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rpm random drop

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Campo

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Apr 12, 2015
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Caceres/Spain
I am having some rpm dropping randomly and only for a few seconds. Fuel pressure is not going down when happening.
I have changed and revised all the fuel lines and distribution in according with rotax.
When happening I am switching off every electric devices and the ignitions to identify the problem. I am not finding water in the fuel.
Where could be the problem? Some bad contact in the electric system? Where? Should I continue checking water in the fuel?
Thank you
 

Dana

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What engine? What kind of fuel pump? What carburetor? Could be electrical, could be dirt in the carburetor, sticking carburetor float, air leak...

Dana
 

Campo

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Apr 12, 2015
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Location
Caceres/Spain
Rotax 912 with only 80 hours. I charged the floaters of the carburetors and they were clean.
if dirty I think that the fuel pressure will drop.
air leaking? Do you mean vapor lock or could be other reason? I don't think it is vapor lock. Fuel return line is in accordance with rotax. Thank you
 

Dana

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No, I wasn't thinking vapor lock, but an air leak either in the fuel system, carburetor, or intake manifold. Under what conditions (full throttle, cruise, idle) are you seeing the problem? On the ground or in the air? Level flight or climb? Did it just start happening, and if so did you recently change something? Is it an occasional thing or frequent? As far as I know, the Rotax ignition system is completely independent from the rest of the aircraft electrical system, so switching off other devices should have no effect.

Dana
 

TFF

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The ignitions are powered straight off the stator. The ignition modules have start and advanced. The other power comes from another set of windings unless it has a ships power alternator too. Fuel pressure also might have a dampener built in to stop needle bounce; it might not read fast enough for one or two air bubbles.
 

Campo

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Apr 12, 2015
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Location
Caceres/Spain
It is happening during cruise and full throttle, in the air, level flight and climbing. It has been happening one year ago and I changed completely the engine and now I have changed the fuel lines and fuel manifold. I will concentrate more in the fuel: water? dirty? bubble?. But how can I see it? I will check the float chamber and the gascolator before every flight.
It is happening randomly and for a few seconds but it is very uncomfortable when you are flying over forest, mountains or during the take off. Thank you very much for your comments.
 

Dana

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How far does the rpm drop? Does it seem like it's quitting for a moment or just running rough? How often does it happen, is it getting worse? Any particular weather conditions? Have you ever found water or contaminants in the gascolator during preflight?

Intermittent problems are the hardest things to diagnose, having a similar (slightly different, and different engine) problem myself right now I feel your pain.

Dana
 

TFF

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If it does not do it at part throttle, float might need adjusting. Slide not sticking? No water in fuel/ alcohol?
 

Campo

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Apr 12, 2015
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Location
Caceres/Spain
Rpm are dropping around 200 at 5000 rpm only for several seconds. Not all the time. When I am reducing to 4800 rpm it stops.
I found some water long time ago. I am going to check with more detail (The problem is that I am closing the tanks in the hangar and when I open, the fuel has to travel from the top wings to the header tank and the gascolator)
Alcohol? I didn't know about it.
I am leaning to think in the following: water in the fuel, venting system in the tank (strange because it is happening with both tanks opened), some restrictions in the fuel line between the header tank and the engine.
This is a link with the problem:
https://youtu.be/ooqUpXs4UgY
I will post my findings. Thank you very much
 

PTAirco

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Sep 20, 2003
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Corona CA
A general fuel flow problem? Have you done a ground fuel flow test? Disconnect fuel lines at the carbs and let the fuel run for at least a full minute; measure and see how much fuel you get and see if it is sufficient for cruise power. You should do this with the aircraft level and also with the tail down/nose high.
 

odwil

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Feb 21, 2015
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denair ca.
it could be condensation forming in the tanks from sitting and separating out and it is just enough to cause the problem and not enough to show up when you check for it
 

Dana

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Stupid question, but I couldn't tell from the sound in the video... is it definitely an actual rpm drop? Not just a flaky tachometer?

Dana
 

Campo

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Apr 12, 2015
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Caceres/Spain
In the video it is difficult to hear but it is really dropping the rpm in the engine.
Today I have been checking the venting tube in the tanks and the fuel flow from the gascolator. Everything seems to be OK but I am not sure.
The problem is that it is randomly happening and I don't know exactly when.
 

Campo

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Apr 12, 2015
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Caceres/Spain
Yesterday I checked the fuel flow to the gascolator in every tank: aprox 1 litre per minute (this is more than enough). I checked the carb cups and I didn't found water or dirt. I am leaning to thing that the problem could be some kind of contamination in the fuel. When hangaring the plane, I close the fuel faucet in every tank. Therefore, when I am checking the water in the carburetor, I am not checking the water in the tanks but in the last fuel lines. Yesterday I have left the tanks open and I am going to check the water in the gascolator. Thank you
 
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