[5th_bike]

Another thought - corrosion at cable contacts. I don't know where the starter motor cables all exactly connect but, if you have enough voltage at the source (car battery) then somewhere in the circuit could be a bad/corroded contact that prevents the starter motor form getting enough current.

There are at least seven places where the corrosion/bad connection could be:
1) connection to the battery
2+3) two connections at the starter relay
4) one contact inside the starter relay
5+6) two connections at the fuse box
7) connection from fuse box to battery.

I don't know (and too lazy to figure it out) if the two cables from the starter are in one unit with it or it they connect at the starter motor, that would be +2.

My car once had trouble starting,5 years ago or so, all I got was clicks from the starter relay (maybe someone would be willing to to record that sound for the GZ250 [hint, hint]). It was a bad connection of the +12V at the battery itself. Cleaned it, sanded it, reconnected it - no problem to date.

edit 1: addition
edits 2-4: typos