My carburated engine is idling fast?

I have a car with a carburated engine (1981 Honda Prelude) and sometimes when I get it up to around 3000 rpm it gets stuck there, although it will slow down when I brake, when I engage the clutch it shoots back up to 3000 and stays there while I'm idling. If I remove the air filter and use my hand to block the air intake valve, the engine stalls out and this seems to fix the problem. But: (1) what's going on? (2) why does this fix it temporarily (3) what can I do for a more permanent fix? Thanks!