Carb Ice

[G-MDAY]

Hi all

Carb Ice… Again
I’m not sure if I’ve broached this subject in this forum before, but here we go. This situation has occurred twice — once in October, in very similar weather conditions with regard to moisture in the air.
First time:
About 20 minutes into the flight, I decided to do a fast low pass at my previous airfield. Carb heat was on for the descent, but it was a very fast, long descent followed by a climb with the carb heat off. Almost immediately, I got rough running with quite aggressive vibration, and it lasted for a good few minutes. Eventually it cleared, and I headed back to my airfield with my tail between my legs — keeping a very good lookout for potential landing sites .
Second time (Monday this week):
It was much colder than October but not freezing — a dull, damp day with plenty of moisture in the air. I had a long-ish taxi for fuel, then quite a long taxi to pick up a departure on Runway 03. I did all the checks and actually applied carb heat while waiting for approaching traffic. Carb heat off, I lined up on 03, and at about 50 ft with WOT I got rough running again. I was still making a little power but was climbing very slowly into rising ground (a large residential area). I applied carb heat at about 200 ft and continued — the rough running continued for quite some time. I didn’t lean the mixture.
Once at about 800 ft, I elected to return to the airfield, and the rough running stopped. I flew another five times that day with no such problems occurring.
I have questions, and I bow to the collective experience of the group. What do you all think, and how can I combat this? I currently have my own opinions but want a kind of consensus.
Thanks all for your input.

[cessnut]

Sounds more like water in fuel.

[GAHorn]

Yup. Doesn’t sound like carb ice. Sounds like possiblhy contaminated / water in fuel.

It always occurred After a descent (with Carb Heat On)…and During a full-power climb..??
Did the descent occur with SOME power applied..?? or was it done at/near idle during the carb heat application…?

One scenario that occurs to me: Is your airplane hangared? or stored outside? Is it possible that rain or a wash job been recent?

I’ve discussed this in the past …(and not everyone agreed with me, but this is a fact)…. the Sceet or Scat hose that supplies air to the carb from the muffler-shrouds…. can collect water and allow it to “pool” in low-lying loops of that hose. A pitch-up with high power settings can Suck that water into the carb-intake…and cause serious rough-running.

The preventive measure is to take an awl or ice-pick and poke a hole in the lowest portion of the Sceet or Scat hose so that any water from rain or a wash job will Drain out at that low-point. Don’t worry about introducing an air-leak with that small hole. It will be insignificant and those hoses aren’t air-tight anyway.

Beware of low-lying loops of intake hoses that cannot drain.

[G-MDAY]

The aircraft is kept inside
And not flown in rain at all
Washing of the airframe is done sparingly and never with a power washer
So I’m not sure about the water aspect of this but I’ll check

Thanks for your input

[n2582d]

What is your carburetor temperature reading during these events? Is your carburetor temp probe in the airbox or carburetor throat?

[Bruce Fenstermacher]

What is your carburetor temperature reading during these events? Is your carburetor temp probe in the airbox or carburetor throat?

Wild assumption that he has a carb temp probe and gauge Gary. I've owned 5 aircraft, two of which were 170s and have never had a carb temp probe and gauge.

[GAHorn]

I’ve seen only one 170 with one. All it did was make the owner worry and run around with carb heat on most of the time.

I’ve got 1K+ hours in C-170x and have Never had carb ice in a 170 despite doing a lot of flying near the Gulf Coast. (Houston is notorious for humidity.)

When I instructed in C-150s in Houston we experienced quite a bit tho’. I suspect the different engine installations made the difference.

[n2582d]

What is your carburetor temperature reading during these events? Is your carburetor temp probe in the airbox or carburetor throat?

Wild assumption that he has a carb temp probe and gauge Gary.

… Up at about 2000ft trimmed out plane flying lovely… about -1 or -2 air temps, I am a regular carb heat on off etc while keeping a close eye on the carb temp. So I lean out the mixture … .

