The International Cessna 170 Association

I noticed flying in 90-100 degree temperatures that my engine seems to run a little rough when I apply carb heat for landing. First question is this normal? Second question is carb heat needed at these temperatures? Bill K

It sounds to me like you may be running slightly rich in general. Higher temps mean higher DA which may require slight leaning of the mixture to compensate. Adding carb head further enrichens the mixture which may be causing your roughness. Check your RPM during shutdown. You should get a very slight increase in RPM when you pull the mixture to cutoff. Also, you can make sure your spark plugs don't show signs of a rich mixture. As far as whether or not you actually need carb heat at those temps, I have heard that a minimum 30 degree tempurature drop in the carb venturi can be expected which puts the temp well above freezing temps. However, if for nothing but good habit, I wouldn't recommend not using it.

LBPilot82's analysis and explanation is exactly what I have come up with; the mixture is further enriched when you apply carb heat and when you do it, slight roughness might occur. If I recall, the book recommends leaning prior to takeoff above 4,000 DA , so there are very real factors that come into play regarding smooth running and full power as things heat up.

My policy has always been to apply it before I turn base as part of my prelanding activities. I just do it as part of my routine, recognizing that really, it probably isn't required. I can only remember 3 times when I clearly had carb ice, and when I did I knew it. Two of those times I was taxiing during very humid conditions (which are rare here in Montana !) and when I did my runup I thought my magnetos had gone bonkers. Applied carb heat, cleared it out, poof, once again a smooth running O-300.

In your part of the world (i.e. Pennsylvania i.e. not God's Country i.e. Montana.... ) I know it can get very humid; so remember, carb ice can happen on any day, even when its warm/hot and keep that in the back of your mind if you think you might have it. Most importantly, fly the 170 as often as possible because it's the coolest plane in the air !!

I rarely use prolonged application of carb heat.
When preparing to make a large power reduction, I apply heat and observe my tachometer. If I see a normal reduction (and perhaps roughness due to the increased richness) and no other evidence of carb ice having formed and needing removal.... I remove carb heat and continue without it. (I've only rarely observed carb ice in my 170, although I've seen it more frequently in others.)

If I find carb ice forming regularly, persistently, and I plan to make long, low-power manuevers, I apply carb heat and keep it applied.

In 42 years of flying carbureted engines I've never found it necessary to fly continuously with carb heat applied so as to also require leaning for that condition.

I have a carburetor temperature gauge and as a result have not been using carb heat, or just cracking it, during approach and landing this warm Maine summer. My thinking is why richen things up needlessly. Is this a baseless concern? Am I correct in this reliance on the gauge? Engine is O-300.