no compression cyl 1&2 912 ULS

[turboturner]

I discovered this issue pre-flight. Engine ran fine on previous flight 3 days prior. Strange that both forward cylinders would lose all compression simultaneously. I there a seal that separates the forward & rear cylinders?
Comp #3:136, #4: 130. Zero on 1&2. Obviously an internal problem. Thoughts, please? Thnx.

[DesertFox4]

May be sticking valves. How exactly did you discover the issue? By hand turning the prop or running the engine and it ran different from the previous run?

[Dave S]

Turbo,

Zero compression is indicative of valve(s) that are not closing. Normally would not see two fail instantly at the same time. Two 100% dead cylinders - the engine should run like junk. If the engine ran fine at an earlier flight, this is very strange.

Curious what kind of compression tester you are using? Are the pressures you indicated in PSI?

[turboturner]

thnx for writing, gents.
Compression gauge is in PSI. I discovered the problem by hand-turning the prop & noticed the “reduced” compression. That the two fwd cylinders would lose compression simultaneously is indeed strange. Yes, could be sticky valves, but both at the same time? Engine will not start.

[Slyfox]

a strange one but if running 100ll very possible. I would remove the valve covers and check to make sure the valve are being pushed in. compare to the good cylinders. I don't have the firing order on hand would that be the two front cylinders like a lyc. next would be to do a differential compression test to determine if it's intake valve, exhaust valve or piston rings. that should get in the right direction. like I said earlier, you can take the covers off. Then smack each valve to see if they are stuck. try another compression check.

[rv9ralph]

I had this happen to one cylinder on my early 912UL. It turned out to be the valves needed lapping. Called a Rotax Maintenance Tech, we pulled a cylinder and he lapped the valves.

To test, when doing a leak down compression check, listen to see where the air leaks... intake manifold/carberator it is the intake valve, out muffler it is the exhaust valve. Into the case and burbles the oil, it is the rings.

[Delta Whisky]

I've had a stuck valve - twice - and staking it fixed the problem. In both cases it was #2 cylinder. There may be something about the front cylinders that makes them more prone.

[Slyfox]

I've had a stuck valve - twice - and staking it fixed the problem. In both cases it was #2 cylinder. There may be something about the front cylinders that makes them more prone.

just what I was thinking. take and rap on the end of the valve and check for compression, you probably will feel the difference when pulling the prop through. afterwards if the engine will run take and put about 2 ounces of mmo Marvel mystery oil in the fuel for every 5 gallons and run it that way for a while and it should clear it up. another would be to decarbon in the same way. maybe a whole can of cleaner(seafoam) to a small amount of fuel maybe a gallon of fuel. good luck

[turboturner]

TY gents for your thoughtful advice - very logical tips. Apologies for my tardy reply. Have been talking to 2 Rotax certified techs. They offered similar advice. Having the problem on the 2 front cylinders simultaneously is throwing everybody off. Still troubleshooting. I recovered compression in the faulty cylinders by adding oil directly into each. Ran the engine: normal ops. Next day: no compression on cyl 1. After re-checking compr on other cyls (WOT & throt clsd, individually w/o oil then after adding oil) cyl recovered its compression again! Leak down chk all cyl inconclusive. For info I run it on super unleaded (not 100LL) - thnx for asking. I like the idea of a good cleaning run. If it works it’s less hassle than shipping the engine for a top overhaul (which I will do if req’d). Will keep you posted. Thnx again. Carl.

[avidflyer]

Clear back in high school shop (auto mechanics) class we were taught this: if you had low compression, and added oil to the cylinders and it didn't make any difference, it was the valves. If the compression came up, it was the rings because the oil was able to temporarily seal off the rings. It's not real hard to pull a cylinder off a 912, but if one has no mechanical experience at all, it's best left to someone else. JImChuk