leak down low on one cylinder / oil usage

Kevin Stewart wrote:

I had an increasing oil burn in my previous 912UL. It turned out to be the piston oil control rings. The springs behind the rings had worn grooves in the back of the rings which prevented them from pushing tightly against the cylinders. Unfortunately it's only possible to see if this if you remove the cylinders. I didn't have any other symptoms other than the ongoing oil consumption. The plugs always looked good.

What you describe sounds like possibly what I have. What did you do if anything?

I had the engine rebuilt with a new set of piston rings, however the oil consumption increased to about 120ml per hour after doing this and despite running it for many hours to try to get them to bed in, the oil consumption never settled. In the end I bought another engine. It was a shame as the old engine looked brand new after it had been rebuilt.

It was rebuilt by a reputable Rotax specialist who agreed to look at it a second time. He couldn't find anything wrong, everything measured within the manufacturing tolerances.

I think that if I had this situation again, I would just keep adding oil unless the consumption was unreasonably high or I had other symptoms. If there are other symptoms that clearly point in a particular direction then I would investigate that area. If a particular cylinder is under suspicion, then it isn't too difficult to pull the cylinder and take a look at the rings.

What plugs are you using? Plug gaps and do you use the thermal paste? have you checked the valves to see if there is any debris trapped under the valve edge?

Roger Lee wrote:

What plugs are you using? Plug gaps and do you use the thermal paste? have you checked the valves to see if there is any debris trapped under the valve edge?

I have always used NGK DPRE8E 4339, gapped to .025 and thermal paste. My secondary concern is the oil consumption, how would a valve that is not seating affect the oil consumption?

Walt,

Not sure how this will go over on this forum but it's worked for me on a few occasions.

But I've had to use Avgas on any cross country trips I take (a lot) and my "debris" was most likely lead but it could have been carbon.

I've had on a few occasions a low cylinder (80/64) and even though my "fix" that I discovered is used on the legacy engines I found that it has worked for me on my 912ULS with 1400 hours as well. On the low cylinder I put the air on it after removing the valve cover, then taking a small plastic or rubber hammer, tap the valves. Just hard enough to lift the valve off the seat and let it pop back down. This may knock any "debris" off the valve/seat(s) that may be on there and the pressure moved from 80/64 to 80/79. I emphasize tapping because you don't want to smack a valve into the piston! The first time I had this happen I pulled the head and lapped the intake valve back in. The next time I tried the "tap method" and it saved me some work. In all instances I could hear air blowing back through the carbs indicating an intake valve leak. So when you do your check, you might make sure your in a quiet area and then listen at the carb intake or at the exhaust pipe for any air leak by while you have air on your #3 cylinder. If there's no air noise, then as others have mentioned it may just be normal wear on your rings. I believe we are allowed up to 20% differential per the Rotax manual. Just stay clear of that prop!

On the oil burn issue I have pretty much the same thing. I did go in and replace the intake valve seals just in case they were the problem. They were hard but I'm not sure this improved anything. After an oil change my engine doesn't use any oil for the first 25 hours. As the oil starts to get darker it will start to use about what your experiencing. When it was new it might use 200ml of oil between oil changes at 50 hours. Now it's up to 400-600ml so pretty good I'd say. I reference ml because that's what's on the side of the oil bottle.?

Hope this helps