All chain saws have a centrifugal clutch, and the Stihl MS 250 model is no exception. This rugged saw, which is as useful in the bush as it is in the back yard, requires the frictional force generated by the clutch shoes against the drum to drive the chain. The shoes, like brake pads on a car, eventually wear out, which is the main reason that the clutch slips. Oil contamination also can cause the slipping.
The MS 250 Clutch
The Stihl MS 250, like all chain saws, has a centrifugal clutch, which has two parts. The shoes are attached to the engine crankshaft, and the drum is part of the chain sprocket. There are three shoes, and they are connected by springs that keep them retracted until the centrifugal force generated by the spinning of the crankshaft drives them apart. When the rotational speed is great enough, they contact the drum, and friction makes the drum turn, which in turn drives the chain. When you release the chain saw's throttle and reduce the engine speed, the springs retract the shoes, and the chain stops.
The surfaces of the clutch shoes are coated with a high-friction lining, much like the brake shoes in an automobile, and this lining wears down slightly each time the clutch engages. Eventually, the lining becomes too thin to do its job, and the clutch begins to slip. This happens for two reasons. One is that the lining becomes so thin that the shoes don't make complete contact with the drum, and the other is that the surfaces of the shoes become slippery. The result, as far as the saw's operator is concerned, is that the chain stops turning whenever it makes a cut.
When the clutch shoes wear out, it's usually best to replace the entire clutch assembly because the inside of the drum is probably worn out, too. Start by unscrewing the spark plug and locking the piston with a locking tool or a piece of rope. After prying the E-clip from the engine shaft, pull off the drum and chain sprocket. Then you'll have access to the clutch shoes. Loosen the screws holding the clutch shoes to the crankshaft by using a socket wrench. Because you locked the piston, the engine won't turn when you do this. Put in a new clutch assembly by reversing the procedure.
When servicing the clutch, you have a chance to perform maintenance on some other key parts. One is the chain oil pump, which operates by means of a worm-drive gear attached to the engine crankshaft just behind the clutch. Pull it off and replace it if the gears are worn to keep the chain oiler operating smoothly. Another part that may need servicing is the needle bearing, which fits inside the hub of the clutch drum. Pull out the needle bearing when you remove the drum, and treat its bearings with some lubricating grease to keep them rolling and the chain sprocket turning smoothly. Smooth operation of the sprocket also prevents the clutch from slipping.
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