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In this article the procedure for setting the valve clearance on all Stihl 4-Mix trimmer engines will be demonstrated. Keeping the valve clearance within spec will ensure that your engine operates at maximum power and fuel efficiency, and will make sure that it always starts as easy as it should.

Disclaimer: The author will not be responsible for any personal injury, engine damage, or loss as a result of the use of these instructions.

The tools needed for this task include: T27 driver, 5/8 spark plug wrench, 8mm socket wrench, and .004 inch feeler gauge. Note: The feeler gauge must be tapered at the end, so that it will fit between the sides of the rocker arm, which are unusually narrow on these particular engines. A standard .004 inch feeler gauge can be easily modified to fit this task with a pair of tin snips, as in the picture.

The first step in this procedure is to remove the spark plug boot, followed by the engine shroud, which is fastened to the power head by 3 T27 screws, one on each side of the clutch housing, and one on the rear of the engine. After the shroud and boot have been removed the spark plug must be removed with the 5/8 spark plug wrench. Once the plug has been removed, the valve cover is pulled, using the T27 driver, as in the picture.

Once the valve cover has been removed, inspect the gasket for signs of damage or deterioration. If the gasket shows signs of wear, replace it. The gasket depicted below is in good condition.

Now, watching the valve rockers, slowly pull the pull cord until the piston reaches Top Dead Center. If the valve rockers showed signs of movement, keep turning the engine over until the next TDC, to ensure that there is no valve overlap. On the flywheel, approximately an inch from the right end of the magnet strip, is an arrow; carefully line the arrow up with the lowermost screw of the ignition module to ensure completely precise alignment, as depicted below.

Once the valves are in the correct position, slide the feeler gauge between the rocker arm and the valve stem/spring, as shown below.

The feeler gauge should slide in with slight resistance; if there is too much resistance, or too little resistance, adjust the valve clearance with the 8mm socket wrench. To adjust the valves, either tighten (right), or loosen, (left) the nut on top of the valve rocker arm, as below.

When tightening the clearance, be sure to use very small increments, and check the clearance every 1/4 turn. Again, the clearance on both Intake and Exhaust valves is .004 inch. Once the valves are adjusted to spec, replace the valve cover, after inspecting the sealing washer between the bolt and the valve cover to ensure that it is not crushed or damaged; the example below is in good condition.

Reinstall the valve cover, being careful not to over tighten the bolt. Finally, reinstall the spark plug, the engine shroud, and the spark plug boot. After all the components are in place, it is advisable to start the engine to test the effectiveness of the adjustment, and, providing that the engine starts and operates satisfactorily, you are finished!:ThumbUp:
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