I have a 12A from a 84 GS as stock as possible. Cold starts are quick and easy, hot starts the same. However, if I run her, then let her sit for a while (achieving luke-warm status), she is very difficult to start and blows alot of smoke when she finally decides to. Any ideas on what could be causing this?

 

Getting flooded due to gas siphoning out of the bowls of the carb. Check the bowl levels when
you shut it off and then right before you start it. See if they are low on one or both bowls.

DivinDriver had an issue with this, he can probably fill in the details.

 

My 85 12a is the same. I started it and drove it to a nearby convienience store, the choke had "just" released....and I nearly killed the battery trying to start it when I was done at the store.

Pulled the top of the air cleaner and held the gas pedal wide open and it fired up. Flooded...I made it worse by giving it the usual few pumps on the pedal like I do when its cold. Sorry, not much help. but thought I would relate.....

 

Too much pressure in the tank. Take off the gas cap and listen for an exhale. The pressure build up can be so strong it can push through the fuel pump and into the carb. It tends to flood the carb when the power is off because the float bowl vent solenoid is closed. And you know what happens next.