The wild assumption is that he’d take notice of the carb temperature when it seems the engine is about to quit, especially since it’s way out “in right field” so to speak. This from Paul’s Dec. 20, 2024 photo:

IMG_1593.jpeg

[Bruce Fenstermacher]

Gary, I apologize.

I momentarily got wild thinking that Pauls 170 would be one of the minority of 170s with a carb heat gauge. Heck maybe my impression is wild that most 170s won't have a carb ice gauge.

As my judgement seems to be wildly off, I will go out on a limb and say that it is wild that you either remembered Paul had a carb heat gauge from viewing his picture 1 year ago, or that you even thought to go look to see if he might have posted a picture before and that once found the picture showed a gauge.

Wildly good work, as usual Gary.

There I think I've gotten that wild word out of my system. Where did that come from anyway.

[schnepel]

You also might try rolling the airplane one (1) wheel at a time up onto a small 2” x 6” board for example in order to raise one side of the aircraft so that the fuel tank on the side of the wheel on the board is higher than the opposite wheel still on the pavement/grass. Drain the high tank sump, and see if any water comes out, you might be surprised, and find water that’s collected in the tank over the years even if you've been draining the sumps before every flight. Repeat the procedure for the other wheel and check that tank also.

I had a very similar situation several years ago when I flew power line patrol in a Cessna 140A for a major utility company here in NW Florida. Kept having rough running when I was maneuvering (sharp turns when the line I was following would turn in another direction). I discussed the issue with a gent who told me to try raising each side of the aircraft seperately to see if any water came out. I was surprised after I drained out more than 1/2 cup of water in each tank. That opened my eyes, because I always drained fuel from the sumps before flight as well as after each re-fuel. Evidently the ridges in the aluminum tank were trapping some water. Just a thought.

[G-MDAY]

I didn’t visit the aircraft this weekend …. Too cold but I’ll be going next Friday and I’ll have plenty of checking to do….
Thanks all for the input .

Paul

[n2582d]

No need to apologize Bruce! I wish it was a good memory that led to the question about Paul's carb temperature gauge. Truth is I got lucky after I read his opening line, "I’m not sure if I’ve broached this subject in this forum before, but here we go." Reviewing his past entries to see if he had broached the subject earlier, I came across the line about his carb temperature.

I think Paul's gauge might be more useful if it was mounted where it was more easily seen or maybe rotated 90º clockwise -- although that would look weird. I also see that Paul has a manifold pressure gauge -- one that's ready for that turbo-supercharger STC with 75" of boost. UMA 7-300-35 is a 3 1/8" gauge that reads up to 35" which might work better for detecting manifold pressure changes on a normally aspirated engine. And that blank instrument hole could use a Insight G4 Single, CGR-30P or a Garmin GI-275. An engine monitor would be helpful in diagnosing the engine roughness -- (is it a sticking valve, carb ice, or water in the fuel?) -- but would ruin the aesthetics with a digital gauge in the midst of all those classic analog instruments.

[G-MDAY]

Yes I also have an AV30C …… fuel flow and an EGT/CHT (Install not fully completed yet) the space on the dash will be filled with a Garmin CDI

[rschreiber]

This sounds very similar to an engine issue I had several years ago. I’d take off, get a rough running engine with some power loss around 100-200’ on takeoff as RPM’s would increase. Once level on the downwind, I’d reduce power and the engine would smooth right out. The problem was a partially collapsed spring on one of the exhaust valves.

[G-MDAY]

Just following up on this post. 1. Plate fitted to oil sump hole in the front cowling 2. checked all Spark plugs cylinder 1 was pretty black …. cleaned all and replaced. 3. Cold Compression check, worst was 79/80 4. checked for holes and correct function of the carb heat. 5 Checked for water in Fuel. 6. Checked mixture control. 7. Boroscoped checking and photographing valves. We found nothing noteworthy with anything we think in this case it was incorrect leaning during a long taxi and insufficient build up of heat in the system causing a carb ice situation …… on application of full throttle …. which seems feasible "