This is a common issue with the Jabiru. Yes, the .040 orifice is correct for doing a differential test on the Jabiru using a 12mm spark plug adapter. It's not at all uncommon for the compressions to be way off on them. Take a soft mallot and rap on the top of the rockers a few times to pop the valves in and out to knock the residue off the valve faces. I often times staked the valves with 80 psi applied to the cylinder. They will seat and the compressions will come right back up. Dynamically, they aren't leaking. What you are seeing is the product of running it on a steady diet of 100LL with no additives to help soften the lead or cause it to slough away easier.
One Jab 3300 I used to maintain in a Sonex was always run with some Marvels as a fuel additive and always checked good on the first pass. Another identical engine I maintained in a CH-750 just got a steady diet of 100LL with no additives. The compression checks always looked terrible, typically 30/80 - 50/80, then I'd rap on the valves a bit and they would always come back up and pass with readings in the upper 70s. Both engines always performed smooth and flawless and neither ever required any cylinder or valve work while I was involved with them. To the best of my knowledge since I left the area, that is still the case with them. I would suggest he stake the valves to get good compression readings. Maybe use some MMO as an additive to the fuel to soften the lead deposits. And plan to fly it for a very long time with minimal maintenance. You're chasing a non-problem. Yes, my KR with the O-200 dusted the Sonex pretty easily. He could out climb me at high altitudes, but once we pitch over to cruise, it wasn't even close. :o) -Jeff Scott Arkansas Ozarks > Sent: Saturday, May 01, 2021 at 3:28 PM > From: "Flesner" <fles...@frontier.com> > To: krnet@list.krnet.org > Subject: KR>Jabiru 3300 engine? > > > This question is for those that have knowledge / experience with the > Jabiru 3300 engine. A friend has one in a Waiex that has some low > differential compression reads. First I had to sort through his quick > disconnect fitting problems. I got that resolved and my first question > are the reads valid with a .040 orifice in the gauges? Jabiru site > gives no info on this, just a differential read of 25% or 80/60. > General info is .040 orifice for cylinders with less than a 5" bore. > > Next question. Are valves a problem on these engines? He has an oil > change in five hours and I suggested he fly it till then as engine runs > smooth and develops full power and we'll check them again. If they get > worse he can take action. If no change and engine continues to run > well, I suggested he remove the heads to be re-worked come late October > when in the first week of November he goes out of action for five months > with a shoulder replacement. He is down for a spell at that point anyway. > > Good advise? Bad advise? Who knows advise? > > I can, by the way, dust him in my KR with an 0-200 and he has 130 hp. > But that's another story.............. > > Larry Flesner ________________________________ -Please see LIST RULES and KRnet info at http://www.krnet.org/info.html -Change list delivery options at https://list.krnet.org/list/krnet.list.krnet.org/ Affinity List Info Board -Search recent KRnet Archives at https://list.krnet.org/empathy/list/krnet.list.krnet.org/ -Search John Boyea's decades of archive at https://www.mail-archive.com/krnet@list.krnet.org/
