Hey gang,

Had to swap the carb for new last fall and installed a carb temp gauge, never had one before. I expected the temps to be lower than they are, it never really reads lower than ambient air temp. What happened to the cooling happening in there?

Yes it's installed correct, analog gauge with thermocouple mounted in proper spot on the carb.

It reads correct, power on in the hangar (cold engine) it reads ambient temp.

Shows rise with carb heat application.

But in cruise it's about ambient temp, never cooler. Pull the power back and the temp increases, and on the ground after landing the temp could be almost 10ºC above ambient. I can attribute this to low MP sucking warm carb heat air past the flapper in the air box (old cessna airbox, no internal seals). But in cruise should I see lower temps?

Thanks y'all. Not gonna stop my season, already did first camping trip of the year!

Does your 170 have an induction air configuration where the ducts run through the oil sump?

Good question, Yes

Then my guess is carb temp matching OAT in flight is coincidental and won’t always be the case. It’s probably working correctly.

What engine? What is the RPM drop with full carb heat at 2400 RPM?
DENNY

Yeah the carb heat is working fine. I thought I’d see more of a temp drop in there is all.

Drop at 2400rpm in cruise? Gotta be close to 100rpm

My Insight G2 engine monitor showing both OAT and carb temp. Engine is Franklin 165.

May not be a bad idea to do an ice bath test on the carb temp probe to check for accuracy. Remove the probe from the carb, reconnect to the harness and submerge in a class filled 70% with ice, 30% water. The gauge should indicate close to 32°F. You can perform a similar test with boiling water, looking for roughly 212°F as a second data point.

darata wrote:May not be a bad idea to ...

Nice to see you still watching the forums Dave! Thx-

Depends on whether the probe reads air temperature or carburetor body temperature. The analog Cessna carb temp gauge that was in my 182 appeared to read carb throat air temperature. In cruise the gauge always read well below ambient and when I applied carb heat it would rapidly rise off-scale.

The JPI probe I currently have is solid brass, without a thermal break between the body and probe tip. It screws into the threaded boss in the carburetor, so I assume it mostly senses the carb body temperature. That’s ok, I guess, since ice will accrete on the body. But I’d prefer to know just the air temperature instead.

Mostly, I'd just continue to use carb heat as you would with no carb temp gauge. In airplanes with carb temp gauges, I've never done anything different than without.

Your mileage may vary.

MTV

Agreed. Nothing changes. New data point, and sometimes more data isn't necessarily better.

I think it may be more body temp that throat temp. Still curious where all the hot air is coming from at low MP, temp rises as if carb heat applied. Appears to be sucked unfiltered warm air from inside the cowl at low MP. Same as before, just never knew it. Tempted to just remove the thing, it's not telling me anything I need to know, nothing different than before. I honestly thought I'd be seeing the carb's throat temp, didn't think much into that during the install. Carb temp is carb temp right? Apparenlty not. THanks for the sounding board everyone.

Here’s a theory from a non-expert. Low MP means low air flow. If you have induction air heating through the oil sump as air flow decreases wouldn’t you expect to see a temperature rise?

Edit: I had my increase/decrease crossed up and corrected it.

The temp is measured before the sump. So if there is a heating in there you won't see it.

Low MP means the engine is sucking the hardest, any airbox leaks would pull air. That's my theory. I've sealed up the airbox best I can but it's old. I'll have a new one made whenever the motor comes up for overhaul some day.

redacted wrote:Low MP means the engine is sucking the hardest, any airbox leaks would pull air. That's my theory.

Your theory is reality regarding the low MP = high vacuum behind the throttle, but i'm not sure it translates to low pressure in the airbox. I'm also partial to the theory that something electrical is going on with your probe that's preventing it from providing an accurate carb temp that jives with OAT.