When you operate the starter button and it spins over without cranking the engine or grabs and then releases without starting the engine, the starter drive is usually at fault.

It operates like a one way clutch and takes a beating after numerous starts over the years. It's not a difficult job to replace but requires removal of the primary cover after draining the primary oil first. Use a new gasket to prevent leaks when reassembling the primary cover and torque the bolts in the proper order to prevent warping the cover.

Here's a typical starter drive on a big twin Harley (89 and up).

http://i149.photobucket.com/albums/s45/bills985/starterdrive89up.jpg


This is what the starter drive itself looks like and costs about $50 for the part.

http://i149.photobucket.com/albums/s45/bills985/Starterdrive2.jpg