Dori1,

There could be a number of issues. First of all 70hrs of run time if the normal flight is 10mins this is likely over 400 flights. Its past time for a new ring. The engine has likely been down on power for some time.

I usually will say most of the ringed engines will go 250-300 flights....before its time to replace the ring. Yes you can replace just the ring without the cylinder but as with all things the sleeve will wear and as a result it will be a shorter time the second ring before it needs a replacement. Say 400 flights you might get 80% of that before you need the new ring again.

Another issue could be some carbon or "coke" build up on the exhaust valve itself. This could be keeping it from closing. Another option could be the valve is damaged somehow and is leaking when its suppose to be closed.

The ring wear is common with all ringed engines. Replacing the ring is cheap and easy. You just have to take the engine apart. A new ring is about $20 US. You should also do bearings. The ring wear happens faster if you are in a dusty dirty environment. It also happens faster if the engine is heavier loaded...slightly too big of prop. But to go 400 flights on a ring you have lots of time on this engine.

If the ring is worn then engine will not run properly. The YS engines gets supercharge effect from the pistons down stroke and pressure inside the case. If the ring is not sealing well enough this supercharger action is not working properly so a full charge doesn't get to the engine for combustion, and since the ring is leaking on the up stroke now the charge that is not "full" is leaking past the ring and not being used for combustion. So it will tend to make lots of smoke and not run properly. Especially in the mid power ranges it will suffer and quit.

I would say you should have YS Parts and Service look it over. Connecting rod could be worn, and other things like the rear disc valve may need some service too. I know sending this engine from overseas to the USA is difficult but it would likely be more cost effective that just buying a bunch of parts and then not having the experience to open the engine up and see what needs to be done. Likely everything is fine but 400 flights is a long time and should see a checkup by someone experienced.

Troy Newman
Team YS