Disconnect the cable from the throttle housing and make sure it moves freely inside the cable housing (also ensure the throttle body is not sticking at the bars), if its free return it back to the throttle body, but dont tighten it all the way up. Try it and if its still sticking open the carb top and remove the cable and slide. Give the slide a light rub with 0000 grade wire wool and a light oil.
With the slide removed from the carb, hold the carb top and twist the throttle, the spring on the carb slide should compress and return easily, if not thats your problem. If it does return quickly and smoothly refit the carb body and slide ensuring the needle is seated correctly and also the needle is not bent.
As stated above, make sure the slide is fitted the correct way round.
Sorry for the "sucking eggs lesson" but sometimes it better to begin right at the start.