If you mean the throttle butterfly it should be held open by a spring at rest.
When running a governor mechanism in the crank case or an air flap in the cooling nacelle closes it against the spring to regulate the speed. Maybe the spring has fallen off?
Yes...the spring is gone, reply 37. I was told to start the mower with the butterfly closed. Which I couldn't.
You got me wondering now.
Just been out and started my mower.
Popped the air box off and shone a torch up the air intake when the mower was cold and prior to starting it. The butterfly was fully closed (as in blocking air from entering the carb). As it warmed up it opened the butterfly to allow air in to the carb. As a double check I looked at my strimmer and the bike, which both have a manual choke. When I flicked the lever to choke, it blocked the air from entering the carb, just the same as the mower. Choke being rich, air being lean.
As a triple check I looked on YouTube.
https://youtu.be/aNi9sJA4PFIIncidentally, it may start without choke but will struggle to rev until warmed up.