YS FZ91 Problem

[Ed White]

Folks,

We have a club member who is really interested in
pattern and bought a ARF pattern plane and put a YS
FZ91 in it to fly Sportsman and Intermediate.  The YS
has been a problem since he bought it.  It starts
easily and runs great on the ground, idles beautifully
before take-off, has good power in the air and flys
the intermediate sequence well.  Then he sets up for
landing and on the downwind or base leg the engine
quits.  I don't mean occasionally quits, I mean every
stinking time.  

This is a very low time engine and the only time I've
seen it land under power was when the throttle linkage
slipped and he couldn't make it idle.  We've checked
for pressure leaks, run the same magnum fuel I use in
my FZ140, tried the idle leaner, tried the idle
richer.  He sent the engine back to YS who installed
an "updated" piston.  All to no avail.

I finally got him to give me the engine to tear down
and look at.  I haven't got very far into it, just
pulled the valve and airbox covers and diaphragm.  I
was sorta hoping to find an air leak but so far
nothing obvious.  Any thoughts/experience from the
community?

Thanks,

Ed

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