Whats Wrong With The Idle On My Briggs and Stratton Classic 35?

I have a Briggs and Stratton Classic 35...someone just gave me it last night, apparently its 18 months old...wouldnt start, when it did stayed on a few seconds then died...anyway, put some fresh fuel in it and checked the oil, cleaned up the spark plug...

Starts up ok, cleared itself out of a load of white smoke and now starts first time no problem...however it struggles to run smoothly and basically seems to have 2 settings when using the throttle, on full and cutting out completely, above half way it goes onto full throttle...below that it cuts out.

The air filter seems to be missing, and the small springs both seem to need replacing as they seem to have lost their tension...not sure if either of these are related...