Even Compressions

[BigPete]

Grounded (again)

 

At 276 hours my poor little Jabiru is grounded once again due to a burnt exhaust valve.

 

This time it is cylinder 4 (last time at 87 hours was No 1) (2 down and 2 to go??) :;)4:

 

I have suspected of late that I was headed for trouble - hence this thread.

 

The big problem is - as I see it - is that a low compression is not always a "true" indication of a problem. Thus we are deceived into believing that nothing is wrong when it actually is. ;)

 

In other words - if it were "normal" to have 4 (or 6) good compressions - then the minute we didn't get them we would know there was a problem and earlier action could be taken. ;)

 

A simple valve grind/dressing rather than a complete valve replacement.

 

Nothing had changed when I pulled the motor thru EXCEPT I heard a hissing noise......:;)1: (by now too late...)

 

Funny thing is - allthough others have similiar results in this forum - I felt that there was a problem. I felt the same thing with the first burnt valve.

 

I must learn to follow my instincts a little more....

 

regards

 

 

[Guest brentc]

I say again, a compression test is not what you are after, however a leakdown tester is what everyone needs for their Jabiru engines! If you have a mate or a co-flyer at the airfield, why not go halves in one? It will help identify these problems early before damage occurs and give you a chance to modify mixture etc before you continue to burn out the valve seats.

 

If you haven't got it already Peter, get the larger needle / jet combination that everyone is talking about! I know of a fellow I was talking to earlier today and he was sent down half a dozen needle combinations and he's going to test them all in conjunction with fuel flow and EGT to get something that works for his configuration. Interestingly he found that by leaving off his naca duct hose, the engine ran way cooler than expected which was interesting.

 

 

[BigPete]

Thanks Brentc - I fitted the larger jets as per Jabiru bulletin at around 240 hours, but I guess it was a little too late.

 

The plugs did show that the mixture was probably a bit rich (nice even black) but at this stage I'd rather run it that way. The good news is that the bore itself is as smooth as - with no sign of anything adverse. ;)

 

One of the club members here is a motor mechanic and he suggested to leave the "mags off" and run the motor on the starter and listen for "eveness" of the cylinders as they compressed. - You could hear the uneven "beat" of the motor as the weak pot came through. :;)4:

 

regards

 

 

[BigPete]

Well - I'm still on the ground.

 

After what seems like forever my poor little Jabiru is still headless. :black_eye: Bad weather and work overload on the L2 doing the repair has taken longer than I thought. :;)4: The latest news is that the valve guides are worn enough to require replacement. So another wait while Jabiru send the extra parts. My motor was one that required the rocker chambers to be vented. As it was the number 4 pot (which runs the hottest), we believe that the oil pooling and overheating in the chamber caused the extra wear. ;)

 

The good news is that I should be back in the air (at the latest) on Friday 22nd of Feb - the eve of the fly In. ;)

 

regards

 

 

[Guest J430]

Make sure that the guides are shrunk into place then reamed to the correct size with an accurate jig. The valve guides are rough bored so you can not just bung em in and hope for the best.

 

If you get that bit wrong its all down hill............

 

 

[Yenn]

brentc. I have the details of how to build a leakdown tester, which I found on the net, but they are a bit vague. Could you p;ease explain how they are supposed to work, for my edification.

 

 

[Guest Andys@coffs]

Ian

 

Leak down testers work this way. Two identical Air pressure guages sit between your Air compressor and your cylinder under test (one spark plug is removed, Air pressure hose connected to head via a suitable spark plug thread adapter that generally comes with the testers, at least good ones) . 80psi of air is then applied.

 

Between the two guages and the connection to the head is a small orrifice that limits the rate of air that can be applied to the cylinder.

 

If there is a leak in the cylinder the small opening doesnt replace the lost pressure fast enough and you end up with a difference on the 2 guages. The amount of difference is proportional to the quality of the piston to cylinder seal and the in and out valves to head seal.

 

Pretty basic, but very effective.

 

Andy

 

 

[slartibartfast]

Andy - I presume the cylinder to be tested is first rotated to be at TDC on the compression stroke?

 

Thanks for the info. I wasn't sure either.

 

 

[Guest Andys@coffs]

Slarti

 

Yep, test is done at TDC, though I can see no reason why you may not choose to test the entire piston to cylinder seal over the complete compression stroke if there was something wierd their like a scuffed liner etc. BUt that said generally done at TDC at or very close to operating temp. Just be careful, as Brentc found out, 80psi has the ability to drive the prop to create a divit in your scalp if not careful. Basically its a 2 man job to do easily

 

Andy

 

P.S Leakdown testers on the US ebay site are around $40US. By the time they are landed at your door youll get change out of $100AUD, or at least I did. Unless your a superb scrounger I suspect the build your own route will cost more. The trap to be wary of is to ensure that the cylinder head adapters covers the jabby plug size as its not a more common size. The testerI purchased came with about 4-6 adapters (in a sky blue plastic vac formed case) The standard (cheaper ones) that only have 1 or 2 generally wont fit a jabby plug.

 

As Brentc has said, time and time again, $100 proactive vs $2k-10k reactive.... Its not a difficult decision really.

