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It's what you know that isn't so


rookie

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One of my dad’s favorite expression was “it’s not what you don’t know that hurts you, it’s what you know that isn’t so. That lesson was brought home to me last week. I was on final for 32 when a CAP flight called “turning base for 32”. Sure enough there he was, no sweat, plenty of space, just do a 360.

 

As I was adding throttle to make a 360 the engine became very rough and wouldn’t power up. Not to worry, I’m on final with enough altitude and besides I know what the problem is, particles in the float bowl. Been there, done that and bought the T-shirt.

 

After landing I pulled the bowls and sure enough, a small sliver of rubber approx. . . .030 X .125 in the left bowl. Buttoned everything up and went home. The next day I flew again to check out my cabin winterizing project. As a precaution I did a full power test during run-up and everything was fine. However on takeoff at approx. 500ft the same thing happened, rough and low power

 

As I started turning right to set up a downwind for runway 4 I pulled carb heat, engine ran smoother. I tried adding power with carb heat, engine seemed to respond well. I decided not to land but stay close to the airport and test some more, even did some touch & go’s (with carb heat on). All seemed to go fine.

 

After landing I rechecked the bowls and per Rogers advice checked the Banjo fitting. Everything was clean as a whistle. I won’t know for sure until I fly again but for now it sure looks like both incidences were likely carb ice.

 

I’ve always heard and it’s been my experience that we don’t get carb ice with this engine; that maybe one of those things we know that isn’t so. I checked my notes and on both days had temp of 28-30F and RH apprx. 79% .

 

al meyer

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The basis of the quote is from Mark Twain (although he may have gotten it from your dad :) ). Of course "what you don't know" in flying can hurt you, but most pilots will ask about what they don't know, it's what they (think) they know for certain that may bring them to harm.

 

Thanks for the reminder that it might not be particles in the float bowl every time.

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There's been a couple incidences of downed CTs in the north eastern states where the engine has stopped and would not restart. One of which was definitely showing symptoms of carb ice. The thing is, they turned off the carb ice after the condition cleared, but didn't think about what may have caused it in the first place. They tested the engine after the wreck and it fired up fine, further lending credence to carb ice.

 

A suggestion to anyone who gets carb ice in a rotax: not only should you turn on the carb ice, but if you are at low power, you should open the throttle some to get warmer air flowing in the carb and decrease any venturi effect over the butterfly valve.

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Just a thought,

 

My 912ULS does not have an air box. Filters are installed on the carbs, open to warmer cowl air. May be that helps prevent icing?

Also. it has water jackets that circulate hot cylinder water around the carbs.

I don't know if it's an option for the older engines, but Roger may.

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