You'll have a rethink about blasting thru any turbulence at normal cruise after that.
Moderate or above, actually.
Common misconception is that you are purely looking for a drop in rpm when you check an aircraft that has carby heat.
You will observe an initial drop due to the reduced density of the ingested air (less oxygen molecules per stroke = less bang/burn), but you should then wait for at least 5 seconds to see if the rpm begin to increase.
Cause?
Low rpm, high humidity,
Perfect conditions to develop carb icing.
Ice present has the same effect as closing the throttle.
You will not achieve the stated performance.
If the rpms rise, positive indication, but you will have cleared the ice, no drama. If you have a fair way to taxi and you had a positive indication, re-check prior to brakes release. A return of rpm to above the initial setting when you return the carb heat to OFF could also be a positive indication.
Secondary confirmation of full power availability is by checking static rpm as full throttle is achieved on take-off.
RPM too low= not enough power from the engine. Not good.
RPM too high = either prop worn beyond limits (not good) or headwind. Unless you have 60 kts headwind, you shouldn't see 2500rpm at the start of your take-off roll.
