It might be that you have an air leak that isn't showing up in the warm air or isn't too much of a leak when the air is warmer. A leak after the carb will show up a little better when the air is cold. Also check the gasket around your manifold to see if it might leak a little when it is cold. It could be running way way lean.
Can you tell a differance during the mid part of the day when it is a little warmer?