Engine Trouble

Category: General Tech Discussion one month ago

I'm new to Rotax motors. I wanted to get a better idea of potential causes of engine abnormalities while flying. I'll have a pair of 914s in an Air Cam.

Rough Running Engine:

My understanding is carbs out of balance may produce this but what causes a carb ot go out of blaance? Throttle cable slippage? Does it manifest as a rough engine in flight at high or low RPM?

What are some other known causes of a rough running engine aside from carb balance?

High CHTs: My unerstanding is the engine is air and liquid cooled. If there is a coolant leak, I'm guessing this would manifest itself as higher CHTs in flight that may require a shotdown. What are other causes of higher than normal CHTs in flight?

Turbo failure failsafe. My understanding is the engine continues to run in the event of a turbo failure. How common are these and is it advisable to continue running the engine in a failsafe turbo failure condition, if so are there any limits as to how ling?

Carb heat: I'm likely getting the carb heat exchanger, are there any known issues with that set up? Is it 100% effective against potential carb ice in the 912/914?


  • Re: Engine Trouble

    by » one month ago

    Carbs get out of sync because of cables stretching and slipping and tolerances changing over time. http://ctflier.com/blogs/entry/24-the-anatomy-of-a-carb-sync/


    Engines can run rough for a million reasons like spark plug problems or grounding issues or gearbox wear, or a single ignition system failure (dual failure causes stoppage of course). Bad fuel can also cause it. The engine will also run more rough at idle because the spark timing is not ideal (the timing is optimized for WOT) and a very low idle can cause more rough running than a higher idle. You can run an even lower idle smoothly if you get a fuel injected engine because the timing adjusts automatically but the 914 is not fuel injected.


    If the coolant leaks then CHTs will get much higher because the fins are not big enough for passive air cooling to keep it cool. If you lose coolant then it will run long enough for you to land but it will overheat quickly. But since you fly a twin I would probably shut down the engine that loses coolant so you don't cause damage to it. Other causes of high CHTs would be over working the engine either because you keep it at WOT for too long on a hot day or because your prop is pitched badly and you are lugging the engine. Or if you let your radiator get too dirty or it is too small then its cooling ability is reduced. If your coolant boils then it will become ineffective even if it doesn't leak and the engine will still overheat as if you lost coolant pressure. Do some research so you get to know what kind of coolant is best for your mission (distilled water, waterless, or semi-synthetic).


    Rotax is very good at keeping itself cool but it is not impossible to overheat. I live in California and the hottest time of day during the hottest time of year I have to climb 10 kts faster than Vx to keep the CHTs from climbing above normal (I do that when OAT is between 100 and 110, but maximum allowed for operation is 120). But remember oil temperature also matters and oil is important for cooling just like the coolant and air flow! The coolant only keeps the heads cool. Oil and air are responsible for keeping the rest of it cool.


    I don't use the 914 so I don't know a lot about the turbo but I would guess failure is not very common. I would think most turbo failures would not cause problems to the engine as long as the turbo just stops and it is not overboosting. Does the 914 operator manual say anything about that? But remember that you will lose some horsepower and if you have two engines and only one has a working turbo then you'll have asymmetric thrust problems. So maybe it is better to just treat a turbo failure as a failed engine so you don't throttle up for a go around at high density altitude and lose control because one engine is putting out 115 hp and the other is putting out only 80.


    Rotax carbs are resistant to icing because they are placed in a hot part of the engine. I don't know about the setup you talk about but nothing is 100% effective against carb ice and you will still need to take action if you suspect icing is happening. There are some optical icing detectors that I read are almost perfect at detecting carb and are more accurate than the simple carb temperature sensors ice but they don't prevent ice.

  • Re: Engine Trouble

    by » one month ago

    Rotax in Canada told me that carb icing is not an issue on the 914 and that testing has demonstrated that no carb heat is required. This applies to 914 only, not 912.

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