I suspect your heat muff / shroud is leaking air...especially if it is a
closed system.

BJ

figurado wrote:

I just bought a 1973 Cessna 172. During runup at 1700 rpm there seems
to be no noticeable drop in rpm when carb heat is applied. We
inspected the operation of the cable, and flapper and inspected the box
and everything looked fine. We inspected the ducting for blockage and
found nothing. What might explain this? Thanks in advance for your
help.