Quote Originally Posted by turboturner View Post
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.