20hp Mercury idling issue

[IceWater]

I have a 1985 Mercury 20hp motor. I rebuilt the carb this spring and it works great...for a while. I turned the mixture screw all the way to the right & 1 3/8 turns back. Set like that the engine starts first pull, isles in neutral and idles for trolling. However, after running it for over 15 minutes the engine will run rough and then stop. Can be a real bugger to get going again. I usually have remove the cowl and manually throttle it up to get it going, so probably flooding? Any thoughts or comments are appreciated