Cessna carb heat linkage issue warning

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Gone Flyin

Pre-takeoff checklist
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Gone Flyin
Putting my plane back together after my recent annual, I found an issue with the carb heat my A&P and A&P/AI missed altogether so I thought I'd pass it along for others to check.

carb 1.jpg


This is the air filter and carb heat box for my Cessna 150. I believe this may also be used on other model Cessna's, as well. The cable on the right side of the black box is the carb heat actuator leading into the cabin.

carb 2.jpg


With the locking bolt and washer removed you can see the lever that flips the butterfly up and down is clearly worn. In my case, it was just flopping back and forth.

carb%203.jpg


My last oil inspection came back with "silicone in the oil". When I called they said this means I have dirt in the oil probably from a defective air filter gasket. Well, the gasket may have been fine and the dirt was getting in from the butterfly being in the heat position allowing unfiltered air to enter during flight or taxi.

I was able to make a temporary fix. There is an aftermarket kit for around $100 that replaces the shaft, bearings and lever. That, or I will make a new lever.
 
Good heads up. Another thing to check on my next pre-flight. Was the lever loose on the shaft? I.e. could you feel it flip back and forth if only able to wiggle it back and forth with your hand and not visually check it out?

Secondly, I won’t comment about the differences between silicone and silicon…..
 
Nice catch! Let us know what the next oil analysis shows. -Skip
 
Matthew Rogers said:
Good heads up. Another thing to check on my next pre-flight. Was the lever loose on the shaft? I.e. could you feel it flip back and forth if only able to wiggle it back and forth with your hand and not visually check it out?

Secondly, I won’t comment about the differences between silicone and silicon…..
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Yes, I could move the butterfly up and down from the hole underneath and not have the lever move.

Silicon. My mistake. That makes more sense, thanks.
 
Gone Flyin said:
They are my new mechanics : (
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Then, the old mechanic(s) put a self-locking nut on a shaft with a cotter pin hole?
EDIT: A check of the parts manual seems to indicate a steel self-locking nut is specified. Go figure.
 
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I'm no expert, but I don't see how unfiltered air going into the engine results in "dirt" in the oil. Unless, perhaps, you have other major problems.
 
Wagondriver said:
I'm no expert, but I don't see how unfiltered air going into the engine results in "dirt" in the oil. Unless, perhaps, you have other major problems.
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Piston rings do an incomplete job of sealing the combustion chamber from the crankcase. The term is "blow by," and the net result is that dirt (dust, sand, etc) winds up in the engine oil. Oil analysis shows high levels of silicon when there is a leaky air filter or carb heat (unfiltered air) used in the ground environment.
 
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That lever should have been clamped tight to the shaft ..... in this case it was not .... either a missing washer or wrong nut .... the threads bottomed out before there was clamping pressure on the lever .

That wear is not caused by the normal use of the carb heat controls.

Engine vibrations during flight would cause the loose lever to rattle around and wear out like that.

.
 
Your cable attach bolt (I use Bug Nuts) may be binding and not freely allowing arm rotation.
 
Year C-150?

I personally do not consider just putting on any kind of nut and tightening down
an “Acceptable Repair”.

The worn part of the Lever should be renewed or the Lever replaced.

This condition could have been detected earlier.

The stopping action should take place at the Box and not when the Knob
hits the Panel.

This is generally referred to as “ Springback” or “Cushion “ and applies to
the Throttle and Mixture Control as well.
 
Kevin Holbrook said:
Piston rings do an incomplete job of sealing the combustion chamber from the crankcase. The term is "blow by," and the net result is that dirt (dust, sand, etc) winds up in the engine oil. Oil analysis shows high levels of silicon when there is a leaky air filter or carb heat (unfiltered air) used in the ground environment.
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Blow by putting dirty air into the oil? That's why I said unless you have other major problems. You would have to have a huge amount of blowby AND really dirty air for the oil to pick up enough "dirt" to show up on an oil analysis!
 
Wagondriver said:
Blow by putting dirty air into the oil? That's why I said unless you have other major problems. You would have to have a huge amount of blowby AND really dirty air for the oil to pick up enough "dirt" to show up on an oil analysis!
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Silica (the source of silicon) is found in just about any mineral dust. It gets sucked into an engine, past a defective air cleaner or via the unfiltered carb heat air, goes into the cylinder where it causes considerable wear (it's really abrasive) and gets ground so fine that the rings roll right over it. It gets into the cylinder wall's crosshatch and the crankcase oil washes it off into the crankcase while the piston is near the top of its stroke. It's damaging stuff, which is why oil analyses look for it, and the proper response is to find out how it's getting in there.

