Carb heat = no rpm drop

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.

In article ,
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.

Is it possible the tach is sticking? Does a strobe on the prop confirm no
RPM change?

Had the same problem with the 182. Sent the muffler down for overhaul and
more important, sent the shroud down for overhaul.

25 RPM drop before; 200 RPM drop thereafter. $500 fix.

Jim



"figurado" wrote in message
...

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.


--
figurado

A '73 172 has a little open-ended box on the right front exhaust
stack. Carb heat air is drawn from this thing. If you are running the
engine with the cowl off, there's so much blast from the prop that the
carb won't get any heated air, and there'll be no RPM drop.

Dan

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

Give the good folks at the Cessna Pilot's association a call. Bet
they'll have an answer. They are good group to join too. There are a
ton of knowledgable people on their web forum.

http://www.cessna.org/

Helen