912ULS won't burp

[Floog]

My friend's 912 won't burp until after maybe 60 blades. Compression is what pushes the oil from the crankcase to the oil tank. His plane is a nose dragger and the tank is about 1 to 2 inches higher than mine (KF7 conventional). The engine is older but low time. The compression feels weak compared to mine. Concerns would be: How do you know the true oil quantity? He bought the aircraft second hand and has never done a compression/leak down test. I thought that would be a great place to start. Any thoughts appreciated.

[Dave S]

Floog,

If the engine has some time on it....probably a pretty good idea to do a compression check. Might consider the type, grade and condition of the oil too - if it is old, tarry and thick it will take longer to burp.

FWIW......try the burp after shutdown before the engine cold soaks....I started doing that long ago and find that hot oil (meaning thinned somewhat) flows a lot quicker than cold oil. Obviously I do burp & check before cold startup - but it only takes a few blades since most of the oil stays put in the tank after the hot burp...at least with the S7 configuration (also a trigear) and current Rotax oil filter on the 912.

[kmach]

dwell or pause on the compression stroke when burping, let the rings push the oil back to the tank. I also burp after shutdown to reduce the amount of strokes on a cold start burp.
A compression check might be in order for your friends. what oil and oil filter and how many hrs on his engine?

[DesertFox4]

My model 4 912 uls with the high mounted oil tank take on average 100 blades to burp after sitting a couple weeks. Engine compression is fine with 900 hours total time.

[jiott]

I am just thinking out loud here, but my understanding of the burping is a buildup of pressure in the crankcase due to piston compression leaking past the rings into the crankcase. That is why if you dwell on each compression stroke it will burp faster, also why a hot burp works faster. Maybe your friend's older engine has lots of crud built up on the rings and ring gaps and therefore does not leak compression gas past the rings very fast, especially with cold thick oil. Also the valves may not be sealing as well and leaking out into the exhaust and intake if the compression feels low. Just some thoughts.

[jrthomas]

I had the same issue. It turned out to be a "non-issue". I have a model 4 with the high mounted oil tank. The engine is a '95 brand new engine that like DesertFox needed 100 blades and more to burp. I thought something had to be wrong so I tried everything I could think of to cure it. Compression check showed 80lbs, I purged and repurged, looked for leaks but nothing helped. Oil pressure came right up spinning with the starter. Finally I called Roger Lee. If you're not familiar with him, he's well known as one of the sharpest Rotax people out there. I can personally testify that he's also one of the nicest. He assured me that nothing was wrong. It's basically the high mounted oil tank. His advice is to pull it thru a few times to check for hydraulic lock when cold. Go fly and burp it and check the oil afterwards while it's hot. I've followed his advise and this works fine for me. My engine runs great. James Thomas

[Floog]

I forgot to add he's using a 'different' oil ? other than AS Sport +4. And a Tempest filter. So all of this prolly adds up to making it more difficult to burp. Propping it hot is the way to check it. Most likely nothing wrong with the engine at all. Great feedback!! Thanks you guys.

Great to hear you're almost finished, Jim!

[kmach]

I forgot to add he's using a 'different' oil ? other than AS Sport +4. And a Tempest filter.

I will advise to use Aeroshell sport +4 and a genuine rotax oil filter. This also may help with the burp.

[Slyfox]

my feeling on this, I can care less. I don't usually burp before run. I will turn my engine over about 18 compression strokes and then get inside and turn it over until the oil pressure comes up with mags off. that's it. if I want to check oil level, I do it after a flight. I shut the engine off, pull out the stick, it WILL be at the proper level.

I will burp the engine before changing oil, I don't pre warm the engine. If I happen to do an oil change in real cold weather, I plug in the engine warmer.
another note: If you want to burp the engine with better results, remove the oil canister lid. I've have noticed that the burp will happen a lot quicker.

[herman pahls]

I check the oil after the flight.
Pull 15 blades to make sure engine not locked before the 1st flight of the day.
If there is not a puddle of oil on the floor, it is still where it belongs.
I started wishing for a Lycoming if 100 plus blades before burp were required.
Good idea Slyfox to see oil pressure before letting it start