Oil analysis can find some of this stuff in the low parts per million. Considering that in 50 airtime hours those six pistons in your IO-520 can each make more than seven million complete strokes, it's not hard to see that silicon can show up. If one consistently operates off dirt or gravel or sandbars, there's lots of opportunity to get stuff in there.

I once found a 185 missing its flexible "boot" duct between the air cleaner duct in the lower cowl and the engine's airbox. Wide open, all the time. The owner wasn't concerned; "it's a floatplane," he said. It was, and floatplanes don't see much dust. But birds like to build nests in cozy places like that duct or airbox, and a nest in there could easily be fatal. Engine failure right after takeoff, maybe, over the rocks at the end of the lake, when the nest gets sucked into the throttle body throat. Ugh.
 
I just looked at my 1966 C150F and the carb heat cable is connected to a lever arm that is welded/riveted to the butterfly shaft. I could not see whether it was a weld or rivet, or press fit, but there was no extra shaft or nut, just a small bump reminiscent of a large rivet connection. Did not bother to take the cowl off, but does not look like anything is wrong and the carb heat cable and function seems the same as always, so I am interested to find out what year C150 the OP had and see if the part numbers match up.
 
Matthew Rogers said:
I just looked at my 1966 C150F and the carb heat cable is connected to a lever arm that is welded/riveted to the butterfly shaft. I could not see whether it was a weld or rivet, or press fit, but there was no extra shaft or nut, just a small bump reminiscent of a large rivet connection. Did not bother to take the cowl off, but does not look like anything is wrong and the carb heat cable and function seems the same as always, so I am interested to find out what year C150 the OP had and see if the part numbers match up.
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Some were welded. I have found some brazed, in older airplanes. Some had a boss welded to the lever and the boss was pinned to the shaft.

There are other things that go wrong with that setup. Most often I found the clamp bolt---the specially-drilled bolt that the control wire passes through and rides in the lever---worn halfway though, or more. Engine vibration does it. The bolt hole in the lever gets wallowed out, and I've found the hole so elongated that the bolt is about to wear its way through through the edge of the lever. The control wire vibrates inside the cable housing (the sheath) which is just tightly coiled wire, and gets grooves cut into it that eventually cause it to break. The clamp that holds the housing I often found loose enough that the housing could slip back and forth in it.

These airplanes are old. Nothing lasts forever. They need looking after, properly. The older they get the more care they need. Unfortunately, they often get too little.
 
Matthew Rogers said:
I just looked at my 1966 C150F and the carb heat cable is connected to a lever arm that is welded/riveted to the butterfly shaft.... I am interested to find out what year C150 the OP had and see if the part numbers match up.
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Cessna150M from late 1975

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This is one version of the repair/replacement part from Aircraft Spruce. The lever appears welded to the shaft. The locking nut is now on the opposite side where the cotter pin is. It includes new style bushings, which are also a source of wear.

This one is $100 but I have seen others going up to $500!

There is also an AD that requires the mechanic doing the annual to check the air filter frame seal as it can deteriorate and allow gasket pieces to enter the carburetor.

https://www.govinfo.gov/content/pkg/FR-1995-02-14/html/95-2786.htm

I replaced the entire filter frame for this annual, rather than deal with the old gasket.

That was $80 and included a new foam filter. Worth it IMHO.

BTW: I was always taught to close the carb heat immediately upon landing so as not to suck in dirt from the ground. Carb heat does not pass through the air filter, as you all should know.
 
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Found an inch of ‘mud’ in a C172 oil filter. Pollen! The owner did a lot of low and slow sight seeing during the desert bloom. Followed the POH religiously. Not likely to ice up in those conditions especially with a Lycoming.
 
BoButt said:
Found an inch of ‘mud’ in a C172 oil filter. Pollen! The owner did a lot of low and slow sight seeing during the desert bloom. Followed the POH religiously. Not likely to ice up in those conditions especially with a Lycoming.
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Are they saying pollen in the air went through the carb and intake .... past the pistons and rings .... and ended up in the oil filter ... ???

.
 
Pollen seems like it would be a pretty good fuel being a mixture of sugars and fats, with some lipids as well. Flour causes explosions at the right air-fuel mixture so I would expect the super fine pollen grains to burn equally well alongside gasoline.
 
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