Subject: Idle speed

Idle is pretty smooth at 1200-1300 rpm when warmed up enough not to die,
maybe after 1 mile of riding.
After about 15 minutes I see the idle drops to 1000. still smooth but it
really doesn't like to idle there, any slower and it will stop.

Is it normal for the idle to drop off a few hundred rpm when hot??
what could be causing that?

Thanks
Pete
71 Ambassador