Launching a powerful gardening tool such as Stihl FS 55 or more productive FS 38, often becomes a problem for inexperienced users who ignore the basic rules of operating two-stroke engines. Many owners make the fatal mistake of pulling the starter as hard as they can right away, resulting in a broken ratchet or broken cable. To avoid costly repairs and ensure a long service life of your equipment, you need to clearly understand the physics of the processes occurring inside the cylinder during the first start-up.
The correct sequence of actions depends on the temperature state of the engine: a cold start requires enriching the mixture, while a hot engine requires a completely different procedure. In this article, we will analyze in detail the algorithm of actions for various situations, explain the role of a diaphragm carburetor, and show you how to avoid typical mistakes that 90% of beginners make when working with a gasoline scythe.
Preparing the fuel mixture and checking systems
Before attempting to start the engine, you must ensure that the fuel mixture is prepared correctly, since two-stroke engines do not have a separate oil lubrication system. Gasoline and special synthetic oil for two-stroke engines are mixed in a strict proportion, usually 1:50 (2%) or 1:40 (2.5%), as indicated in the instructions for the specific model Stihl. Using pure gasoline or incorrect dosage of oil will lead to scuffing of the piston group and rapid failure of the unit.
The quality of the components used plays a critical role: gasoline must be fresh, with an octane rating of at least AI-92, and stored in a metal canister for no more than 3-4 weeks. Old fuel oxidizes, loses volatility and forms a tarry coating that clogs the carburetor jets, making starting impossible without completely disassembling and cleaning the fuel system.
In addition to fuel, a visual inspection requires checking the integrity of the air filter and the absence of mechanical damage to the mowing head housing. If the filter is clogged with grass dust, the engine will choke and the mixture will become too rich, causing the spark plug to flood.
⚠️ Attention: Never use motor oils for four-stroke car engines, as they do not burn completely and form carbon deposits on the spark plug and in the muffler, which is critical for high-speed engines Stihl.
Make sure the fuel tank is no more than 90% full, leaving room for vapor to expand, and tighten the tank cap until it clicks into place. The tank ventilation system must work properly, otherwise when the fuel runs out, a vacuum will form inside and gasoline will stop flowing into the carburetor.
Cold engine starting algorithm
The procedure for starting a cold engine that has not been running for several hours or days is the most difficult and requires precise adherence to the sequence of actions to create the desired concentration of fuel vapors. First, place the scythe on a flat, hard surface, making sure that the cutting element does not touch the ground, grass or foreign objects that could prevent the shaft from turning.
Press the fuel primer (primer) 5-10 times until there are no air bubbles in the clear fuel hoses and they are filled with liquid. This action is necessary to force the supply of gasoline into the carburetor chamber, especially if the tool is new or has not been used for a long time.
Move the combination control lever to the cold start position, which on most models Stihl indicated by a closed damper symbol or inscription Start/Choke. In this position, the throttle valve completely blocks the access of air, creating a strong vacuum when the starter is cranked.
☑️ Cold start checklist
After installing the damper, grab the front handle with your left hand, pressing the scythe body to the ground, and grab the starter handle with your right hand. Smoothly pull the starter cable until there is resistance (the ratchets will work), and then make a sharp and strong jerk upward, without releasing the handle suddenly, so as not to damage the mechanism.
The engine should make a characteristic “catch” or first flash sound, after which the combination lever should be immediately moved to the intermediate position Run/Throttle Lock (half-open damper). If you do not do this and continue to pull the starter in the “Choke” mode, fuel will overflow and the spark plug will “flood,” which will require a long drying time.
Starting a hot engine and restarting
The situation when the engine is hot (for example, after a short stop for rest or refueling) requires a completely different approach, since the volatility of the fuel in a warm carburetor is already high. In this case, absolutely prohibited use pumping (primer) and close the air damper, as this is guaranteed to lead to over-enrichment of the mixture.
To start a hot engine, move the combination lever directly to the run position. Run or I, corresponding to the open damper. If the engine does not start after 3-5 pulls, you can lightly press the throttle trigger to reset the possible mixture, but do not hold it down constantly.
Why does it stall immediately after starting?
If the engine starts and immediately stalls, most likely the idle speed adjustment has gone wrong or there is an air leak. Try adding a little idle speed with the screw LA or L (clockwise by 1/4 turn), but do not exceed the norm so that the scythe does not start moving the shaft at idle.
If after several attempts the engine does not catch, let it cool for 10-15 minutes and repeat the procedure as for a cold start, but without using a primer.
