National Aeronca Association Bulletin Board

My carb heat control is very stiff and I fear that it may break at some point and therfore get stuck in one position or the other. I have tried oiling the end near the air box but this has not helped. I see that Aircraft Spruce has a control available but I am not sure if it is legal or that it will fit my aircraft. Any thoughts?
Regards,

The first thing I would do is to disconnect the cable at the carburetor heat box, and work the flap by hand. If the flap works easy, then it is your cable that is bad. If the flap is hard to work, then the resistance is in your heat box, and that needs to be addressed. IF your cable is the culprit, I'd just replace it. I have seen cable controls sold that are lined with teflon, and I'd go for one of those. I'd buy a cable from Aircraft Spruce, if they have one that is teflon lined. If they sell a teflon unit that is "certified", then go for it. If not, I'd still go for it. I'd venture to say that there are lots of small aircraft out there with lawnmower cables operating the carburetor heat control, not that I'm advocating using one of those cheap cables. I've never yet seen any IA that would not license your plane if the carb heat control was questionable, heck, they only check it for operation, so who knows what kind of cable is installed.
Just my 2c worth.

Mike

Sam,
Go down to NAPA and look at the marine application control cables. They are one of the few cables that come in the length required.
Paul

Friends,
Thanks for the helpful comments. My plan is to first disconnect the line from the airbox and see if the the trouble is in the airbox control or the cable itself. If the cable is still stiff then I plan on pulling the wire out of the cable assemly and applying lubricant and reinstalling, if it is still stiff then I will look for tie down points with sharp bends and correct them if possable. If all fails I plan to order another control cable from Aircraft Spruce. I will report the outcome on this forum.
Regards,
Sam

Hi All,

I applied turbine oil to the outer coil assembly from the carb-heat control to the carburetor which resulted in smooth function of the control cable. No need to replace the assembly as yet.
Regards,
Sam

You might try a cable luber. Meant primarily for the motorcycle market, you can find them on e-bay for about $10. Aluminum block with a rubber insert. Insert a "straw" from lube can and squirt away. Works pretty well. I believe I saw one at Harbor Fright recenty.

Brian Walker
N1197E '46 7AC..DC

sburke wrote:My carb heat control is very stiff and I fear that it may break at some point and therfore get stuck in one position or the other. I have tried oiling the end near the air box but this has not helped. I see that Aircraft Spruce has a control available but I am not sure if it is legal or that it will fit my aircraft. Any thoughts?
Regards,

The next time you look at the carburetor end, examine the center wire carefully for grooves worn by the outer spiral housing. You may have to carefully straighten the center wire and pull it clear out of the housing through the cabin. Old cables get grooves worn in them from dirt and vibration. If it is grooved, it is weakened and could bend or snap without warning. Replace it. This goes for operational mixture controls, too. jrh

Building on the previous comments related to the inspection and lubrication of the carb heat cable, Bill Pancake told me a story about one time when he was flying and the carb heat cable broke where it exited the sheath near the air box. What you may not expect but what happened was that the carb heat flapper began ossolating rather quickly between the open and closed position. As you would expect the engine rpm also cycled resulting in a higher than normal stress level during landing. Beyond inspecting and lubricating the cable on a regular basis there is a cure for this potential problem. You can mount a spring on the airbox carb heat lever that will function much like a spring in a toggle switch to hold the valve either open or closed. I will attempt to attach a photo to this post of the spring that Bill installed on my plane. We also installed a spring on the mixture control lever so that if the mixture control cable should break the mixture control would return to the fill rich position.

Sam,
Carl and I had a nice visit and lunch with you at Santa Ynez a few weeks ago and the Chief project made it home in one piece.

If you get ready to replace the cable check out NAPA and their marine application cables. They are non corrosive and come in the extended lengths needed for the carb heat cable application.
Paul

Safety improvements--fantastic!!

John,

I like your idea for having return springs installed in case of broken control wires. I would assume that the spring tension would cause the air intake box to go to carb heat and the mixture spring would cause the control to go to full rich. Can you post some detail photos showing how the mixture return spring is installed on the carburetor?

Regards,
Sam N6404C 1947 L16A

Sam, the mixture spring is connected so that if the mixture cable should break, the mixtrure will go to full rich. The spring on the carb heat on the airbox just retains the carb heat in the position it is in when the carb heat cable breaks. The purpose of the carb heat spring was just to keep the carb heat flapper valve from fluttering open and closed. The mixture arm moves quite easily on the carb so the return spring does not have to be very strong. I just went through my "spring drawer" and found a suitable candidate and then fabricated a bracket to anchor the free end. Bill installed the spring on the airbox. As shown in the photo, one end is anchored to a fixed screw on the airbox. The other end is connected to the end of the carb heat lever. The spring functions much like the spring in a toggle switch to keep the flapper valve either in the full open or the full closed position.

I looked for a photo of the mixture spring. The attached picture is the best that I could find. The photo is taken from the front left side of the engine. The carb is a Marvel-Schebler MA-3spa. The brass fitting angling down is the primer injection fitting. To the right of it is the carb mixture lever arm. The mixture cable is attached to the top of the mixture lever from the right side of the engine. The spring is attached to the mixture lever just below where the mixture cable is attached. The spring extends to the lower left side of the engine. A bracket was fabricated to anchor the opposite end of the spring. The bracket attaches to the drilled at tapped hole on the bottom side of the lower left engine motor mount flange. If the mixture cable should fail, the spring will pull the mixture lever to the left, which is the rich position for the mixture lever.