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Sick 912s


Mike

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How's CHT and EGTs?

 

So far I am thinking:

 

Probably running too rich. (Likely candidate)

-Carb metering might need checked

-Choke is messing up

-Induction air restriction

 

 

Does it have a MAP gauge? What is it reading?

 

When was the last time the carbs were inspected? How many hours of use since then too?

 

What grade of mogas? How old is it?

 

I need to know the specific oil. Rotax recommends a lot of oils.

 

Do you have pictures of the plugs? There are a lot of "black and fouled" that all look VERY similar, but each is caused by completely different symptoms.

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If the plugs are black its a rich problem. Even flooding if cutting out. Carbs are the issue. The fuel injected models won't see this problem unless the injectors foul...

 

Sometimes if you get this in one or plugs and not all of them, it can be a plug problem. If they fire cold they wont burn all the gas and they foul. But if all plugs are black, then its a carb problem.

 

There are several different types of black plug problems. The most common is mixture issues, but oil deposits can look very similar. Here's a chart in cars as an example: http://www.aa1car.co...spark_plugs.jpg

 

It's highly unlikely that it's anything other than a mixture problem, since it's across all the plugs, but it never hurts to check. For example, mogas starts to break down after 6 weeks, and doesn't burn well after a while. Ethanol also is extremely hydrophillic, and pulls moisture right out of the air to collect in the fuel.

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Hi Mike,

 

Black and oily or black and dry soot? To check the plugs don't do it after idling. Run it on up to 4K rpm for at least 3-4 minutes and then pull throttle and turn the engine off right then and there. No idling. Then check them. What would worry me is if it is all the plugs on both sides as you say or just one side. If it is just one side we have either an air leak or a bad carb, but both sides at once? Is the mag drop equal and how big a drop and at what rpm? Do it at 3500 if you would. You need to put a set of gauges on the carbs and not an electronic sync instrument. Fuel contamination with diesel fuel will do this and usually make the oil tank oil level increase. Time to start ruling out each as a possible cause. You need to determine if it is electrical or fuel system. Some sort of air intake issue could possibly (slight chance only) be a cause. Something blocking air flow for instance. Has any maint. been done lately before this happened?

Time to start a logical diagnosis to rule thing in or out.

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Thanks for your help everyone we will go back to the engine and check through your ideas and will let you know if we find/or not find anything...

 

Mike

Ps my first thought was maybe diesel contamination as we run on mogas unleaded normally filled from cans "he " says no but i will be checking that .. the air filter system is 2 separate k&n type filters so would need to both be blocked Unlikely

 

Cheers Mike

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I'll add 2 things. I had a CT that would miss a little bit on climb out or under heavy load. I went trhough all the trouble shooting, and found a sunk float. Replaced that and it didn't fix the problem. I had a sparkplug go bad after just a few hours run time. I put a used set of plugs that were just removed from another engine in and the problem went away.

The other thing, and I know people have already said a plugged filter, but to much oil on a K&N filter can cause the engine to run rich.

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CTLSi:

 

If the fuel is breaking down from age, it will not burn very well, leaving behind deposits from various sources, and the fuel burn not being hot enough to scavenge. Mogas is inherently unstable due to the blends chosen by refineries, and ethanol makes the problem significantly worse. As stated, fuel without ethanol lasts about a year before it's unusable, and fuel with ethanol lasts about 6 weeks. Avgas, on the other hand, has a shelf life of 15 years.

 

http://www.fuel-test...thanol_gas.html (this says ethanol mogas lasts 90 days)

http://www.echo-usa....nol-Fuel-062512 (this one says, second page last paragraph, in as little as 30 days in the worst conditions)

http://generalaviati...10-mogas-myths/ (see myth #2, fact section, about adding ethanol greatly reduces life)

 

If you run mogas, you need to keep the fuel cycled. Don't let the plane sit for months. Not only does the fuel go to crap, but the water in the fuel will start corroding parts, and wreck havoc on the engine if you tried to start it without purging. Ever try to start a lawnmower after a long winter? It chugs at first!

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Possible, but not very probable. You may want to have the fuel checked. It could be cross contaminated with diesel. Have you noticed the oil level in the oil tank rise?

 

 

Do you have any update?

I would like to know mag drop at 3500-4000? Plug gaps (.023-.027), Dumping all the fuel and starting fresh? Put a set of carb sync gauges on and getting the results? Fuel pressure?

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  • 5 weeks later...

After lots of checking and testing it turns out to be the fuel pump over pressurising causing the carbs to flood ,it apparently sets up oscillations on float needle valve increasing the float chamber levels, there was two possible fixes, to replace the pump or new needle jets with up rated springs, new needles fixed the problem.

 

thanks all for your help Mike

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Good outcome!

 

Best to remember that our Bings were originally designed for motorcycles, and probably with the assumption that all fuel supplied would be gravity fed. So it's not surprising that a bit too much pressure can overcome the needle and seat arrangement.

 

Anyway, glad that's all it was.

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