Right well if you have a PH2 172 it
could be this
Excuse the picture
(do this with the engine off!!)
Unscrew the clip around the large black elbow
Pull of the elbow to expose the butterfly (the flap)
Now get a mate to put the ignition on (dont start it)
Get him to put the throttle down to the floor
Look inside the area around the butterfly if it is dirty this needs to be cleaned, use carb cleaner and a cloth or similar to remove said dirt.
You should notice the car idles that little bit better.