Severe carb icing ... probably.

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tpowers

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tpowers
So here's my tale from last weekend. It was 80 degrees on the surface, 60 at 6,500' which is the altitude I was cruising at in my C150, unseasonably warm, nice day. The ground-speed I was getting was only 70 kts, which was 20 knots less than my indicated airspeed. I decided I wanted to descent to 4,500' to see if I could pick up some speed.

I started the descent as usual, apply some carb heat first (not full and that may have been my mistake), wait a few seconds, then bring the power back slightly to keep the RPM's in the green as it descends. I was probably at about 65% power during the descent and descending at about 300 fpm. For the moment I decided to keep the fuel mixture where it was knowing that the carb heater was enriching the mixture.

At about 5,000' the engine started to sputter a bit run rough. I applied full carb heat and it smoothed out. I also increased power and started to adjust the mixture to try to achieve the best power setting.

Very quickly the icing continued to get worse with the engine going back and forth between sputtering and not sputtering (running fine). I started to climb slowly, something told me that even though there was no visible moisture here that there's "something" about the atmospheric conditions here that is causing my carburetor to ice up big time.

Eventually it got to the point where I could keep the engine running smoothly if all three of these conditions were met: 1) full carb heat, 2) full throttle, 3) full rich mixture. It's #3 that's confusing me maybe someone can shed some light on it.

In general if I'm running with carb heat on continuously I want to lean the mixture a bit to compensate for the richer mixture carb heat gives you. However when I tried that this time if I was anything less than full rich, even just slightly, the engine would sputter and begin to cut out.
I was able to climb a bit and the icing cleared out and I landed without any further issues. Since then I've had an aircraft mechanic look the engine over for signs of other potential causes and he could find nothing wrong.

Anyone have any ideas why I needed to be on full rich?
 
maybe it's setup too lean?....do you get a 50-75 rpm rise at shut down? If not there's your problem.
 
See the other carb ice thread and my comment about what happens when a little water in in the carb bowl. Your description is that you had more air than fuel. That doesn't indicate carb ice as much as a fuel supply issue.
 
Ok, probably nothing to do with your situation, but I had similar situation in an Aeronca Chief with a Stromberg carburetor. It would run rough, but with carb heat on it would smooth out. This persisted for awhile, but Strombergs are known to have icing issues. Me and my friend both flew it for a couple of weeks after this started, mostly on flights less than 30 minutes. Then one day I was flying and it started getting rough. I had been flying about an hour. I pulled on carb heat and it smoothed out. I pushed the carb heat back in and it was rough again. This persisted over the next several minutes. I made a turn toward a big airport just in case, and eventually the engine started getting more and more rough. It was a controlled field, and I still have the recording where I told him I had just lost my engine. It quit! The questions of how many souls on board, etc. are always scary. I was able to glide in, but had no idea what had happened. Then a friend of mine walked over and told everyone to be quiet. He opened the fuel cap and we heard air rush in. When we thought back, the issues had started when we changed out the glass in the fuel gauge and replaced the gaskets. The previous owner had put the wrong fuel cap on the plane, one that didn't vent. The only thing that saved him, and us for a long time, was the fact that it was venting through the improperly sealed gauge. On short flights, it would start starving the engine, but carb heat would enrichen it enough to smooth it out. On the day it quit, I just went further than we had been flying.

My long-winded story is just to say, it might not be the obvious carb heat!
 
Checkout_my_Six said:
maybe it's setup too lean?....do you get a 50-75 rpm rise at shut down? If not there's your problem.
Click to expand...

Hmm I didn't notice a rise at shutdown. With the exception of this one occurrence I have a lot of play in the mixture. I can usually pull it out quite far before it starts to stumble.
 
Stewartb said:
See the other carb ice thread and my comment about what happens when a little water in in the carb bowl. Your description is that you had more air than fuel. That doesn't indicate carb ice as much as a fuel supply issue.
Click to expand...

That's what I was thinking as well but outside of this one occurrence I haven't had an issue. No water was detected in the fuel, I had recently fueled up before the flight, it's all gravity fed so there's no fuel pump. However the one thing that I thought of was that even though I didn't see any water when I tested it, and even though I drained from the sump, I did have the plane parked for a week with less than 1/2 tank which could permit water to condense in there. So I can't rule out water contaminated fuel entirely.
 
Interesting, thanks. Yeah carb heat is designed to solve one problem but has a side affect which in some situations that might solve (or cause) other problems.

