VERY low ime 912 ULS - leaking exhaust valve

Probably lead building up on the valve seats. I’d remove the rocker arm and spring and try spinning the valve against the seat.

I'd be surprised you have a bad valve with only 70 hours on a new engine. If it truly is a bad valve it would be a true anomaly. 

Troubleshoot first.

Most likely these two things causing the issue. First you may have the cylinder on the wrong stroke which would have the exhaust valve open and air coming out the exhaust. 

Next the rings have slots in them and turn as the piston moves up and down. Once in a great while the slots may line up so when you do the compression test the air leaks out. I see this maybe twice a year. Just go out and run the engine for 5-10 minutes and then try your compression test again. When I do this it always re-adjust itself.

Do these before you worry about a bad valve. It's cheap, easy and doesn't cost money and a common cause for low compression test pressure. 

I know a lot of folks like Decalin, and I have no experience with it personally.  However, last year a friend of mine with a 914 powered aircraft switched from automotive fuel to 100LL plus Decalin. The engine had about 150 hours at the time.  In the first few hours of using Decalin he had one cylinder that you could feel was low compression when you pulled the prop through. His turbo wastegate also started sticking. Just like Oliver describes, the differential compression test showed that cylinder was in the low 70s, and the borescope inspection was unremarkable.  We cleaned up and lubed the wastegate, which was covered in a white substance from the Decalin. He switched back to auto fuel and flew the aircraft for an hour and the problem with that cylinder cleared up. 

It should be noted that this aircraft had used 100LL without Decalin before this, maybe 25% of the time without any problems.  So Decalin was the only new factor.  I have read that Decalin may cause these problems early in its use as it may dislodge existing lead deposits. But I sure did not like the white substance that everything was coated in from using Decalin.  The heads in the 9 series Rotax engines run considerably cooler than conventional air cooled engines. I’ve have wondered if these cooler temperatures allow the valves to more easily accumulate the lead conversion byproduct Decalin and similar products produce.  

Thank you very much for your responses.

I would like to clarify that I am certain that the leakage is through the exhaust valve not the piston rings as I am hardly getting any burping in the oil reservoir while there is a very noticeable hiss and air flow coming from the exhaust. I am also certain that I am at TDC of the compression stroke.

I just spoke with Brad at Advanced Powerplant Solutions.

He stated that it is extremely rare that such a low time engine needs cylinder head work.

He is rather suspecting that something might be stuck on the valve seat.
Before we do anything else, we should therefore bring the piston to TDC, increase the pressure to 100 psi and then stake the valve a few times. If this improves the leakage, we should run it hard for an hour or two and then repeat the measurement.

So, that's what we'll do next.

Oliver

Hi Jeff,

There are many many people that have been  using Decalin for years without any issues. 

I'd  do what Bret said to try or go run the engine for a while and check it again.