While Stihl string trimmers use a small two-stroke engine notorious for its vibrations and loud noises, the unit should not vibrate excessively in other areas such as the shaft or the gearbox, which are designed with a reduced vibration system. Excessive vibration may imply loose parts or parts about to go.
The shaft on a Stihl trimmer connects to the power head generally through four screws. The vibration on the engine can gradually shake loose even tightly screwed-in bolts. For this reason, Stihl recommends checking for loose screws and other parts after every use, so you can spot them early and tighten. Check all of the screws connecting the shaft to the power head, the throttle control lever to the shaft, and the shaft to the gearbox.
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The gearbox connects the mowing head to the drive shaft. Any noise that may appear to emanate from the shaft may actually be coming from the gearbox. If dirt, weeds or other material get trapped in the gearbox, it may force the drive shaft to work harder, thus exposing a problem in the drive shaft or further up the engine. To isolate the problem, remove the mowing head and check the gearbox and drive shaft for damage.
Drive Shaft Problems
The drive shaft rotates inside a hollow, protective tube. Like other engine parts, the drive shaft needs lubrication to keep spinning smoothly. If it's spinning rough or poorly inside the tube, it may force the shaft and other areas to vibrate during operation. Pull the drive shaft carefully out of the tube and check it for blue coloration. Replace the drive shaft if it's blue.
Clutch and Flywheel
While not all Stihl trimmers use a clutch, they all use a flywheel to help power the ignition system and cool the engine. A damaged flywheel will cause the entire engine to vibrate more. If the trimmer head hit something hard like a rock during full speed, the flywheel may get bent. This is often referred to as a sheared key. This key mates the flywheel to the crankshaft; if it's bent, the flywheel will spin poorly. If your unit uses a clutch, check the clutch plates, shoes and springs for signs of thinning and other damage.