Rotax burping sound

[Warmi]

Hey,

So I have been burping my 912 uls for seems like ages now before every first flight of the day and just recently ( in the last , say 4 months ) - started hearing like a faint hissing sound when rotating the prop.

Everything works fine and engine burps in about 30 pulls or so ( depending on outside conditions ) but at least twice I heard that high pitching hissing sound like air escaping somewhere in the engine compartment - first time I thought I was hearing things but now it happened again.
Any idea what could that be ?

Thanks

[Sling 2 Pilot]

Jeez, 30 pulls? Mine burps with about 6-8 if warm, 10-12 if cold.

[Warmi]

Jeez, 30 pulls? Mine burps with about 6-8 if warm, 10-12 if cold.

Well by 30 I mean 30 prop pulls , not rotations ... it would be about 10 full prop rotations.
It has pretty much always been like that since 150 hours ( now at 400 )

[3Dreaming]

Pull slow and easy, not fast and quick. It is the air passing by the rings that is pushing the oil back to the tank. The more time with each pull to let the air past the rigns the quicker it will burp.

Now for the hissing. Does happen with each blade, or only on certain blades?

[Warmi]

I think it is with certain blades only but I’m a not sure - it is very faint to the point that I am having hard time hearing it.
It seems to be coming from the passenger side of the engine.

And as I mentioned, I burp the thing often , half of the time with cowling off and I heard it so far maybe two, three times at most so it is not frequent by any means.

[3Dreaming]

If it is not every blade, the 4 culprits that come to mind. A loose or leaking spark plug, a leaking head to cylinder connection, a cracked head, or a leaking exhaust valve. Which side is your exhaust located? I would check or replace the spark plugs, because that is fairly cheap, and you can do that yourself. Second have someone put an ear to the exhaust as the engine is being pulled through listening to see if the hiss is coming from the exhaust. A compression check would be in order for the head issues.

[Warmi]

The compression test was done early this year and was perfect - anyway, I will follow your advice and try to check all these things ( spark plugs, have somebody listen for where exactly does it come from etc )

Now if this hissing sound would have been at every blade , would that indicate something else ?

Thanks

[FastEddieB]

Just thinking out loud...

...”blades” don’t correlate directly to crankshaft/piston position, due to the gearing involved between the prop and the crankshaft. If that’s what was being implied.

As a data point, if burped from cold, 30 blades is not unusual for my Sky Arrow. Even having just run, probably 10 to 15 is average.

[3Dreaming]

The compression test was done early this year and was perfect - anyway, I will follow your advice and try to check all these things ( spark plugs, have somebody listen for where exactly does it come from etc )

Now if this hissing sound would have been at every blade , would that indicate something else ?

Thanks

I guess when I say blade, I really mean every compression stroke. If it is every fourth blade that indicative of a singular issue related to one cylinder. Every blade would indicate something else, and thinking about possibilities I don't think it would be good.

[AGLyme]

My Rotax (Flight Design) mechanic doesn't rotate the prop -- really... he pulls very slowly (maybe a 1/3rd of a rotation) until something is just right in there and viola' he creates the burping sound.

[Warmi]

My Rotax (Flight Design) mechanic doesn't rotate the prop -- really... he pulls very slowly (maybe a 1/3rd of a rotation) until something is just right in there and viola' he creates the burping sound.

Are you saying he gets the burb with 1/3 of a rotation ?

[MrMorden]

As a data point, if burped from cold, 30 blades is not unusual for my Sky Arrow. Even having just run, probably 10 to 15 is average.

Why burp the engine if it has just run? Mine always has oil on the flat of the dipstick ("Acceptable range") if the engine is warm. I check the stick and only burp if the oil is off the flat. After all, it will only go higher with burping...

[AGLyme]

Sorry, just saw this question... Yes, 1/3rd of a rotation... he does it very slowly... I haven't been able to copy it.

[Nomore767]

I always remove the oil tank cap before burping less air pressure to the tank and the gurgling sound is louder.
Just my technique.

[TimTaylor]

I always remove the oil tank cap before burping less air pressure to the tank and the gurgling sound is louder.
Just my technique.

Me too. Can you even hear it otherwise?