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Low compression on two cylinders.

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Low compression on two cylinders.

Postby johnwp » Mon May 18, 2020 11:25 pm

Hello All.

I am new to Jabiru engines but being an A&P I am anxious to learn. I recently bought a Zenith 701 with a 2200, with hydraulic valves, with less than 150 hours TT. I ran a compression test today and two cylinders were great at 75 and 76 over 80. But two were really bad at around 45 each. I can hear the air escaping through the exhaust. Borescope shows the exhaust valves look perfect. I suspect they need to be lapped. Can I simply remove the heads and lap the valves? If so what should I look out for? Will I need new head gaskets or does it even have head gaskets? Any advice from an experienced Jabiru mechanic would be greatly appreciated.
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Aircraft: Zenith 701

Re: Low compression on two cylinders.

Postby diablo » Tue May 19, 2020 7:56 am

johnwp wrote:Hello All.

I am new to Jabiru engines but being an A&P I am anxious to learn. I recently bought a Zenith 701 with a 2200, with hydraulic valves, with less than 150 hours TT. I ran a compression test today and two cylinders were great at 75 and 76 over 80. But two were really bad at around 45 each. I can hear the air escaping through the exhaust. Borescope shows the exhaust valves look perfect. I suspect they need to be lapped. Can I simply remove the heads and lap the valves? If so what should I look out for? Will I need new head gaskets or does it even have head gaskets? Any advice from an experienced Jabiru mechanic would be greatly appreciated.


You say that you have low compression on two pots; which pots ? As it is an early hydraulic lifter (serial number would help), it could well be that valves are not closing correctly. There are high leak hydraulic lifters that are available from Jabiru and are aimed at dealing with this problem so it is best to check. A very straight forward check you can do is to see whether you still get air leaks through exhaust valves when engine is cold.

Removing the heads is relatively straight forward and simple.It does not require other than basic mechanical skills to do and as far as parts, all you should need is an exhaust gasket if you have the older large finned heads (early lifter). The procedure for removing heads is as follows ...

1. remove baffles, plugs and any wiring (cht probe etc ... ect)
2. loosen oil feed T and remove oil feed pipe
3. remove tappet cover and loosen off tappets (you should be able to remove pushers at this point)
4. remove exhaust and inlet manifold from head
6. Remove head bolts and remove head

There is no gasket. Inspect heads and check valve is sealing correctly by injecting fuel (with a syringe) into exhaust and inlet port and checking for leak around valve guide.
Sent from my "Electronic Numerical Integrator And Calculator" E-N-I-A-C
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Re: Low compression on two cylinders.

Postby johnwp » Fri May 29, 2020 4:17 am

I was able to raise one cylinder from 45/80 to 75/80 by lapping the exhaust valve. (simple job) The exhaust valve in the other cylinder was too far gone. I, just today, got my new valve from Arion Aircraft, LLC. I'd like to add that Arion Aircraft is a great company to deal with; very personable and knowledgable. I will report back after installing the new valve. I love the simplicity of this Jabiru engine. I just finished a complete Top-Overhaul on a Continental TSIO-520 and after that, this Jabiru is a pure pleasure to work on.
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Aircraft: Zenith 701

Re: Low compression on two cylinders.

Postby johnwp » Fri Jun 05, 2020 10:01 pm

I installed the new valve and the compression check was 73/80 cold. I am a happy camper and I proclaim the engine Airworthy. I love the simplicity of this Jabiru engine and I'm just going to fly it and keep a close eye on everything. I know the old engines have a bad reputation but for now, at least, I cannot see why. Time will tell.
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Posts: 3
Joined: Fri Apr 24, 2020 11:53 pm
Aircraft: Zenith 701


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