no compression cyl 1&2 912 ULS

[Dave S]

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.

Carl,

No apologies needed -we feel your pain.

I'll toss out a couple additional thoughts, based on your new observations.

Ralphs comments are right on for evaluating valves vs rings.

Normally (and I am not sure this gremlin is normal), when doing a standard compression test, adding oil to a cylinder is useful to evaluate ring sealing; and, will generally raise the compression readout if rings are passing excessive blowby. Oil won't seal a leaking valve surface so oil addition is useful in diagnosing rings. What does not normally happen is the compression value going from zero to healthy, but the compression will improve enough to measure. An engine with a lot of excessive blowby will normally make the belly of the plane very oily.

You mentioned that the leakdown check for all cylinders was inconclusive. FWIW, the 912ULS engines, if the cylinders are healthy in all respects, will produce fairly narrow compression leakdown results on all cylinders because the cylinder and ring tolerances are very tight - like 78/80 which is less than 3% leakdown - should never see large differences, like 20% for instance.

In the case of an intake valve sticking open, or a broken intake valve spring on engines (not airplane engines) I have observed intermittent backfiring through the carb as the the cylinder fires. Like Ralph said, you will hear a hiss through the intake if an intake valve does not seat.

If an exhaust valve doesn't close properly, the hiss will be heard at the exhaust pipe, per Ralph's comment. Burned exhaust valves don't change and adding oil won't help them - they only get worse. A sticking one (usually meaning the stem sticking in the guide because of crud buildup) can be intermittent. High hour large continentals and lycomings are said to develope "morning sickness" when the valves stick intermittently on startup but the syndrome goes away when the engine warms up and the valves seat properly - since you are not using leaded fuel, shouldn't be an issue but carbon accumulations in the guides from fuel or oil can cause sticking too.

Chunks of crud (usually a chunk of carbon flaking off somewhere) can get pinched between the valve face and seat causing intermittent lack of sealing - this stuff usually wears out or blows out.

In the realm of the most unlikely to the point of being rediculous, I have seen one auto engine troubled with seemed to be intermittent valve sticking - turns out it was a short threaded stud that someone had apparently dropped into the intake and found its way into the valve pocket where it was banging around in the pocket under the valve head and occasionaly holding the valve open - but you know - that is not supposed to happen.

A non invasive preocedure which is really good to do, if you have access to a bore scope, is to examine the interior of the cylinders with the borescope with each piston at bottom dead center. While cylinder scuffing, which certainly screws up rings, is not very common on 912s - it gives a person a chance to rule it out (or verify that it exists).

Tough problem, hoping for good luck for you finding out what the heck is going on.

[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.

[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!

[Delta Whisky]

Carl - I use only 100LL (not because I want to) and the Decalin is used to avoid or at least minimize lead deposit build up. Lead build up will do the same to engines as carbon will but (in my experience) more frequently and in more places. Decalin causes the lead to end up in a different compound during or after the fuel is combusted and it flows out with the exhaust rather than building up. I've not come across it being used as any kind of cleaner - can't talk to that - but my tanks have not (yet?) shown any deleterious effects.

[turboturner]

Thx for clearing that up DW. I just learned that the 912 is best run at high RPM: 5400+. My (ltd) research seems to support this. I usually run mine in the 5200 range. This might help explain carbon buildup. Comments, anyone?

[Delta Whisky]

Carl - I sure hate to put this out on the internet but in my POH I wrote that when on 100LL, the minimum recommended cruise RPM is 5200. BUT - I can't locate where I found it originally so take it for what it is worth. I will say that having written it, I routinely fly at 5200 -5300 (and, like 5300 the best). I'm pretty sure that the 5200 for 100LL is for scavenging purposes and therefore wouldn't apply to mogas. BUT - don't anyone quote me - I don't want to be the one that started an internet rumor!!

[jiott]

Nothing I've ever read by Rotax that says 5400 rpm is best. 5200 is fine; been running there for 1300 hours. Its a sweet spot for me for fuel economy and speed. No carbon buildup noticed at all.

[jrevens]

Nothing I've ever read by Rotax that says 5400 rpm is best. 5200 is fine; been running there for 1300 hours. Its a sweet spot for me for fuel economy and speed. No carbon buildup noticed at all.

Jim,

Have you done any service/maintenance work on your gear box (PSRU) in those 1300 hours? Also, what kind of backlash can you measure at your prop tips? I’m just curious, and interested in the experience of those with more hours than I have on my engine. I was listening to a conversation at OSH regarding a recommendation to keep the rpm at something like 5450 during normal cruise to help minimize wear on the gears that can occur when the prop is more lightly loaded & more backlash is possible.