I have a Subaru Robin EX17 that runs great as long as the choke is on. Any idea's why ?
This is the classic symptom of a clogged pilot jet. See item number -5 in the attached diagram link. I have never worked on a Subaru motor before, but from the diagram it seems pretty basic. The pilot jet supplies fuel at low speed or when the throttle is closed. On most carbs, the pilot jet is accessed through the float bowl. The Subaru's pilot jet appears to screw in through the top of the carb.
The pilot jet orifice is tiny. It becomes clogged when the engine sits unused for awhile. The fuel in the carburetor evaporates, leaving behind some crud that hardens inside the jet passage. Flipping the choke lever on allows enough extra fuel in to keep the engine running, but it won't run well.
Remove the pilot jet and hold it up to the light to see if the hole is open. If you cannot see light through the center of the jet, it's plugged. A squirt of carb cleaner and a little poking with a small sewing needle should work to clear the passage. Squirt some more carb cleaner into the hole on the carb where the jet mounts, and maybe use a little compressed air, too.
While you're at it, remove and clean the main jet and emulsion tube. These are items -20 & -22 on the diagram and are accessed through the float bowl.
The photo is a main jet from one of my Honda scooters that I was working on last week. It had the exact problem described above after it sat unused for 18 months. I attached the photo just to show what a clean hole looks like.
EX17 Carb Diagram