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. :D
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

[DesertFox4]

I started wishing for a Lycoming if 100 plus blades before burp were required
100 blades is normal after sitting a couple weeks especially on a model 4 with the higher mounted tank. Seen this on brand new Rotax engines and is no indication of engine health either way as jrthomas stated after talking with Roger Lee, so not sure what is implyed by the statement in quotes above. Roger is not the only highly experienced Rotax mechanic to confim this normal Rotax specific characteristic.

I've also found the most accurate oil level readings come from post flight checks and if you are flying as often as someone like slyfox then his described pre-start procedure works well and I have employed it many times. I find with my building taking precedent over my flying lately that long periods of inactivity require burping pre-engine start just for piece of mind.

[Dusty]

Re:burping post flight
This is defiantly the quickest way and the oil level difference between hot and cold is minimal.However I can't bring myselfe to NOT check the level on preflight but a few blades is then all it takes to reburp.
(KF4 high tank)

[jiott]

I found that you can really reduce the number of blades to burp, even on a cold engine, if you dwell on each compression stroke-just hold the blade against the pressure, maybe very slowly turning it to hold the pressure until the pressure bleeds off and the blade will turn easily to the next compression stroke, and repeat. Its not the rate or speed or number of blades that matters, but rather the time held against compression to cause blowby past the rings into the crankcase. This eliminates the frantic blade turning on the ramp which makes everyone wonder what kind of weird engine is that. Of course most of you already know this, but maybe some new to Rotax haven't figured it out yet.

[Slyfox]

yes I agree, take it slow at each compression stroke, but like I said, make sure and remove the cap off the oil tank. That made a difference when burping mine.

[jiott]

If the cap is on the oil tank I have trouble hearing it burp; cap off no problem at all.

[Av8r3400]

Using the described compression dwell, cap off method, I have never had to pull more than 15-20 blades, cold, on either of my high tank Model IVs.

[jiott]

Come to think of it, the faster you turn the prop the more oil you are pumping out of the tank, thru the system and back into the crankcase, which slows down the process of blowing oil out of the case back to the tank, which then takes more blades to empty the crankcase and burp. Kind of self defeating. Slower turning with dwell is just the opposite, which speeds up the whole process.

[PaulSS]

My first post, so be gentle :)

I had to Google Rotax 'burping' as I had no idea what you guys were talking about. I don't (yet) own a Kitfox, or any other Rotax-powered machine for that matter, but have flown behind one in the Tecnam and still had no idea. My Googling led me to this Youtube video, which I think gives some good ideas of what, why and how. Enjoy your burping:

Burping Rotax (https://www.youtube.com/watch?v=HsIfNt7_SB4)

[jiott]

Paul you better hurry and order your own Kitfox! What better thing could you do where public burping is not only socially acceptable but mandatory.

[PaulSS]

Haha, believe me; I'm looking. I've seen a IV that looks great but has the straight 80hp engine. IF I take the aircraft back to England then I reckon it would be flown from a very short farm strip, so now I'm all over the Zipper modification and trying to find out the state of slipper clutches, beefed up starter motors etc (I'd like to convert it to 115hp just to give me more options). I knew literally nothing of this until a few days ago but have had some down time in China, so this website has been thoroughly viewed.....my eyes are sore from so much reading :D

[Micro Mong Bldr]

The book on my HKS with a similar setup says to run it for 3 minutes before checking the oil level.