not sureon the type of starter you have, but if there is a square shaped spindle that fits into a hole on the starter pulley, then it is likely the starter clutch is sticking. it is a small housing dscrewed onto the crankshaft that uses ball bearings on a cam race. when worn they can stick. if you do not have a square shaped spindle then disregard this and i will think of something else.
dustydes
June 2008