Lowflynjack said:
Ok, probably nothing to do with your situation, but I had similar situation in an Aeronca Chief with a Stromberg carburetor. It would run rough, but with carb heat on it would smooth out. This persisted for awhile, but Strombergs are known to have icing issues. Me and my friend both flew it for a couple of weeks after this started, mostly on flights less than 30 minutes. Then one day I was flying and it started getting rough. I had been flying about an hour. I pulled on carb heat and it smoothed out. I pushed the carb heat back in and it was rough again. This persisted over the next several minutes. I made a turn toward a big airport just in case, and eventually the engine started getting more and more rough. It was a controlled field, and I still have the recording where I told him I had just lost my engine. It quit! The questions of how many souls on board, etc. are always scary. I was able to glide in, but had no idea what had happened. Then a friend of mine walked over and told everyone to be quiet. He opened the fuel cap and we heard air rush in. When we thought back, the issues had started when we changed out the glass in the fuel gauge and replaced the gaskets. The previous owner had put the wrong fuel cap on the plane, one that didn't vent. The only thing that saved him, and us for a long time, was the fact that it was venting through the improperly sealed gauge. On short flights, it would start starving the engine, but carb heat would enrichen it enough to smooth it out. On the day it quit, I just went further than we had been flying.

My long-winded story is just to say, it might not be the obvious carb heat!
Click to expand...
 
My story about a little water in the carb bowl is a pirep. When it happened to my plane it drove me batty for about two weeks before a western Alaska pilot friend commented that he used to get water in his carb often enough that he wanted to install a quick drain on it. We all have strainers and think carb water is impossible. I'd never heard of water in the bowl. Neither had my mechanic. When we drained it after talking to my friend? Sure enough. Once that was done the plane ran perfectly again. Two weeks of tail-chasing and it was fixed in 10 minutes.

Here's a typical Marvel Scheduler carb bowl. The brass pipe in the middle is the main jet. Above that in the bowl you can see the fuel pickup. The brass thingy with the slot is the mixture control. If fits into the fuel inlet and rotates to restrict fuel into the main jet. Just an FYI pic I have on file.
 

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Hmm I'll look into that. I'm not sure if that AD applied to my model (1971 L series) but if I don't have vented caps (or at least one) and the under-wing vent got a bug in it that might explain it.

Checkout_my_Six said:
check your fuel caps also....you have an AD for one of them to be vented.
Click to expand...
 
Stewartb said:
My story about a little water in the carb bowl is a pirep. When it happened to my plane it drove me batty for about two weeks before a western Alaska pilot friend commented that he used to get water in his carb often enough that he wanted to install a quick drain on it. We all have strainers and think carb water is impossible. I'd never heard of water in the bowl. Neither had my mechanic. When we drained it after talking to my friend? Sure enough. Once that was done the plane ran perfectly again. Two weeks of tail-chasing and it was fixed in 10 minutes.

Here's a typical Marvel Scheduler carb bowl. The brass pipe in the middle is the main jet. Above that in the bowl you can see the fuel pickup. The brass thingy with the slot is the mixture control. If fits into the fuel inlet and rotates to restrict fuel into the main jet. Just an FYI pic I have on file.
Click to expand...


I remember instructors at A&P school taking about entrained water, as a very small amount of water that can be dissolved in fuel (primarily Jet A IIRC) can 100LL do the same? Can it come out of solution and collect in the carb bowl with temperature swings?
 
bnt83 said:
I remember instructors at A&P school taking about entrained water, as a very small amount of water that can be dissolved in fuel (primarily Jet A IIRC) can 100LL do the same? Can it come out of solution and collect in the carb bowl with temperature swings?
Click to expand...
I have no idea how mine got there. I was flying regularly, my strainer works perfectly, I rarely find any water in the sumps.... I'm just sharing the story hoping it'll help.
 
tpowers said:
Hmm, you're right, my model is affected. I'll go take a look at my fuel caps today. I'm not sure exactly what I'm looking for except some vent holes I assume. Maybe I can make out a serial # or something etched on the cap.

