Carb Heat Problems

JLowe

JLowe

Pre-takeoff checklist
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Jun 7, 2012
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143
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Addison, TX
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Jon
I've asked multiple A&Ps about this and still have no resolution, so I am turning to you fine people. Here's the issue. I've got a 172 and after pulling carb heat out, it likes to slowly vibrate on it's own most of the way closed. So if I pull it out on downwind, usually by base I notice it's only open about 1/4th, and I have to pull it back out.

First A&P adjusted the cable, had me pull it in and out a couple of times and adjusted where the cable connects in the engine compartment.

The second A&P said that the system was normal and that he didn't know of anything he could do to fix that issue... I just stated I've gotten into the habit now of pulling it out every couple of minutes -- but if I was in a situation where I needed to keep it on longer it would not be ideal.

I read on some other forum or post about a spring somewhere which may or may not be standard equipment. Any ideas on what I should check out to see about getting this fixed? Works fine on the ground, but with the engine running the vibration just slowly works it in.

Thanks!
 
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If you could give us a model year and engine it will be easier to help. My first instinct without looking at it is the cable itself is worn out. The cable and the surrounding sheath have a bit of built in friction and after years the become "sloppy" for the lack of a better term.
 
flyingmoose said:
If you could give us a model year and engine it will be easier to help. My first instinct without looking at it is the cable itself is worn out. The cable and the surrounding sheath have a bit of built in friction and after years the become "sloppy" for the lack of a better term.
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1982 172P, Engine is an O-360-A4M from... I believe 94.

The first A&P did clean the cable pretty good, and neither suggested the cable may be bad... but that may be something to check out. I would hope that it wouldn't be a 300$ cable!
 
JLowe said:
I've asked multiple A&Ps about this and still have no resolution, so I am turning to you fine people. Here's the issue. I've got a 172 and after pulling carb heat out, it likes to slowly vibrate on it's own most of the way closed. So if I pull it out on downwind, usually by base I notice it's only open about 1/4th, and I have to pull it back out.

First A&P adjusted the cable, had me pull it in and out a couple of times and adjusted where the cable connects in the engine compartment.

The second A&P said that the system was normal and that he didn't know of anything he could do to fix that issue... I just stated I've gotten into the habit now of pulling it out every couple of minutes -- but if I was in a situation where I needed to keep it on longer it would not be ideal.

I read on some other forum or post about a spring somewhere which may or may not be standard equipment. Any ideas on what I should check out to see about getting this fixed? Works fine on the ground, but with the engine running the vibration just slowly works it in.

Thanks!
Click to expand...

Disconnect the cable at the carb air box, pull the cable out (into the cockpit) until you see the wire core, place a small bend in the wire, push the cable back in and re-connect at the carb air box.
 
Tom-D said:
Disconnect the cable at the carb air box, pull the cable out (into the cockpit) until you see the wire core, place a small bend in the wire, push the cable back in and re-connect at the carb air box.
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so pull the cable all the way into the cockpit until there is a core, just a small bend push it back in and reconnect? The bend is just to add some friction?
 
Tom-D said:
Disconnect the cable at the carb air box, pull the cable out (into the cockpit) until you see the wire core, place a small bend in the wire, push the cable back in and re-connect at the carb air box.
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What part of AC 43.13-1B provides the approved data for that "repair"? Sounds more like damaging the cable than fixing the problem.

Personally, I'm with the moose -- start with the friction bearing on the air box to which the lever arm driven by the cable is connected. Trying to work around a loose bearing by adding drag to the cable seems like treating the symptom rather than the disease.
 
Please try to isolate the problem before you do any fixing so you aren't fixing something that ain't broke, or trying to fix a symptom rather than the real problem. Your mechanic should be able to provide advice on how to go about that process, supervise any work you do, and sign the log entry returning the aircraft to service.
 
Sounds like you need a new cable. I had a similar problem in a 150 I used to own.
 
