Jabiru gen4 valve spring.

[Thruster88]

I have a friend with a lsa55 jabiru with a gen4 engine about 4 years old and 50 hours total time. Over the last year or so another L2 friend has attempted to resolve a slight roughness in the engine, it seems to occur at cruise rpm. Carburetor, jets, fuel flow, spark plugs, ignition system changed etc have made no difference, all very frustrating. A few weeks ago the owner and I went for another test fly. As we rotated the engine lost power and became very rough. Taxi back in engine running rough the whole time.

 

I got out and flipped the prop over, no compression on one cylinder. Suspected a stuck valve. Five minutes later flipped prop over and very good compression on all cylinders. Later we did a cylinder leak down and all were 76-77. Next we ran the engine up keeping an eye on cylinder head temps, it is very cold atm. After about 2 mins the engine started to run rough. Flip prop no compression on one cylinder. Got leak down test done as quickly as possible, every cylinder 76-77. Repeat two more times, same result, no compression then comes good very quickly. So we Suspected a hydraulic lifter problem.  Order parts.

 

Removed rocker cover from problem cylinder and found the big valve spring was broken. Pulled cylinder to check for other damage, none found. We will replace hydraulic lifters on that cylinder and all valve springs in the engine.

 

The valve spring was broken in two places, suspect this happened at two difference times, one break small problem,  two breaks big problem. 

 

So in a way it was a lifter problem, the little valve spring only is not as strong as the hydraulic lifter. This was on inlet valve. Engine was starting and ideling perfectly. 

The valve springs are very cheap but this has been a costly ordeal. I post for educational purposes. The valve spring can be seen in three pieces. 

 

 

 

[facthunter]

Pretty crap metal for that to happen and better if it was wound progressive or conical to avoid harmonics. Nev

[onetrack]

They're simply standard automotive valve springs, aren't they? If they're OEM, you'd shudder at how they're manufactured, and how little QC goes into them.

 

The last engine I rebuilt, I used valve springs from SBI (S.B. International), supplied by NASON - and SBI products are aftermarket, high performance springs, made in the U.S. And the engine I rebuilt wasn't a high performance race engine, or an aero engine, it was just a regular industrial diesel engine. I fully expect the SBI springs to perform better than OEM springs.

 

https://www.sbintl.com/Product/DetailsENG?productId=20

 

Edited Sunday at 12:47 PM by onetrack

[facthunter]

Gen 4 is Chinese isn't it?  IF you know what you are doing you can change the Valve springs without removing the Cylinder on engines like that. Nev

Edited Thursday at 05:43 AM by facthunter
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[Thruster88]

1 hour ago, facthunter said:

Gen 4 is Chinese isn't it?  IF you know what you are doing you can change the Valve springs without removing the Cylinder on engines like that. Nev

Yes, we plan to change the valve springs without removing the other 3 cylinders. Less surgery is always better.