Leaning with carb heat

[Jim K]

Remember that after you turn on the carb heat, you should lean the mixture, too.

This message in the carb ice thread jogged my memory about a question I had regarding carb heat on the cessna 172M I occasionally fly. I pull in carb heat whenever I'm below the green arc, as that's what the "owner's manual" tells me to do. Academically I understand this also makes the mixture richer, but I have no idea by how much.

I'm uncomfortable running the engine in this double rich condition, but that's what the poh says to do, right? Should I be leaning? The book doesn't mention it. I've had the thought that they might want the mixture extra rich and easy to light in case there is suddenly a bolus of water entering the cylinders as it melts out of the carb. What's the consensus among cessna flyers?

 

[GeorgeC]

It varies; try it at different altitudes/temps/power settings and watch the tach. There is such a thing as being too rich.

 

[Mtns2Skies]

With the O-470, in the winter running with about half carb heat helps the EGT's even out, otherwise the difference can be as much as 300* between cylinders because of poor distribution. In this case, I do indeed lean for the carb heat.

 

[luvflyin]

This message in the carb ice thread jogged my memory about a question I had regarding carb heat on the cessna 172M I occasionally fly. I pull in carb heat whenever I'm below the green arc, as that's what the "owner's manual" tells me to do. Academically I understand this also makes the mixture richer, but I have no idea by how much.

I'm uncomfortable running the engine in this double rich condition, but that's what the poh says to do, right? Should I be leaning? The book doesn't mention it. I've had the thought that they might want the mixture extra rich and easy to light in case there is suddenly a bolus of water entering the cylinders as it melts out of the carb. What's the consensus among cessna flyers?

I’ve flown many Cessnas that didn’t have a problem with carb heat making the mixture richer. But I’ve flown a couple that had problems with fouling plugs. One was the C177 Cardinal I owned. Another was a 172 I rented often after my plane was gone. I leaned those when adding carb heat. I didn’t go through the whole leaning process, out until engine stumbles and then back in a little, every time. But I knew from doing that just how much to back off the mixture. If you think about doing it, don’t forget to add ‘mixture rich on a go around’ to whatever GUMPy type thing you do for landing checks.

 

[Dan Thomas]

From the Cessna R182 (182RG) POH:

 

[Stewartb]

If running at low power settings you should already be leaned pretty aggressively.

 

[Stewartb]

With the O-470, in the winter running with about half carb heat helps the EGT's even out, otherwise the difference can be as much as 300* between cylinders because of poor distribution. In this case, I do indeed lean for the carb heat.

You need to experiment with a Cessna restrictor plate for your air intake.

Is your balance tube insulated?

 

[Jim K]

If running at low power settings you should already be leaned pretty aggressively.

Generally the only time I'm outside the green arc is after I'm abeam the numbers & descending for the runway, at which point I'm full rich. When I turn off the carb heat after touchdown, the engine picks up a few RPM. I always wonder if that's because going from hot air to cool air is effectively the same as opening the throttle a bit, or if it's so rich that it's on the edge of dying. Probably a bit of both I suppose.

 

[Mtns2Skies]

You need to experiment with a Cessna restrictor plate for your air intake.

Is your balance tube insulated?

What's a Cessna restrictor plate?

Nope - balance tube is uninsulated.

 

[Stewartb]

You need to lean for the power setting. I’d advise against leaning more for carb heat because if you have to go-around the engine will stumble unless you enrich first. I know guys will say we’re all trained to push heat and mixture in prior to adding throttle but in the real world it doesn’t always happen that way.

 

[Stewartb]

What's a Cessna restrictor plate?

Nope - balance tube is uninsulated.

Cessna’s winter kit included restrictor plates for the cowl front and the air intake. The intake restrictor covers the air filter and has approx 1-1/4” holes in it to pass air at a reduced volume to compensate for higher density air temps. Keep in mind that cold air temps make the engine run leaner than normal. Carb heat is correcting that. You may need to increase fuel flow. That was the magic move for my plane.

Insulating the balance tube addresses fuel condensing there in cold temps and making mixtures at 5 and 6 excessively rich.

My balance tube and breather tube are insulated year round. With that I don’t need a restrictor on the air filter and I have very equal cyl temps. I don’t need or use cowl restrictors, either. I do cover the majority of the oil cooler.

Here’s the parts catalog page with the winter kit pieces-