Rough running engine @ 4800 and lower RPM

[Stacy]

On the ground, the air temp was 16 F today. I have a couple of strips of tape; after 15 minutes of warm up, the engine temp was 134. I flew 25 miles to another airport, temp stayed 140 plus. As soon as I cut power, the engine started running rough.

i pulled the carb heat on, and it imedietly ran smooth. I picked up a passenger and took off again. We idled back a couple of times and it ran rough until I pulled the carb heat.

There was no water in the fuel. There was haze in the air. So,,,, was there enough moisture in the air to make it run rough, or do you think it was something else? In the year and a few months that I have owned the plane, today was the only time it’s ran rough and it’s the only time I’ve used carb heat. 

[Ed Cesnalis]

Sounds like carb ice to me.  Good on you for using your carb heat.

[Tom Baker]

You have a CTSW correct? Check the vent tubes on the carbs, and where it runs through the filter box. My guess is the tube is off center in the box with a scallop open to cowling air, or it is open between the carb and air box. Both carbs need to be the same pressure. If the line is out of place or disconnected it allows a pressure difference. Pulling carb heat would equalize the pressure to the carbs. Another thing is the slides get sticky inside the carbs allowing the carbs to be unbalanced. The cold air might contribute to this, and carb heat allows it to free up.

[WmInce]

3 hours ago, Tom Baker said:

You have a CTSW correct? Check the vent tubes on the carbs, and where it runs through the filter box. My guess is the tube is off center in the box with a scallop open to cowling air, or it is open between the carb and air box. Both carbs need to be the same pressure. If the line is out of place or disconnected it allows a pressure difference. Pulling carb heat would equalize the pressure to the carbs. Another thing is the slides get sticky inside the carbs allowing the carbs to be unbalanced. The cold air might contribute to this, and carb heat allows it to free up.

That's great information to know. Thanks for the tips, Tom.

[Roger Lee]

At 16F not carb ice. I'm with Tom.

[Ed Cesnalis]

34 minutes ago, Roger Lee said:

At 16F not carb ice. I'm with Tom.

that's likely correct but 16F with haze in the air?  you sure?  visible moisture adds something no?

[FlyingMonkey]

Could this also be a mixture problem?  It happened in the cold air, and as air density goes up, the mixture might lean out some.  The carb should compensate for it, but...?  The carb heat would allow warmer air in and richen the mixture back up.

I have noticed my engine idles slightly rougher when it's colder, but it does fine at normal running RPM.  I assume it's the mixture going a bit lean in the idle circuit.

[Roger Lee]

Cold dry air does make carbs run leaner. They run the CT and Rotax engines in many cold snowy  European countries without issue. 

[FlyingMonkey]

17 minutes ago, Roger Lee said:

Cold dry air does make carbs run leaner. They run the CT and Rotax engines in many cold snowy  European countries without issue. 

I didn't mean there was anything wrong with the design, just that maybe in this case there is something awry with the mixture.

[Stacy]

I should have posted the following earlier. I've been up three times since the flight with the ruff running engine. Twice the temps were 10-15 F, once with the temp in the upper twenty's. The engine ran smooth on all three flights.

So, I assume it was carb ice.