YS FZ91 Problem

[Ed White]

Folks, thanks for all the responses!
 
I'll try to answer as many of the questions raised in previous emails as I can.
 
He does have a computer radio, a Futaba 8UAF.  He is not using the Idle Down function or mixing that would affect the idle.  I have the same radio and am very familiar with those functions.  
 
We set the idle on the ground to be around 2000-2100 rpm and set the low speed adjustment as you would on any other YS, so the idle is constant, doesn't speed up or slow down.  Don't know that we ever let it idle for a full five minutes but certainly for more than 2 minutes.  Then has good clean transition and goes to full power.
 
We have tried speeding up the idle.  If it is speeded up enough to keep the engine running the plane is nearly impossible to land because it won't slow down.  Back down on the throttle trim to something manageable for landing and it quits.
 
We've run Magnum #1 and changed glow plugs (OS #F).  And replumbed the fuel tubing.
 
It seems to hold pressure just fine, plenty of pressure even after a dead stick.
 
It was sent back to YS and as I said they installed an updated piston.  I didn't mention that they bench ran it for a full tank.
 
The gaskets I looked at looked good.  They're stock YS.  As far as I know, leak free gaskets aren't available for the 91's.  The regulator diaphragm also looks good, no distortion.  The mating surfaces under the gaskets have no nicks or other obvious leak paths, but neither are they polished smooth.
 
Loose engine mount and or crappy throttle servo - Hmmm, now that is something I haven't looked at.  We may have something here.  I noticed the throttle barrel has a bit more drag to it than I have typically seen on most engines, but not so much that it seemed  excessive.  He has flown this exact same plane (same servo and throttle linkage) with an OS 91 Surpass.  Idle was great, rock solid, couldn't ask for better.  Power was less than pitiful.  
 
I'll continue the tear down and inspection of the engine and then check the servo and mount.
 
Thanks again,
 
Ed


Earl Vincent <ev3464 at sccoast.net> wrote:
Ed, I know you said it was a club member that is interested in pattern, does
he have a computer radio and does he turn on any kind of mix or landing
switch that may have a throttle low set in it, this is not assuming he has a
four channel Conquest. Us pattern flyers are not the only pilots that have
hi end radios.
Earl Vincent
----- Original Message ----- 
From: "Ed White" 
To: 
Sent: Thursday, January 29, 2004 12:11 AM
Subject: YS FZ91 Problem


> 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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