Things to try before removing a cylinder with low compression?

Hello all,

I have one cylinder with compression in the low 60's on a 3300A with 300 hours. When checking compression I can hear air escaping when I remove the dipstick. This tells me air is flowing past the rings.
This is cylinder #2 which always runs cool (never over 300 degrees F). One thought is that the the rings are gummed up with lead (I have always burned 100LL). Hopefully it is not a broken ring.

I am looking for suggestions before I tear the cylinder off. I do plan on borescoping the cylinder to check for anything obvious. Other suggestions I have heard include:

1. Fill the cylinder with Gumout or Sea Foam and let it sit a couple days to remove lead deposits. Then change the oil, fly and re-check the compression.
2. Add Marvel Mystery Oil to the oil and the fuel to help clean out lead deposits. Maybe do both #1 and #2.

Thanks in advance for any other tips or feedback on the above ideas.

Zenith Zodiac 601XL

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Mike bush webinars talk about this some

There are lots of them on youtube

You may want to go fly a few hours and retest.

If the rings can float they may just be lined up with the gaps. Mike says low compression on its own doesnt hurt the motor.  My 2 cents, worth even less



i believe Jabiru recommends a cylinder get removed periodically for general inspection... i do not however.  i remove the valve covers often however.  if there is a problem with valves, you will see it immediately inside the cover.  remove the entire side and compare.  i realize your rings are the problem, but mention it as a longterm preventative habit and it is easy.

if you are beyond warrantee, you can drain the AVgas and run 10 gals. of Ethanol Free Mogas 88+ with a lead scavenger.  and/or Marvels.  Marvel Mystery Oil is great stuff.  mixed properly - small amount if i recall, like 2 fl. oz./gal.

i agree with Fred that low compression is not harmful.  

as long as the exhaust valve is seating to get it's time to cool... just keep an eye out, and remove the cylinder when it's convenient.

John mentioned a winter Jabiru clinic coming up.  perhaps we can encourage them to show a full cylinder overhaul.  i was unable to confirm with them if this would be covered during their clinic.

others may add to, or even object to, my suggestions... but i feel with jabirus, we all need to continue to monitor because the engine is amazing as long as one is willing to pull the cowling and visit and revisit.

all the best,


CH750 STOL/Jabiru 3300


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