Carb Ice

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170C
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Carb Ice

Post by 170C »

I'm curious what indications most have experienced when getting carb ice. In almost 30 yrs of ownership I have only had 2-3 carb ice experiences. All were discovered when I felt unusual vibration in the rudder pedals. I had not noticed a decrease in rpm although I probably should have. Application of carb heat cured the problem. Yesterday on a breakfast flight, in really humid conditions, all of a sudden my engine (O-300B) started running really rough and a decreased rpm. I applied carb heat, but most likely not for long enough. Roughness continued as I began a diversion to the closest airport. After maybe a minute or two engine smoothness resumed. With a smooth engine I elected to return to my home airport and the engine ran smooth, as usual, for the 30 minute return flight and an inspection with the upper cowling off didn't reveal any adnormalities. It certainly got my attention since I had never experienced this previously! I have not heard others discuss their carb ice experiences other than seeing reduced rpms so this is why I am asking.
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Re: Carb Ice

Post by GAHorn »

Sudden roughness which clears up is usually either water in the fuel or a slow/sticking valve.

Carb ice I’ve experienced a dozen times or so but mostly only in small Continentals such as the C-65 thru 90’s and the O200. In those engines it usually presents with a reduction in cruise RPM and may not be noticed for awhile. The only time I’ve ever had carb ice in an O300 it also was noticed as a reduction of RPM at cruise. We were enroute to the New Orleans mid-year board mtg and I had cruised at 2450 for about 30 mins and noticed the RPM was about 2400. I added power and tightened the throttle friction. About 5 mins later it was again at 2375-2400 and I pulled full carb heat. It stumbled dropping RPM to about 2300 then recovered to about 2400. When I shoved the carb heat knob back into the cold position, the RPM rose to about 2550, so I reduced it back to my favored 2450. No other carb ice has ever been experienced by me with this engine.
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ghostflyer
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Re: Carb Ice

Post by ghostflyer »

I fly in a very humid area and when I had the O-300 fitted ,carbs heat was always applied as I entered the circuit area. After reducing power it would ice very quickly ,especially on finals as it was always very steep due to noise abatement. In the early days I often landed dead stick . 60,secs on the ground ,it would run as nothing was wrong. This is where one learns a couple little tricks in aircraft handling .BUT after fitting the “shakey “4 banger ,I have had no issues with Carby ice whatsoever . Except when flying through a very moist cloud once ,the carby heat was applied. Even both engines were carby feed both had very different characteristics.
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Bruce Fenstermacher
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Re: Carb Ice

Post by Bruce Fenstermacher »

Usually I notice a loss of some RPM. 50 or 100 RPM. Usually the first reaction is to squeeze in a little throttle to gain back the RPM thinking the throttle had some creep. Shortly there will be another drop and usually I've had my hand on the throttle so I know it didn't creep this time.

Pull carb heat on full and leave it on. 5 minutes won't hurt anything. You will likely get some more rpm drop when you do pull it on. Engine may run ruff but will clear. After 5 minutes, If you don't think you are in immediate icing conditions you might push the carb heat off slowly to see if you regained your full RPM. If you feel there is any chance your still in icing conditions put the heat back on full.
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Re: Carb Ice

Post by cessna170bdriver »

My O-300 has had carb ice in cruise a couple of times, 500-1000 feet below an overcast (low temp-dew point spread). I noticed a very slight change in engine note, couldn’t see an RPM drop, but I did notice about a 1/4” Hg drop in manifold pressure. Application of carb heat caused a temporary roughness, then manifold pressure returned to what it had been. After a couple of cycles, I just left the carb heat on.
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Re: Carb Ice

Post by N2625U »

Bruce Fenstermacher wrote:Usually I notice a loss of some RPM. 50 or 100 RPM. Usually the first reaction is to squeeze in a little throttle to gain back the RPM thinking the throttle had some creep. Shortly there will be another drop and usually I've had my hand on the throttle so I know it didn't creep this time.

Pull carb heat on full and leave it on. 5 minutes won't hurt anything. You will likely get some more rpm drop when you do pull it on. Engine may run ruff but will clear. After 5 minutes, If you don't think you are in immediate icing conditions you might push the carb heat off slowly to see if you regained your full RPM. If you feel there is any chance your still in icing conditions put the heat back on full.
Exactly my experience too a couple of times with a Connie O300C engine. Upon application of carb heat lose some more RPM, may run a little rough then smooths out, then it slowly comes back some as the ice is melted.
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Re: Carb Ice

Post by 170C »

Thanks for the reply's. Seems most carb ice occurrences comes on slow and get worse, but are more of a loss of rpm rather than a sudden rough engine. George suggested it might have been a sticking valve. I know a lot of O-300 drivers have experienced sticking valves and some say the vibration was violent. So far I haven't had a known sticking valve. This vibration certainly got my attention, but wasn't violent. Hoping it was ice, not valves. Will do a lunch run tomorrow and see how it runs.
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Re: Carb Ice

Post by N2625U »

My experience with sticking exhaust valves is it happens when the engine is cold. Sometimes called 'morning sickness' along with other things not fit for print in this family forum. One thing about a sticking exhaust valve is it will vibrate! It seems that it knocks out one cylinder and a neighboring cylinder will have partial power.
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Re: Carb Ice

Post by Bruce Fenstermacher »

My experience is cooling of some type such as cooled slowly after a flight, is required to stick a valve is position. A 4 or 6 cylinder Continental, running with a stuck hard open valve much over an idle causes the most severe shake and resulting loss of power you want to feel in the air. Shaking so hard you can't see the instruments hard. It does not compare to the roughness caused by carb ice or water ingestion of melted ice.

A valve that is tight, and maybe ever so slightly slow too close, can cause roughness in a cold engine that is hard to detect and confused with the start of carb ice, but it won't clear with carb heat. However it will clear with engine expansion due to engine heating over run time which may coincide with the application of carb heat.
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Re: Carb Ice

Post by 170C »

To close out this post, I flew on Tuesday for 1.4 hours with no issues. Air was much dryer than this past Saturday. I'm going forward with the theory that what I experienced was somewhat major carb ice. Again, thanks for your input.
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Re: Carb Ice

Post by hilltop170 »

I have had card ice several times with the O300-D. Each time, I saw a drop in EGT before I noticed any drop in RPM.
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