Dave,
I have a YS-140 sport that I have been very happy with so far. Yesterday it just stopped running mid-flight. I was at half throttle and done one snap roll at the top of a loop, completed the large loop and flew 50+ feet level when the engine just quit. It was high enough to land easily but I don't want a repeat. The engine had three gallons of fuel run last year (20/20) when it was new and was stored for the winter. When I did get it out it was a little difficult to start but ran fine after a tank. I flew with no problem. The second time out for the year was when it stopped. I ran a tank on the ground with no problem. The tank and lines appear clear and the clunk seems OK. It is a central hobbies tank with their NMG muffler in a QQ 72" Yak. Is there something else I should check? I hate to be stupid and just fly it and have the engine cut out at the wrong time. Incidently, when the engine was new, it had a problem with the throttle barrel rotating and you advised me to send it in for service. It has worked great ever since. I thank you for your advice last time and for this time. Thanks

Hard to say, but I would guess that due to the storage there could be debris that is breaking loose inside the engine and messing up the regulator. Strip it down and clean it and let me know what happens after that.

Dave,
When I removed the cowl I found that the teflon tube between the NMG muffler and the header was kinked and had become partially blocked. The header came loose the day before the engine quit and I tightened too much and twisted the teflon tube very slightly. It became kinked and then split thru the side. Some of the exaust was still going thru the muffler as the sound was not changed. I still cleaned out the regulator. It will take a few days to get a new tube and try the engine. Should I get a silcone tube instead of the teflon connector that came with the NMG muffler from Central Hobbies? Thanks.

Man... the silicon will last 30secs compared to the telflon

blabla is right.

Dave,
I replaced the tube to the muffler and then had another dead stick on approach. A friend who has much more engine knowledge than I do suggested resetting the valve clearance and he helped (did it while I learned). The exhaust valve was open almost 1/8 inch. Further checking revealed that the valve was sticking at times. There was no marking or scars on the piston head. The connecting rod or valve stem appear to be straight but the valve does not move freely as the intake valve/spring does. We reassembled the engine and it does run on the ground but I am not trusting it in the air. I already know the answer-- send it in for service, but I thought I'd let you know where I am with it. Thanks for your advice.

The valve seat may be loose in the head.

It has to come in, sorry.

Dave: I have installed many valve seats in much larger engines. The head has to be heated up to 450 degrees. The seat is on dry ice ...very cold. We drive the seat in and when the head shrinks down and seat expands....its there for good. Is your prcedure similar. If a seat becomes loose when head is at normal temp...the bore where the seat goes in was machined too large. A manufacturer defect. Capt,n

I don't install them so I have no clue what YS does.