Remove the spark plug boot from the spark plug. The plug boot is located in the center of the top of the engine.
Remove the spark plug by turning it counterclockwise until it is free.
Examine the firing point. If it is encrusted with carbon deposits, clean them away with a brass bristle brush. Reinstall the plug and reattach the boot. Start the engine and move the choke lever to the "Run" position. If the trimmer runs smoothly, you have corrected the problem.
Recall the last time you replaced the spark plug. If it has been more than 100 hours, Stihl recommends replacing it. The FS 70 is factory-equipped with an NGK CMR 6 H plug. Acquire a new plug from your Stihl dealer or an auto parts retailer and install it. If the trimmer behaves properly, you have addressed the problem.
Start the trimmer. After it has warmed up, place the choke lever in the "Run" position and release the throttle trigger. If the engine stalls, it is an indication that the low-speed idle setting is too lean.
Examine the rear of the engine. Look for three screws marked "H," "L" and "LA."
Turn the screw marked "L" one-quarter turn counterclockwise and start the engine. If it runs properly, you have fixed the idling problem. If not, continue turning the screw in one-quarter-turn increments until the engine idles smoothly.
Locate the screw marked "LA." Slowly turn it clockwise. This will further improve the idling behavior. Stop turning the screw when the cutting head barely begins to spin.
Things You Will Need
- Spark plug wrench
- Brass bristle brush
- Small slotted screwdriver
- If these steps fail to correct the problem, it may indicate that a more serious condition exists. Stihl recommends returning the unit to your authorized Stihl dealer for service.