Motorcycle Repair: 1983 Honda Starter works but slips, missing teeth, machined surface

Question
I have 2 1983 Hondas doing the same thing, a CM 250 and a VT750 Shadow.  Both bikes the starter will turn and has been replaced, but the starter seems to "slip" it engages for a moment then makes a noise like it ceases to engage then back to engaged for a 1/2 se and back and forth.  The Bike shop has tested both the starters and are stumped.  The engines just does not get turned long enough to turn over in the mix.  They have checked the gear that the starters turn and it appears fine, no wear no missing teeth.  
Any Suggestions?

Answer
Robert,there is such a  thing as a one-way clutch used for starting on the back of the flywheels.The starter chain spins the flywheel to start then once the engine fires the clutch freewheels on a machined surface. The CM250 clutch part#28120-201-040 is about $53.00 the VT 750 is more. It consists of two parts 28120-ME9-000 and 28125-ME9-013 the brg and the retainer hub. Check these first and see if they need replaced.