Some good suggestions above. I believe the trouble is a blocked/dirty idle port in the Gurtner carbs throat. When these little fuel delivery holes are plugged, The vacuum that is created by the motor is not enough to keep the motor fed and it wont idle.
Closing the choke creates a higher vacuum in this area and the main jet tube will begin to feed the motor and it keeps running but it blubbers and smokes more.
The first thing you can try before you start to take the bike apart is to start the bike on the stand and rev the motor as high as it will go, then quickly apply the choke lever till the motor almost dies then release the choke and let the motor rev up high again. Repeat this several time and see if the bike will idle. This action can sometimes clear dirt out of the assages by using suction from the motor.
Here is a picture of the two idle passage holes in the throat of the carb looking down through the area where the throttle slide has been removed.
Out of site is a larger brass nipple which is the main jet feed tube.
You can verify if the passages are clear by spraying carb or brake cleen in this outer hole (pointing with pen) using the little red tube that comes with the can. It can be hard to observe the two holes gysering the spray because the brass main jet tube will produce most of the spray and more often the not right in your face.
You can use the tip if an ink pen to plug the brass tube so you can better see the smaller holes.
A good tip for removing you Gurtner carb and air box is to loosen the main clamp at the intake then rotate the top of the carb towards the passenger side of the bike.
Then remove the throttle linkage top plate screw. (shown here as the rusty beat up screw)
This will allow you to slide out the whole throttle slide and linkage so you can just let it hang there on the bike.
Then you can slide off the whole carb/airbox assembly and move it to the bench.