Motorcycle Questions? Ask a Mechanic.
It sounds like your Idle Air Contol motor has stuck or failed. It provides 100% of air to the bike when the throttle plate is closed. If the IAC pintle does not set at the proper step it will cause your issue.
It is located in the top center of the intake manifold directly behind the breather back plate. There is a single hole you'll see from the inside of the manifold. This hole is the hole that the IAC pintle should adjust the air flow of to control idle.
When turning the bike on and off you should see that pintle moving. If not, it may be bad. It's about $50 and is fairly simple to replace.
I hope this helps. Thanks, P.J.