Well my mechanic wasn't a ton of help. I'll be taking it to one of the recommend A&Ps soon for an annual so this will probably just get fixed at that time. I'm going to check for a friction bearing though and see what I can come up with. At least the $40 for a new cable and $85 (1 hr) to install shouldn't break the bank come annual time. I'm more worried about all the other things that could come up.
 
Ron Levy said:
What part of AC 43.13-1B provides the approved data for that "repair"? Sounds more like damaging the cable than fixing the problem.

Personally, I'm with the moose -- start with the friction bearing on the air box to which the lever arm driven by the cable is connected. Trying to work around a loose bearing by adding drag to the cable seems like treating the symptom rather than the disease.
Click to expand...

You are looking in the wrong reference again, there is no friction bearing in the carb airbox.
 
JLowe said:
so pull the cable all the way into the cockpit until there is a core, just a small bend push it back in and reconnect? The bend is just to add some friction?
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That's all you need to do, add a little friction to the cable. the air control door in the airbox rides on 2 bushings one at each end of axel that it pivots on, and is free to flop in either direction.

The cheap cable that is OEM is noted for this problem. It has no liner, or any thing it is the same type that your lawn mower has.

Later models there is a segmented crescent at the air box that was to give the cable a click at each position, those have long ago worn off and now allow the door to flop any where it likes, so you can replace the airbox, they are only $750.00 new from cessna my price with discount.
 
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Tom's advice to bend a kink in the cable wire is bad, and could result in a broken or jammed carb heat cable. Don't do it. The desired friction comes from the lever mount on the air box, so look there first.
 
Ron Levy said:
Tom's advice to bend a kink in the cable wire is bad, and could result in a broken or jammed carb heat cable. Don't do it. The desired friction comes from the lever mount on the air box, so look there first.
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Ron. This sort of thing is often done by many mechanics and it does not hurt the cable or the wire one bit. The specifications for music wire, which is the core wire in a carb heat cable, demands that it can be wrapped around itself without any cracking. It's very tough, very strong stuff. It only takes a tiny bit of a bend to add the required friction against the sheath, and in fact that little bit of bend can add life to the wire, since its life is normally limited by the wear it suffers at the engine end as vibration rattles it inside the sheath and wears little grooves that ultimately lead to a broken wire.

I would suspect that the OP's problem is related to a sheath that has been cut too short and is being yanked as the engine vibrates up and down. The propeller is perhaps imbalanced. In any case, the carb heat cable is probably worn out and should be replaced. This is one part that is no more expensive from Cessna than from MacFarlane.

Tom: MacFarlane makes PMA'd carb heat shafts and bearings for those Cessna airboxes. The original bearings are needle bearings that cut into the shaft as everything vibrates; the PMA'd bearings have an elastomer bearing insert that holds the shaft snugly and adds some friction so that the flapper tends to stay put. I put these things into a bunch of 172s and they outlasted the rest of the box.

Dan
 
Fight fight!

The crescent thing with the notches makes sense, I'll be poking around in that area to see what I can figure out. I'll also see how loose the lever is, but if there's no way to adjust it I may just live with the problem. Do you guys no longer think the cable is bad? The $40 replacement would be an easy fix, and a spare may let me test the bend method...

Either way I'm gonna be taking her to JohnE for some annual around January and from what I've read he is the best person to have tinker with it!
 
Ron Levy said:
Tom's advice to bend a kink in the cable wire is bad, and could result in a broken or jammed carb heat cable. Don't do it. The desired friction comes from the lever mount on the air box, so look there first.
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Not all late model carb airboxes have them.

OBTW the Cessna aircraft have a well written maintenance manual.
your AC will not be an acceptable return to service reference.

plus you don't need to over do the bend. as you suggest.
 
JLowe said:
Fight fight!

