[chuck's]

With the engine warm,engine idling,the choke should be wide open.With engine shut off and warm,the choke should be open.Does the carb have an electric choke?These were adjustable.Some may have rivets that hold the assembly together.As you suspect,does the vaccum choke pull off work?If it doesn't,your choke is probably staying on.Heat riser valve?Does this work and does the passage under the carb get warm?If the valve doesn't work or if the passage is plugged,the air/fuel mixture doesn't atomize properly and can end up with too rich a mixture.
These are a few suggestions to try.
chuck's