Frequent stopping and starting throughout the day can cause the engine to become unstable if it is not allowed to warm up under load. After a successful start, let the tool run at medium speed for 10-20 seconds before starting to mow the grass to stabilize the temperature.
The role of the decompression valve and candles
Many modern models Stihl, such as FS 250 or FS 350, are equipped with a decompression valve, which greatly facilitates manual starting of the engine. When you press the valve button, part of the compressed mixture is released from the cylinder, reducing the resistance of the piston during the compression stroke, which allows even a child or woman to pull the starter without excessive effort.
Using this mechanism requires the correct sequence: first press the decompression button, then pull the starter until it engages the first time, release the button (the valve will close automatically when the pressure rises) and continue starting normally. Ignoring this element on powerful models can lead to injury to the arm or shoulder joint.
The condition of the spark plug also directly affects the ease of starting: carbon deposits, improper gap or a wet electrode can make starting impossible. Check the spark plug regularly, clean it from carbon deposits with a wire brush and adjust the gap between the electrodes to 0.5 mm, if necessary.
| Candle condition | Probable Cause | Action |
|---|---|---|
| Dry, light brown coating | Normal operation | Clean and reuse |
| Wet, black coating | Re-enrichment of the mixture | Dry, blow out the cylinder, check the air filter |
| White coating, melting | Lean mixture, overheating | Check carburetor adjustment, oil quality |
| Oily coating | CPG wear or too much oil | Measure compression, reconsider mixture proportions |
If the spark plug is wet after unsuccessful starting attempts, unscrew it, wipe it dry and jerk the starter several times with the choke open to purge excess fuel from the cylinder.
Carburetor settings for stable operation
If the lawn mower starts, but immediately stalls or is unstable, you may need to adjust the carburetor settings using three adjusting screws: L (low speed), H (high speed) and LA or T (idle). The basic factory setting usually allows the engine to start, but over time or when changing fuel it may be necessary (fine tuning).
Screw LA (or T) is responsible for the position of the throttle valve at idle: if you tighten it too much, the scythe will begin to move at idle, if you unscrew it, the engine will stall. The optimal position is when the engine is running steadily and the trimmer head is stationary.
Screw adjustment L and H requires a tachometer and must be done carefully, as incorrect high speed setting (H) can lead to engine operation at the limit of strength and rapid destruction of the piston group. Usually it is enough to turn the screw L by 1/4 turn to achieve a smooth increase in speed without dips.
When tuning, always move in small increments (1/8 or 1/4 turn) and note changes in engine performance. If after adjustment the engine begins to run worse, return the screw to its original position and try turning it in the other direction.
Common mistakes and safety precautions
One of the most common mistakes is trying to wind a braid while holding it in your hands, which is strictly prohibited by safety instructions. The tool must stand on the ground so that if the starter jerks sharply or the gas is accidentally pressed, the cutting element does not injure the operator.
Also, users often forget to lock the throttle trigger with a locking device when starting, which causes the throttle to remain closed or open in the wrong position. Always make sure that the throttle trigger is depressed and the locking mechanism is locked in the desired position before pulling the starter.
⚠️ Attention: Never wrap the starter cable around your hand or release the starter handle without control - the return spring can injure your hand and the handle can hit the body.
Ignoring the air filter is another cause of problems: running without a filter or with a dirty filter causes abrasive dust to enter the cylinder, which acts like sandpaper on the cylinder bore and rings. Clean the filter regularly and change it if it is very dirty.
Following the sequence of operations and using a high-quality fuel mixture is 90% of the success in starting and longevity of your Stihl lawn mower.
Frequently asked questions (FAQ)
What should I do if my Stihl lawn mower starts and immediately stalls?
Most likely, the fuel supply is interrupted or the idle speed setting is lost. Check the fuel filter in the tank (if it is clogged), the condition of the air filter and try to tighten the screw a little LA (idle). The reason may also be air leaks through seals or gaskets.
How many times do you need to pump swap before starting?
Usually 5-10 presses are enough until the air bubbles in the hoses disappear. If you pump for too long, you can flood the spark plug with fuel, especially on a cold engine, making it difficult to start.
Is it possible to use AI-95 gasoline for Stihl lawn mowers?
The manufacturer recommends using gasoline with an octane rating of at least 90 (AI-92). The use of AI-95 is permissible, but not mandatory, and in some cases (in the presence of old-type leaded additives) can be harmful, although modern high-quality AI-95 is usually safe.
Why does the starter cable pull hard?
Heavy cable movement may indicate fuel in the cylinder (water hammer), carbon deposits in the muffler, or wear on the crankshaft bearings. If the engine is “filled” with fuel, you need to unscrew the spark plug and blow out the cylinder.