Solenoid throws pinion in, engages with ring gear ...starter motor is spinning but not engaging pinion shaft.... 

sounds like starter clutch needs replacing usually an easy fix ,just done mine under a couple hundred 

Thanks gidgi, from the manual it looks like the clutch shaft and springs may be the trouble, i will investigate....

i had the same "sound" as yours (exactly) 

Thanks Kato yes helpful, I have my starter in pieces and can see no broken or worn parts ?? Thanks for the help. Really want to know what has caused this trouble before I order a new starter motor. Thinking that it is just the starter motor clutch in the housing.

Definitely your starter clutch.. I've fixed allot of them, the starter clutch is a 1 way bearing while you have it apart if your starter clutch is rooted you will be able to spin it either way.. New one from easyR is about $60-$80 I think from memory..