The International Cessna^® 170 Association

Dave, I have an 0-320 conversion so mine is a bit different but they should be much the same as the players didn't change much just the name(s). My carb heat comes off the centre of the muffler shroud and is a 2 inch hose (from memory, possibly 1.5) and although I've never measured the heat, I've had icing and it had no problems taking care of business as well application of carb heat on my machine cases a definite drop in RPM.
I'm thinking you must have one of those carb boxes that allows heat to pass through the back of the box and when heat is selected, an internal plate blocks the exit while another plate directs the hot air to the carb. Just wondering how well it would work if you swapped the carb heat source between the cabin source and the carb/cabin heat source.

Interesting idea of swapping the duct locations. I'll add that to the list of experiments. A friend has spent two years optimizing the cowl inlets and outlets on his homebuilt to improve the CHTs and oil temps. One thing is certain: This combination of aerodynamics and heat transfer is not intuitive!

My carb air box is fairly conventional. Filtered cold air enters from the front, heated air from the back. The flapper rotates about 90 degrees, directing one up to the carburetor, and the other out the hole in the bottom.

Just trying to learn something new here Dave and certainly not trying to pester you but it sounds like 3 inch duct for carb heat is somewhat large. Is it reduced in diameter when it's attached to the box ?

No, it's three inches all the way from the muffler shroud to the air box. What is your concern with duct diameter? If the duct is smaller diameter then the air travels through it faster, losing less heat to the outside world, but whether the duct is two or three inches the air is still going really fast so I don't think it loses much heat. Just a guess, though.

Keep asking questions. I'm glad someone is interested and I hope that whatever I find can be of some use to someone else.

Well I 'm just thinking that a 3" duct is overkill for a smaller carb throat but what do I know. I'll have to post a pic of mine, but I'd try moving the supply from the end to the middle to see if that improves things.

It looks like an instrumentation problem. I put a thermocouple into the hot air side of the carburetor air box and found that the air coming off the heat shroud is about 180F, but the carb air temp probe never measured above 85F. It seems to be measuring the temperature of the carburetor body, not just the air.

I suspected that was the problem when I looked at the probe. The probe active element is swaged into the body, so there's no thermal isolation between two. When the probe assembly is threaded into the carburetor it senses the carb body temperature. JPI insists this isn't true, but my measurement says it is.

Where in the carburetor is the carb air temperature probe located? I should think there would be a considerable temperature drop just following fuel atomization.

Just another example of measuring something with a micrometer that one normally cuts with a hatchet.

Carb temp gauges are micrometers in hatchet-job carburetted O-300 engines instrumented with tachs and fixed pitch props. (Where's that tongue-in-cheek emoticon..???)