The brushes m- like the lead in your pencil - wear down - the thing they bear against - the commutator gets pitted - sparks = bad contact.
New brushes might fix it - cleaning the commutator might help - a new motor may be required. I get my replacement motors from the council tip for about £3.
My Flymo was bought in April 1991 and is on its 3rd motor - I shall soon need another...
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