Thanks.
Click to expand...
At least one of your caps has a vent in the middle of it.

should look something like this.....
$_1.JPG
 
I found this document:
http://gasci.weebly.com/uploads/1/2/9/1/12917568/flying_in_ireland_c150_mar14.pdf

which states that some models of 150's use a fuel tank designed for the 140 and that the fuel strainers are in the rear of the tank. This is not a problem on a trail-dragger as water would settle down toward the strainer but on the 150 which sits level it could mean that water can sit in the center of the tank and not be detected by taking a fuel sample.
 
tpowers said:
I did have the plane parked for a week with less than 1/2 tank which could permit water to condense in there. So I can't rule out water contaminated fuel entirely.
Click to expand...

Half a tank of very humid air won't condense enough water to cause your symptoms. -Skip
 
Carb heat, either on or off, no in between. If the engine runs worse when you put it on, leave it on and allow the heat to melt the ice. That's what I was taught, the heat to melt the ice dissipates quickly when the engine isn't running well, so put it on full and give it time to work. Using a little carb heat is like cutting the tip off your condom, not very effective.
 
Haha, interesting analogy. Okay from now on full carb heat.

PaulS said:
Carb heat, either on or off, no in between. If the engine runs worse when you put it on, leave it on and allow the heat to melt the ice. That's what I was taught, the heat to melt the ice dissipates quickly when the engine isn't running well, so put it on full and give it time to work. Using a little carb heat is like cutting the tip off your condom, not very effective.
Click to expand...
 
PaulS said:
Carb heat, either on or off, no in between. If the engine runs worse when you put it on, leave it on and allow the heat to melt the ice. That's what I was taught, the heat to melt the ice dissipates quickly when the engine isn't running well, so put it on full and give it time to work. Using a little carb heat is like cutting the tip off your condom, not very effective.
Click to expand...
UNLESS you have a carb temperature gauge.

If you have one of those, put in just enough carb heat to keep it out of the danger zone, and leave it there. It can be eye opening.
 
My IO-360 does not have a carb heat pull knob. They say it doesn't make any ice.
Does it mean that I need to BUY ice for the Scotch in my hand while on SEO approach down to mins in Aspen? :(
 
BigBadLou said:
My IO-360 does not have a carb heat pull knob. They say it doesn't make any ice.
Does it mean that I need to BUY ice for the Scotch in my hand while on SEO approach down to mins in Aspen? :(
Click to expand...
Well, you could buy a carburetor... cheaper to just buy ice though!
 
I suspect an inlet manifold leak is making it go lean.
Carburetor heat makes it richer.
Full rich makes it richer.
Full throttle reduces the suck in the inlet manifold making the leaked air be a smaller percentage of air flow making it richer.
 
Vance Breese said:
I suspect an inlet manifold leak is making it go lean.
Carburetor heat makes it richer.
Full rich makes it richer.
Full throttle reduces the suck in the inlet manifold making the leaked air be a smaller percentage of air flow making it richer.
Click to expand...

But would that be something that would come on suddenly and go away a few minutes later? After having a mechanic look over the engine and being reasonably satisfied it didn't have any detectable malfunctions I flew it back home two days later (1.4 hours) without issue. Opted to get a hotel and wait until day rather than fly by night and chose a course with lots of safe landing spots just in case but I had no further issues either before this incident or after.

Again, other than during these few minutes I could lean normally without a problem.
 
It would have to be a system-wide intake leak. Not unheard of in big Continentals when the intake rubbers shift at the Y pipe. And yes, a leak there can be intermittent and unpredictable. I'm not familiar with the 300 induction but your symptoms are more extreme than I'd expect from a leak that could heal itself.

Has your engine start changed lately? Need more prime? Had any backfiring?
 
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tpowers said:
But would that be something that would come on suddenly and go away a few minutes later? After having a mechanic look over the engine and being reasonably satisfied it didn't have any detectable malfunctions I flew it back home two days later (1.4 hours) without issue.
Again, other than during these few minutes I could lean normally without a problem.
Click to expand...

In my experience an inlet tract leak can occur suddenly and disappear just as suddenly.
I feel an inlet tract leak bests fits the symptoms you describe for the reasons I have outlined.
I am not a power plant mechanic and have not had extensive experience with your engine.
I do have experience with engines of a similar configuration.
 
Stewartb said:
It would have to be a system-wide intake leak. Not unheard of in big Continentals when the intake rubbers shift at the Y pipe. And yes, a leak there can be intermittent and unpredictable. I'm not familiar with the 300 induction but your symptoms are more extreme than I'd expect from a leak that could heal itself.

Has your engine start changed lately? Need more prime? Had any backfiring?
Click to expand...

Hmm yes as a matter of fact I have noticed that I need a bit more prime than usual.
 
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