The Cessna 120-140 Association Forum

Hi all, I'm the proud new owner of a 1946 140, S/N 10109 up in northern Minnesota. I have just over 12 hours in the airplane now and am trying to work out what's normal and what is something to have fixed. I've noticed that when in cruise (2400 rpm) at between 2500 and 3500 MSL with an outside air temperature of around 32F, I start feeling what I think is the onset of carburetor icing. I start to feel the occasional miss, and it seems like the RPM starts to drop without moving the throttle. Hard to tell if I'm actually losing RPM (or just climbing), but pulling carb heat on fully causes a definite stumble that then smooths out. Once stabilized, I can push the knob back in until it's about a 1/4 to 1/2 inch out from the panel. From then on, with partial carb heat, the engine is totally happy.

I've always heard rumors that small Continentals were particularly prone to carb ice, but it surprises me to be encountering it at fairly high cruise power settings. Anyone else out there had experience like this? Or do I have something else potentially wrong that just feels like carb ice? For reference, I'm running a C85-12F modified with STC SE00979AT for O-200 crank, rods, and pistons, still rated at 85hp.

Yes, they are prone to carb icing. Usually (for me, at least,) the symptom is constantly having to "readjust" the throttle to maintain RPM. If I don't catch on, there is a rude awakening when you finally pull carb heat on when entering the pattern... ("Why did you cut the engine when you entered downwind?")

I strongly recommend you avoid using partial carb heat. If the icing progresses, you may not have enough remaining carb heat to get rid of the ice.

I have a C90, which is a lot like yours, but with a different cam.

So you'd recommend just periodically pulling full carb heat on, letting the engine clear, then turning it completely off again? Presumably frequently enough to avoid unplanned power off 180s in the traffic pattern...

I've got about 12 hours in my '47 and two weeks ago while climbing i hit carb ice -very apparent and it came on suddenly. it was about 34 degrees, humidity high, dew point about 1 degree spread. pulled carb heat and turned around, checked mags to make sure nothing wrong there -took about 6 minutes to clear out. I was on a prog check, and my prog pilot said i should take a look at the system before we go up again. so, pulled the carb heat box, checked plugs, all good (i ended up changing the plugs, but they were quite serviceable - but really old and not available anymore (EM41E). i called my ferry pilot and he said that the little continentals were very prone to icing and not just at reduced power. will monitor conditions for icing potential and check for ice as part of my standard procedure going forward. -perhaps a carb icing indicator is in my future? I have a C85-12

bill_e wrote: ↑Fri Nov 25, 2022 7:56 pm So you'd recommend just periodically pulling full carb heat on, letting the engine clear, then turning it completely off again?

Yes, as soon as you notice it. IIRC, standard practice is full heat and full throttle.

-took about 6 minutes to clear out.

That doesn't sound right. I'd give the carb air box and the carb heat muff a good looking over.

I was at full power climbing through 2000' on my way to the practice area (airport at sea level in the pnw). I immediately applied full carb heat, leveled off, and pulled power back to 2200 rpm. When i applied full carb heat the engine ran even rougher -i looked for a field if things got worse. By the time i joined the 45 to downwind, the engine had settled out and caused no further problem during approach. I pulled the carb box, it's working normally (i'm also an A&P), heat muff looked good too. Looking over the icing probability chart, i was definitely in a high probability for carb icing at any power setting (gorgeous morning). In retrospect, i wonder if i should have maintained a climb at full power with carb heat on? the aircraft is not hangared, but it does have a roof over it's head and it was really cold. before taking off i let the eng come up to the lower acceptable temp band (about 15 min), and i only applied carb heat during the run up and got the normal rpm drop expected. engine developed full power at take off.

use this chart as a guide for possible icing https://www.boldmethod.com/learn-to-fly ... en-to-you/

Yes, this is the prob chart i laminated, and put into both aircraft post icing incident -thanks!

rominoff wrote: ↑Sat Nov 26, 2022 9:51 am I was at full power climbing through 2000' on my way to the practice area (airport at sea level in the pnw). I immediately applied full carb heat, leveled off, and pulled power back to 2200 rpm. When i applied full carb heat the engine ran even rougher -i looked for a field if things got worse. By the time i joined the 45 to downwind, the engine had settled out and caused no further problem during approach. I pulled the carb box, it's working normally (i'm also an A&P), heat muff looked good too. Looking over the icing probability chart, i was definitely in a high probability for carb icing at any power setting (gorgeous morning). In retrospect, i wonder if i should have maintained a climb at full power with carb heat on? the aircraft is not hangared, but it does have a roof over it's head and it was really cold. before taking off i let the eng come up to the lower acceptable temp band (about 15 min), and i only applied carb heat during the run up and got the normal rpm drop expected. engine developed full power at take off.

Having flown in the Great Lakes most of my life, we get icing in the fall, winter and spring. Having flown in the pacific NW very, very seldom the few times I did, I got HOLY XXXX ice the only time in my life. I think your climate is more conducive to icing even than ours. I've experienced icing at full power, typically it clears itself in less than a minute. One rule of thumb for anyone concerning icing, when you have icing and pull carb heat, it's going to run rougher than a normal carb heat application. That being said. Do, not, push, the carb heat back in!!! until it clears. Numerous C-150's had off airport landings because the pilot pushed the carb heat back in before the ice melted and the engine quit. Don't let the rough running engine scare you into pushing the carb heat back in, thinking the extra roughness is going to go away.

I've read that Prist Loflo will eliminate or reduce fuel evaporation cooling in the carb. Fuel evaporation cooling accounts for about half the temperature drop. Verify that it is approved by TCM (or whatever they are called today).

https://www.aircraftspruce.com/catalog/ ... prist2.php