 

This one on ebay in the US is the one i purchased. http://cgi.ebay.com/ebaymotors/New-Dual-Gauge-Leak-Down-MADE-IN-USA-90-Retail_W0QQitemZ300198028041QQihZ020QQcategoryZ43989QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

 

[IanR]

When doing a leakdown test do you "wet" the rings with some oil ? I recall that as being part of the process from way back.

 

 

[Guest Andys@coffs]

As I recall weting the rings is a way to potentially generate a "pass". The test needs to be done at operational temperatures and if there is oil needed to get the leakage down then maybe something needs to be looked at.

 

In the small amount of experience Ive had the issues seem to be more valve related that piston rings on jabbi engines.

 

The operating temp requirement makes it difficult to achieve a full leakdown on all 6 cyls with a good feeling that the last cyl was actually up at the right temp. The 8 would be even more difficult.

 

May be worth reversing the order between tests so that the last cyl isnt always last.

 

Andy

 

 

[Guest brentc]

Wet rings would be preferable yes, however they should already be wet because the engine should be hot (not warm) for the test.

 

 

[Guest J430]

Rings V Valves

 

500 hours and our valves were good enough not to even lap, but we did.

 

The Rings......2 out of 6 had stuck!

 

My Leak down gauge is the same type, the pressure drop is in % so about 25% is the desired loss.

 

I think the setting pressure is 100PSI on them not 80PSI. Also it should b noted that a minimum 12CFM(i think) 2.5HP compressor is needed to keep a decent pressure up when you have even a normal leak down.

 

WARNING!

 

Dont try to use your $100 Super Cheap compressor......you will get nowhere with that

 

 

[Yenn]

Thanks for that, now I will have to find the plans and work it out from there.

 

 

[Modest Pilot]

From around Oct 07 Jabiru went from leakdown type hydralic lifters to non leak down type. I would guess that the new lifters would randomly lose charge after sitting idle for a time. Maybe you only get a good conparision after a short engine run.

 

 

[spollock]

Hi,

 

Just been reading this thread. We've recently done a leakdown test on an 80 hour 2200 using the ebay tester from the US (in blue box), and were a bit disappointed with the results. We found it difficult to get consistent results and noted that moving the prop slightly either side of TDC could alter the readings by 5% or so. We got values from 23 to 28%. It was also difficult to say where the leaks were happening; in all cases we could hear and feel air coming up the oil filler hole, but it might have been leaking somewhere else as well. The question is......what further actions, if any, should we take?

 

Simon

 

UK

 

 

[Guest J430]

Spollock

 

Assuming you used the gauge correctly its not uncommon to get 20-25% leakage and particularly if the engine is cool.

 

For 80 hours that is low I should add. You may want to give it a really good run, full power climb to a substantial height, 5000' or more.

 

if they were all consistent thats a sign its all good. When you have a problem its usually in one cylinder only.

 

Dont stress. Most leakage is via the rings even in a healthy engine, and thats venting out the oil filler tube.

 

Cheers

 

J:thumb_up:

 

 

[BigPete]

My Jabiru is back in the air and running sweet. 4 - yes 4 (oh god yes 4), did I say 4 good compressions! We think its running (my L2 and I) a bit rich, but will leave well enough alone for now.

 

A big thankyou to Jabiru for their "support".

 

regards

 

 

[Guest grantisaac]

Hi Peter , did you make any jetting changes ?

 

 

[Guest Decca]

Great News, Peter, well done!!

 

Decca.

 

 

[Guest J430]

I did a Leak down on the weekend for a fellow with a 4 year old UL with the 2200 engine. It had 265 hrs or there abouts on it.

 

Front Cylinders were 10% or 72/80, and the rear cylinders were 12.5% or 70/80.

 

Not a bad result !!!

 

J:thumb_up:

 

 

[BigPete]

I had the carby jets changed to what Jabiru recommended in their last bulletin. :thumb_up: This was done at around 250 hrs. I hnow have 283 hrs up and I want to do a 300 hour service b4 I head off to Narromine.

 

Looking forward to meeting forum members at Ian's tent (and the free Hot X buns). :big_grin:

 

regards

 

 

[Captain]

Can anyone please advise what is the plug thread adapter size that is needed with the Leak Down Tester for the Jabiru plug hole?

 

 

 

Most on ebay just offer 14 mm, so does anyone know whether they can be readily sourced here if smaller is needed?

 

 

 

Regards Geoff

 

 

 

[facthunter]

Adaptor.

 

Captain, use an old time expired plug, machine the part away that retains the insulator and remove the insulator. I think they are 10 mm. Nev..

 

 

[Guest Andys@coffs]

Quick google of D9EA and thread will show they are 12mm in size. I posted a link to an American ebay auction that has the correct sized adapter earlier in this thread. If you follow that link and then click on the show other items from this seller them pressumably if he still sells them you should see the offerings available. Note that it is important that you use the american ebay and not the australian one as I have yet to see a leakdown tester for sale on the Aust site that has anything other than the std 14mm adapter, and the cost isnt competative.

 

Andy