes small branches or pieces of plastic can partially block the channel, resulting in unstable engine operation. The tightness of the connection between the carburetor and the cylinder also plays an important role.
☑️ Checking the air supply system
Pay attention to the condition of the cold start damper. It should completely block the channel during a cold start and open after the first “setting”. A malfunction of the damper mechanism may interfere with the starting process.
Problems with carburetor and mixture settings
The carburetor is the heart of the fuel system, and this is where most often lie the reasons why it won’t start. Calm 250. Over time, the jets can become clogged with small fractions, and the membranes lose their elasticity. This disrupts the process of preparing the mixture.
If the saw sat for a long time, the gasoline could evaporate, leaving behind a sticky resin that clogs the thin channels. In this case it is required disassembly and cleaning carburetor using a special cleaner. Pay special attention to the strainer and needle valve.
It is also worth checking the factory settings of the adjusting screws L, H and T. Lost settings lead to the mixture proportions being violated. Screw L is responsible for low speed, H for high speed, and T (or LA) for idle speed.
⚠️ Attention: Do not turn screws L and H unless necessary and without a tachometer. Incorrect adjustment of screw H can cause the engine to run lean at high speeds, causing overheating and piston scuffing.
In some cases, the problem is solved by simply bleeding the carburetor. However, if the membranes become stiff or damaged, a repair kit will be required. Air leakage through carburetor gaskets is another common defect that is easy to diagnose.
Compression and condition of the piston group
If everything is fine with fuel and spark, but the engine does not start, it is worth checking the compression. Low pressure in the cylinder prevents the mixture from compressing and igniting. You can check the compression manually (you can feel resistance when you pull the starter) or using a compression gauge.
Normal compression for Stihl MS 250 should be within 8-12 atmospheres. If the reading is significantly lower, this may indicate wear on the piston rings, scoring on the cylinder, or stuck rings. A visual inspection through a backlit spark plug hole will help assess the condition of the cylinder walls.
Often the cause of a drop in compression is the deterioration of the crankshaft seals. If the seals allow air to pass through, the seal of the crankcase is broken, and the fuel mixture is sucked in from the outside without creating the necessary vacuum.
How to quickly check the tightness of oil seals?
Remove the carburetor and muffler, plug the holes in the cylinder. Apply air through the spark plug hole at a pressure of about 0.5-0.8 atm and spray the seals with soapy water. The appearance of bubbles will indicate the location of the leak.
Wear of the piston group is a natural process, especially with intensive use or the use of low-quality fuel. In such cases, a major overhaul or replacement of the CPG (cylinder-piston group) is required.
Exhaust system and muffler
It is rare, but it happens that the engine does not start due to a clogged muffler. Combustion products, mixing with oil, form carbon deposits, which over time can completely block the outlet of the spark arrestor. The engine “suffocates”, unable to eject exhaust gases.
If you notice that the saw loses power and then stops starting, check the exhaust system for leaks. Remove the muffler and inspect the channel for coke deposits. If necessary, clean it mechanically or burn it over an open fire.
Also check the condition of the spark arrestor inside the muffler. If it is destroyed or completely clogged, it must be replaced or removed (although the latter is not recommended due to environmental regulations and the risk of sparks).
Table of main faults and troubleshooting methods
For ease of diagnosis, the main symptoms and methods for solving them are summarized in a table. This will help you quickly navigate the situation.
| Symptom | Probable Cause | Solution method |
|---|---|---|
| No spark, spark plug is dry | Ignition module malfunction or wire break | Checking the circuit, replacing the module |
| The candle is wet, the smell of gasoline | Fuel overflow, engine flooded | Dry the cylinder, blow it out, check the carburetor |
| The engine starts but stalls | Fuel filter or air filter clogged | Replacing filters, cleaning the tank |
| No compression | Worn piston or seals | CPG troubleshooting, replacement of seals |
| Stalls at high speeds | Muffler clogged or screw H set incorrectly | Cleaning the muffler, adjusting the carburetor |
90% of starting problems are solved by replacing the spark plug, fresh fuel and cleaning the air filter. Do not rush to disassemble the carburetor without checking these simple points.
Cold engine starting algorithm
Sometimes the user is faced with a situation where a technically sound saw does not start due to a violation of the starting procedure. For Stihl MS 250 there is a clear algorithm that must be followed, especially during a cold start.
First apply the chain brake, then press the primer several times to prime the fuel. Move the choke control lever to the cold start (closed) position. Pull the starter until the first “click” (pop), then move the choke to the “open” position and continue starting.
Violating this sequence, for example, trying to start it with the valve closed after the first clap, will lead to flooding of the spark plug. It is important to feel the moment the engine switches to operating mode.
⚠️ Attention: Never leave the saw idling for long periods of time without load. This leads to the formation of carbon deposits on the spark plug and piston, which will make starting difficult in the future.
Following the correct starting technique extends the life of the starter group and prevents engine wear. If after all the manipulations the saw does not start, you may need a more in-depth diagnosis from a specialist.
Frequently asked questions (FAQ)
Why does the Shtil 250 chainsaw start and immediately stall?
Most often, this indicates a clogged fuel filter in the tank or a disruption in the air supply through a dirty air filter. The reason may also be incorrect carburetor settings or air leaks through the seals.
How often should the fuel filter be changed?
It is recommended to change the fuel filter Stihl once a season or every time the piston group is replaced. If you notice that the filter has darkened or become sticky, replace it immediately without waiting until the end of the season.
Is it possible to use motor gasoline for Shtil 250?
Yes, you can use AI-92 or AI-95 gasoline, but it must be fresh (no older than 2-3 weeks). Using old gasoline or fuel with an octane rating lower than recommended may cause detonation and engine damage.
What to do if the saw does not start after winter?
Most likely, the fuel in the carburetor has dried out, leaving a tarry coating. It is necessary to drain the remaining mixture, unscrew the spark plug, blow out the cylinder and, possibly, disassemble and wash the carburetor with cleaner.
What spark plug gap should the Stihl MS 250 have?
The optimal gap between the spark plug electrodes for this model is 0.5 mm. You need to check and adjust the gap with a special feeler gauge before each sharpening of the chain or replacing a spark plug.