Me again... I have never had so many problems with a YS engine. Must be losing my touch.

Anyway, Hyde soft mounted YS140L, running Wildcat Helimix 30% nitro, Hatori piped, inverted, OS F plug. Installed in 2M wide body with tank on CG. About 16" of fuel line from tank to engine. Only thing in fuel line is one TEE fitting and a fuel filter. Tank holds pressure for days. Lines in good condition and clunk is free. Fuel reg and filter are clean. Valves are set properly. Engine has very good compression. No air leaks that I can find. All seals / rings new this year.

Engine will shut off on extended down lines of about 5 seconds or more.

Seems like it only happens after at least half the fuel is gone. Cannot reproduce problem on the ground. Engine will idle at 2000-2200 RPM for minutes and will still transition to full.

What I notice: down line at idle. Prop always turns. As I apply throttle in level flight, I can hear the intake note change, but engine is not lit, but prop is still turning. I'll blow a little smoke (probably just clearing existing exhaust from pipe) and then the prop stops completely.

I'm stumped. It won't always do it. When it does die I have no warning. I've considered on board glow drive, hopper tank, higher idle, changing low end mixture, but don't really know why the engine is shutting down so I don't know where to start. My gut reaction is that the long lines are allowing a large enough air bubble to be passed on down lines that the engine dies: bladder tank or header tank would fix. However, others have indicated that they do not have this problem in similar setups. Any ideas on what I'm doing wrong? Thanks in advance.

Not a YS specific reponse I know, but I have had similar problems with a Saito 150. It would run fine on the ground, idle all day nice and slow, bu in the air it would quit as soon as you throttled back in say a loop. It was really frustrating until I one day tried richening the idle slightly. It didn't seem too lean before and you couldn't detect any change on the ground but it stopped it quitting in the air.

I would try that, I can't garuntee anything of course and my experience wasn't on a YS but it might be worth a try. Unfortunatley my set up which sounds identical to yours (L, WC30, hatori pipe etc) works perfectly so there must be something out of place. If you change the fuel lines (always a good idea) don't forget about the one inside the tank...

Bump the idle speed up to 24/2500 and see if it improves.

I tried dang near everything, including wearing a home made tinfoil hat to block alien mind control rays. After three check valves, new plug, new lines, mixture changes, etc., it seems as if the valve lash was too close. It checked at spec with shims, but as soon as I loosened the valves by about 1 turn the engine ran much, much better.

Cheers