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Carburetor Settings for Stihl Trimmers and Brushcutters

SCREW(H)—CAUTION!

Adjustment of the high speed adjusting screw(H) not only affects the power output but also the maximum off-load engine speed. If the setting is too lean (screw turned too far clockwise), the maximum permissible engine speed will be exceeded. This can cause engine damage, brought about by lack of lubrication and overheating in particular. Corrections to the setting of the high speed adjusting screw may be carried out only if an accurate tachometer is available to check the maximum engine speed of 13,000 rpm.

When the engine is tested at the factory the carburetor is set to obtain a slightly richer mixture to ensure that the cylinder bore and the bearings receive additional lubrica­tion during the break-in period. This setting should be left as it is for the first three tank fillings. The high speed adjusting screw may then be turned no more than 1/4 turn clockwise (leaner mixture). Caution: The engine”s maxi­mum permissible rpm must not be exceeded! Note that even slight alterations on the adjusting screws have a noticeable effect on the engine”s running behavior. Only carry out carburetor adjustments after cleaning the air filter and warming up the engine.

If you have no means of checking the maximum engine speed, do not set the high speed adjusting screw (H) any leaner by turning it (clockwise) beyond the basic setting below!

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Safe Basic Setting For Screws (H) & (L)

Carefully screw the two adjusting Screws (H) & (L) down onto their seats (clockwise). Then:High speed adjusting screw (H): back off 1 complete turnLow speed adjusting screw (L): back off 1 complete turn

Basic Setting For Screw (LA)

Turn idle speed adjusting screw (LA) until the trimmer stops. Then turn counter-clockwise one quarter of a turn.

Fine Tuning – Screw(L)

Erratic idling behavior; poor acceleration

Idle setting too lean; turn low speed adjusting screw (L) counterclockwise until engine runs and accelerates smoothly. Now readjust screw (LA).

Exhaust smokes at idle speed

Idle speed setting too rich; turn low speed adjusting screw (L) clockwise until engine speed drops. Then turn screw back one quarter turn and check that engine still accelerates smoothly when you open the throttle. Now readjust screw (LA).

Stihl trimmer Troubleshooting Guide

1. Compression Test – Can you lift the trammer off the ground with the pull rope?2. Fuel Test- Remove air filter and cover and spray starter fluid in the carburetor and try to start. Use a 50:1 fuel mixture with premium fuel3. Spark Test – Remove spark plug clean with solvent & wire brush. Reconnect plug wire and let plug hang close to engine block (away from any gas) pull starter rope and look for spark, dim lighting makes it easier to see. If no spark replace plug and try again. Check off/on switch and wire coming from coil. If still no spark replace coil.4. Trimmer runs on starter fluid not on gas in tank. Check gas tank for old gas, empty old gas look for “syrup gunk” in the bottom of the tank, if present clean and rinse several times until clean (I use carburetor cleaner & gas and any brush that can reach in and not melt) Try to start with fresh gas mix. If it doesn’t start check and replace fuel line and pick-up. If it still doesn’t start remove carburetor and check manifold boot (tube between carburetor and engine) replace if needed and try to start. If it still doesn’t start remove carburetor, disassemble and clean carburetor, (clean outside of carburetor first) you may have to disassemble and soak in solvent over night try not to get solvent or carburetor cleaner on gaskets. Check gaskets, diaphragm gasket should be smooth and puffy not wrinkled or wilted if in doubt replace. Carburetor kits are around $8-$15 and include all gaskets, needle and spring. Reassemble (making sure all gaskets are positioned properly) and try to start if it still doesn’t start replace carburetor.

This Tune Up guide is for the following Models of Stihl Trimmers:

Click to see the Stihl trimmers on ebay: FS100 * FS110 * FS110 R * FS120 * FS120 R * FS130 * FS130 R * FS200 * FS200 R * FS250 * FS250 R * FS350 * FS36 * FS40 * FS44 * FS44 R * FS45 * FS450 * FS46 * FS55 * FS55 * FS550 * FS75 * FS80 * FS80 R * FS85 * FS85 R * FS90 * FS90 R * KM110 R * KM55 R * KM85

  • chad

    Thanks for help. Fs110 stihl weedeater was running rich and uneven. Used your guide above, works good now.

  • davedave

    have had stihl weedeater for ten years, and stihl runs great .

  • CHRIS

    i need help !!!!!!!!!!!!!!!!!!!

  • Bill Walker

    Not clear on Idle speed adjustment. (LA) Says turn until trimmer stops, than turn counterclockwise one quarter of a turn. If you turn until trimmer stops, wouldn’t that be counterclockwise? And than another quarter of a turn counterclockwise How would it idle??