Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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It sounds like the drive shaft may be broken or twisted apart. This is the first thing that I would look at. You can remove the engine from the upper shaft to inspect the inner drive shaft.
I am including a diagram of the trimmer shaft housings. It could be either the upper or lower inner drive shaft. If it is the upper causing the problem then you will have to replace the upper shaft assembly, however if it is the lower inner drive shaft you can replace it as a separate part.
If you don't find that one of the shafts is broken then I would look closely at the coupler on the end of the upper drive shaft to make certain that it is not stripped or rounded out. If you see no problems there and the shafts are not broken then you could have gears stripped inside the lower gear head.
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