You have an air vane governor autochoke system. On cold start-up, the spring keeps the choke closed. When the engine starts, the flywheel fan opens the choke fully via the air vane governor. When the engine is fully warmed-up, the thermostat keeps the choke off so when stopping/starting the engine when hot it does not flood.
If the engine starts then quickly dies, there are two main things to check/do.
1) make sure spring is not missing or disconnected.
2) The main jet is clogged (bowl nut is also the main jet) needs to be cleaned. Check for water/debris in bowl if jet is removed.