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]

Update for all who are interested:
TY again for the additional comments and suggestions. I discovered that I am not alone to have had this issue, some of which has "self-cured". All fingers point to carbonized cylinders and sticky rings. Note that there are no fluid leaks (coolant, oil) to indicate internal breakage. On the advice of a Rotax certified tech: "run it on 100LL & fly it like you stole it". I drained the tanks & re-fueled with 100% 100LL. Also added Sea Foam (thks Steve). Extensive ground run at high RPM, followed by test flight. Engine starts, operates & sounds 100% normally. Compression OK before & after ground runs & flight. More "cleaning" runs to come. Hopefully cured but will add Sea Foam to fuel occasionally to help prevent carbon build ups.

[avidflyer]

Maybe I'm all wet, but I would think that 100 LL would stick up an engine way faster than mogas. But if it works, go for it. JImChuk

[Delta Whisky]

Carl - I run Decalin with every 100LL fill up and highly recommend it. It greatly reduces lead deposits and that could be an issue with rings (it was on a small truck I owned years ago) and it definitely reduces plug fowling. In any event, glad to read that you are back up in the air.

[jrevens]

Makes sense to me… thanks Jim! I don’t believe any of the talk and opinions about it that I’ve heard have come directly from Rotax.

[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.

[turboturner]

thx for the recommendation DW. Do you use it because you run mostly on 100LL? Just wondering if it’s as effective on Mogas. Read that Decalin is a solvent for resin (?). Could it have an effect on fibreglass fuel tanks? In any case, adding a carbon cleansing agent regularly to the petrol seems like a good idea. I’m sold!