Carb heat = no rpm drop

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.

Thanks much for the responses.

I don't think the tach is sticking but it will be checked in a week when it goes in to my local mechanic to deal with a few squawks.

I'm hoping the exhaust and shroud don't need an overhaul but I'll check that also.

The testing was with the cowl on.

One thought is that the full rich position of carb is not rich enough. Any thoughts?

"figurado" wrote in message
...

Thanks much for the responses.

I don't think the tach is sticking but it will be checked in a week
when it goes in to my local mechanic to deal with a few squawks.

I'm hoping the exhaust and shroud don't need an overhaul but I'll check
that also.

The testing was with the cowl on.

One thought is that the full rich position of carb is not rich enough.
Any thoughts?
--
figurado

One way to check the idle mixture is to set the RPM at 1,000 when full rich.
Slowly pull the mixture knob out while watching the tach. You should see a
25-50 RPM increase just before the engine dies. If you do not see the RPM
rise then you may be to lean and it should be checked. If there are any
doubts about the heater shroud and exhaust system in general get it checked
too, with winter coming on you will be using it.

Allen