[425HP409]

Geezer has it right, turn the fast idle screw up, it is located at the bottom of the choke mechanism facing forward. You can access it with a long screwdriver. You need to also make sure the the vacuum pot on the carburetor is working properly, it should close when the engine starts. It should also hold a vacuum. It is called a "Choke Pulloff". It has two functions:
1. When the engine first starts it closes by engine vacuum, which opens the choke plate slightly so the engine will not run too rich and flood.
2. It allows the secondary air valve to open slowly on full throttle acceleration to keep the engine from having a "lean out".
If the choke Pulloff is working properly, raise the fast idle by the screw on the bottom of the choke mechanism. This should allow the carburetor to work properly, and the engine to stay running when cold.