The Cessna 120-140 Association Forum

johnhicks wrote: ↑Thu Dec 01, 2022 2:03 pm Something I would like to hear others opinions on is my my primary flight instructor has always told me to never pull carb heat on during cruise or high RPM as it could lead to detonation. If I've wanted to "check" during cruise, I've always pulled the RPM back first before pulling it on. Anyone else do this?

Not so, detonation is caused by high heat, faulty ignition timing or too low a fuel octane for the engine. Utilizing carburetor heat actually causes a richer mixture due to the lesser air density caused by the heat (less molecules in warmer air). As a result the fuel mixture in the full rich position will actually cool, not lead to higher heat to cause detonation.

Ask yourself this, when does the engine produce more heat, at a lower RPM or when it's running at cruise? When the power is pulled back to a lower RPM less heat is generated, heat that might be needed to melt any ice. As a rule of thumb I always pull carb heat midway on downwind at cruise power in order provide enough heat to melt any ice if present before the throttle is closed.

Has anyone installed a carb ice detection system?

I just had carb ice all the way to Phoenix and again on the way back to LA so I guess one's mileage may vary with climate, leaning technique, carb setup and temp/dewpoint conditions on the day. I seemed to get less icing with more leaning. It hit hard on climb out from SMO at 5000 feet with ambient air temp just above freezing and 8 degree temp/dew point spread. I dont know what to say except I ended up needing it for the remainder of the trip at 7500 ft.Then iced again on the way back. I feel like leaning is critical due to evaporative cooling from excess fuel, but what do I know. Lean and lean again afrer pulling carb heat.

6643 wrote: ↑Thu Dec 01, 2022 11:23 am

tonycondon wrote: ↑Wed Nov 30, 2022 10:41 am I'm over 300 hrs in my 120 and have never experienced Carb Icing in it...

I think if you use carb heat religiously whenever you reduce power below about 2100 RPM you'll rarely encounter carb icing. In about 35 years I can only remember 2 instances where it was a real problem.

An ounce of prevention, and all that...

I missed this, ...........not sure why. Which engine and carb do you have?

You can make a simple carb temp probe from the following items:

Omega: SMPW-K-M & SA2C-K-72
Thermoworks: MTC mini thermocouple meter

I've found it interesting to watch the temperature rise when I apply carb heat. Also, now I know when the manifold is approaching the freezing zone.

Cheers!