The crescent thing with the notches makes sense, I'll be poking around in that area to see what I can figure out. I'll also see how loose the lever is, but if there's no way to adjust it I may just live with the problem. Do you guys no longer think the cable is bad? The $40 replacement would be an easy fix, and a spare may let me test the bend method...

Either way I'm gonna be taking her to JohnE for some annual around January and from what I've read he is the best person to have tinker with it!
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Do the bend first, you'll see how well it works. You can take the advice of a A&P-IA with nearly 60 years experience or a CFIs your choice.

a new cable never hurts, but in a couple years you'll be back to the bending method.

The notched Crescent is a part of the air box, to replace it will cost big time.

I have a video but it won't load here.
 
JLowe said:
:yikes: oh god lol
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Go ahead. Laugh. Propeller imbalance causes all sorts of problems including creeping controls, failing gyros and radios, and it can loosen hardware and start cracks in some expensive stuff. A prop balance job can pay for itself just in the savings in avionics and instrument repairs.

Dan
 
Tom-D said:
Not all late model carb airboxes have them.

OBTW the Cessna aircraft have a well written maintenance manual.
your AC will not be an acceptable return to service reference.

plus you don't need to over do the bend. as you suggest.
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I haven't seen that serrated crescent on any of the 172s beyond the K model. I did see it on the old Taylorcraft airbox. American Champion gets their carb heat to stay put by using two methods: a friction setup on the throttle/carb heat quadrant, and a huge carb heat cable wire that must be .090". Massive. Never wears out.

It's interesting that Cessna often used a detented mixture cable, with a small ball that was sprung against the serrated knob shaft and worked well to hold things still, but didn't do that with the carb heat.

Dan
 
Dan Thomas said:
Go ahead. Laugh. Propeller imbalance causes all sorts of problems including creeping controls, failing gyros and radios, and it can loosen hardware and start cracks in some expensive stuff. A prop balance job can pay for itself just in the savings in avionics and instrument repairs.

Dan
Click to expand...

That was more of a nervous laugh that something else has been added to my list of squawks.
 
Dan Thomas said:
I haven't seen that serrated crescent on any of the 172s beyond the K model.
Dan
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I have a better set up on my lawn mower than Cessna used on there late model 150-172s

the biggest problem I have with the cable is the attachment at the airbox control lever. some have a bolt with a hole drilled in it to attach the cable wire to the lever, when they should have one of these
http://www.aircraftspruce.com/catalog/appages/cablendbnut.php

So, the bolt wears the lever hole until it gets so sloppy the bolt falls out. So some handy A&P adds washers rather than repair the lever and use the right hardware.
 
Tom-D said:
I have a better set up on my lawn mower than Cessna used on there late model 150-172s

the biggest problem I have with the cable is the attachment at the airbox control lever. some have a bolt with a hole drilled in it to attach the cable wire to the lever, when they should have one of these
http://www.aircraftspruce.com/catalog/appages/cablendbnut.php

So, the bolt wears the lever hole until it gets so sloppy the bolt falls out. So some handy A&P adds washers rather than repair the lever and use the right hardware.
Click to expand...

I have seen those bolts break through that hole, too, and have also seen the wire nearly sheared from the nut being tightened too much.

On my homebuilt I use a bolt, but being a homebuilt the bolt is larger (AN5) and is threaded farther down the shank so that the wire hole is between two nuts and thin washers. Less chance of breaking the bolt, and the wire is clamped between the washers rather than the sharp edge of the wire hole and a washer.

Dan
 
Dan Thomas said:
I have seen those bolts break through that hole, too, and have also seen the wire nearly sheared from the nut being tightened too much.

On my homebuilt I use a bolt, but being a homebuilt the bolt is larger (AN5) and is threaded farther down the shank so that the wire hole is between two nuts and thin washers. Less chance of breaking the bolt, and the wire is clamped between the washers rather than the sharp edge of the wire hole and a washer.Dan
Click to expand...
When you consider the costs of the bolt, nuts and washers, the cable nut is cheaper, lighter and better.
 
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