Here's what's happening. You are running the top end rich -correctly. The idle is probably a little rich too. Good. When you get to full idle, the tank is too low to draw fuel, and it leans out and speeds up since your setting is a little rich. Not sure with a fix. You really need to raise the tank or lower the carb centerline. You can rotate the engine from upright to sideways and that will lower the carb - or try a pump and leave the rest the same. Make sure also that you can close the throttle barrell completely with the throttle trim too as a safety issue so you don't have to reach towards the engine every time to shut it off.