Hello Blackchicken
I am the author of the post copied by Kommando.
It sounds very much like your sprag is not gripping the engine sprocket. i have come across this before, albet on a Kawasaki Z1300, same symtoms, starter revolving and not engaging. I quick strip down of the starter sprag (needing a puller making to get something off, cant remember what that was, alternator maybe) an emery up of the sprag engagment race, reassemble and viola engaged every time. the engagment race was too shiny/polished causing the sprag to slip.
What oil are you putting in the primary drive? Try a different oil, ATF instead of 20W/50. I find the heavier oil dosn't help with clutch slip. Or if you have it in bits again, take some 80 grit emery tape to the inside of the engine sprocket and drive gear snout.
I have sleeved a number of drive gears for MK3 owners who have had sprag problems. Cheaper than replacing sprag, sprocket and drive gear. I have also reset the overload device as i have found that these were set too low from new.
Anyone who has reset their overload device will tell you it is not an easy task to get correct. It sounds straight forward in the manual but when using a torque wrench to get the 50ftlbs setting you will find the adjusting nut revolves when checking the breakaway torque leading to false setting etc.
I overcome this by machining off the staking flange and making a thin locknut for the adjusting nut. Much much simpler to adjust and set the device.
Regards
Peter