Unhook the throttle cable from the carb the work the trigger
It should always close easily.
with a screwdriver or similar work the throttle on the carb with no cable connected , it should snap back
If not and there is nothing obstructing it then suspect the throttle return spring on the carb.
Also suspect the carb mounting screws, people do them up like they are holding an aircraft carried against a hurricane which can distort the bodies and cause the throttle to bind.