Don't be afraid to take your carburetor apart. You will have to eventually when it gets clogged and needs to be cleaned. The top two screws will lead you access to the throttle slide and arm. This arm sticks into the choke lever and keeps it in place until raise the throttle slide to the highest position.
It's a very intricate system and you'll need practice getting back together once you take it apart. To see how it really works, take the airbox off of the back of the carburetor and then the metal air filter. In the back of the carburetor, you'll see the choke lever. Next, take off the top two screws and take out the throttle slide and arm. SNAP, the choke should go back. You'll have to practice sliding the throttle slide back in while shoving the arm into the right place. The arm should fit in so that it sticks between the carburetor body and the choke plate. Anytime I work on a del orto, I'll double check the choke.
Your throttle cable could have stretched and there should be adjustment screw not only on the top of the carburetor but near your throttle assembly on your handlebars.