What exactly does this throttle locking bolt do? What is its purpose?  Does it regulate the idle?

Cruise control....sort of.

Holds/slows the throttle mechanism return

Mick67 wrote:Cruise control....sort of.

Holds/slows the throttle mechanism return

So if it is untightened, then does the throttle mechanism returns/rolls to off faster?

When you screw it in the plastic tip is pressed against the internal part of the throttle grip creating a frictional drag that can hold the throttle so the return spring can't shut it off when you let go. 

I adjust mine so that it is just tight enough to hold the throttle position at highway cruising speed.  This way the friction is not so high that it makes it tiring to control the speed in changing conditions, but I can let go of the grip and the speed will hold or very slowly drop.

No longer in production and extremely hard to find these days but the "Flip-a-lever" does the same thing much more easily.  I had one on one of my first K75s. It was a Godsend before I started installing real cruise control.

http://www.gunsmoke.com/motorcycling/r100gs/flip_a_lever/index.html

Point-Seven-five wrote:When you screw it in the plastic tip is pressed against the internal part of the throttle grip creating a frictional drag that can hold the throttle so the return spring can't shut it off when you let go. 

I adjust mine so that it is just tight enough to hold the throttle position at highway cruising speed.  This way the friction is not so high that it makes it tiring to control the speed in changing conditions, but I can let go of the grip and the speed will hold or very slowly drop.

Thank you, this explains its purpose well.

Want one? I’ve got one PM for price.

I have one for sale if needed just make sure you have the threaded hole under the throttle body of switch gear area as I have seen and also struck one of my K100's that didn't have the hole, hence I still have the screw.
post message me re have I found it yet and also the price. Cheers syd