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YS 91FZ dies

Old 10-02-2003, 10:10 PM
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Default YS 91FZ dies


I hope you can help here. I received my YS91FZ back from you last week. It was badly damaged in a crash and tested out fine on your end after the repairs (as I was told on the phone) I have installed the engine back in a UCD3D inverted (a new one). I am using a 20/20 fuel designed for YS engines (a new gallon).

The engine is having the same problems it was before the crash (actually what caused the crash). A friend and I have been frustrated by this. His is an identical setup and his runs perfectly without ever adjusting the engine. The first flight after getting it back seemed fine. I had left the idle mixture and regulator at the settings when I received the engine. Subsequent flights became a problem. On the ground the engine will run forever at idle about 2100 to 2200 rpms without dying. Top end is great at 9600 rpm's on an APC 15x6 prop. Transition/midrange never hesitates. I have not been able to reliably reproduce the problem on the ground.

All of the problems begin once off the ground. It seems that the low end settings (idle or mixture) won't hold once the engine warms up in flight. If I pull back to an idle while flying the engine will slow down and die after 10-20 seconds. It dies every time I try to land. The idle seems lower. It hesitates badly or dies on transition to midrange. If I get it beyond that and get the throttle up, it will run again. Raising the low idle speed to a higher idle doesn't help much and just makes it hard to land. The throttle barrel comes back to the same spot at idle every time so I don't think it is a servo issue. The engine is run without a cowl currently. Leaning out or richening the idle mixture a little doesn't help. Also fuel is coming from the carburator after it dies.

The fuel lines have been checked and replaced. The sytem will hold pressure very well. No debris has been found in the regulator. New glow plugs don't help either.

Sorry for the essay but this is quite frustrating. I don't want to crash it again and right now, I don't trust the engine to stay running.


Chris Massa
Old 10-03-2003, 07:08 AM
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Default RE: YS 91FZ dies

Greetings Chris,

While you are waiting for Dave to reply, I will say that you've describbed the "CLASSIC" symptoms of over-adjusted valves, especially the exhaust. I've worked on several YS engines doing exactly what yours is doing. Every one of them had the exhaust valve too tight. There's also a possibility that your ring isn't seating after the engine warms up and un-loads (while flying). This isn't as common but it does happen occasionally.

Visit my YS webpage, to get a detailed description of how YS engines work, and how to set & maintain them. You will find it to be most beneficial...

Mark Fuess
Old 10-06-2003, 10:37 AM
Dave Shadel
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Default RE: YS 91FZ dies

It sounds as if the regulator is set too lean. Try opening it 1/2 turn and see if the situation improves.


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