Sick 912s

[Mike]

My mates 912 is giving problems, in the climb out it starts to run rough on landing the plugs were inspected and found very black and fouled on all cylinders.....

 

Any help would be appreciated,

[Anticept]

Running avgas?

 

What oil?

 

When was the last oil change? When was the last time plugs were replaced?

[Mike]

Running unleaded mogas standard rotax recommended oil ,plugs in time before replacement

 

Thanks Mike

[Anticept]

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.

[Anticept]

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.

[coppercity]

I would check the carb floats and float level and clean the carbs well.

[Jacques]

clogged /dirty air filter will make all cylinders running rich

[coppercity]

Hi Eric,

 

I could see one side carb issue affect one bank of plugs, but he says all of them are black and having both carbs go bad at one time? What would be the odds?

 

True, that would not be very likely.

[Mike]

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

[Anticept]

How old is the gas, and does it have ethanol? Again, it breaks down after 6 weeks, (non ethanol fuel has a life of about a year).

[Tom Baker]

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.

[Mike]

Not sure how old the fuel is will find out but it will definitely have ethanol in it its almost impossible to find fuel without in the UK

[Anticept]

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!

[mocfly]

Mike,

What is your friends 912 in?

 

[Mike]

Chris its fitted to a Rans aircraft

 

Roger would it be possible for the engine oil to get into the fuel via the pump ?

[Mike]

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

[FastEddieB]

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.

[Runtoeat]

Hi Mike. Was the new Rotax fuel pump installed which caused the over-pressurization? it would be good info to know what the part number is for the higher rated float needles.