YS FZ91 Problem

[Ed Miller]

If all the obvious checks out, here are a few suggestions. Try a
different/new jug of fuel. Replace the check valve with a new one. Deeper,
more serious things to check is compression when hot and listen carefully as
you turn the engine over and pay close attention to the feel on the prop as
the piston rotates past TDC. If this feels fine, remove the plug and check
again, you are looking for a mild bump at TDC if there is a problem,
possible piston to head interference. If compression feels normal  hot and
when turning engine over and you do not hear any hissing past valves (
remove muffler ) or at air box joint, another thing to look for is the
piston hitting the head. You'll have to remove the cylinder/head assembly
and look carefully at the edges of the top of the piston above the ring
land, especially on the exhaust port side, if it is hitting the head it will
be shiny. If this is the case, a second cylinder base gasket will do the
trick. One last adjustment check is the idle air bleed screw, which does
adjust the idle air/fuel mixture. It is possible to have the idle too lean
( screw out too far ) and the regulator screw overly rich ( again screw out
too far ), this would show up as a hot idle problem as the idle tents to
lean out a bit as internal engine and fuel temperature rises.
Ed M.
----- Original Message -----
From: "John Ferrell" <johnferrell at earthlink.net>
To: <discussion at nsrca.org>
Sent: Thursday, January 29, 2004 1:57 PM
Subject: Re: YS FZ91 Problem


> Could it be that the idle is just set too low?
> Does the tank hold pressure OK?
>
> John Ferrell
> 6241 Phillippi Rd
> Julian NC 27283
> Phone: (336)685-9606
> johnferrell at earthlink.net
> http://DixieNC.US
> NSRCA 479 AMA 4190  W8CCW
> "My Competition is Not My Enemy"
>
> ----- Original Message -----
> From: "Henderson,Eric" <Eric.Henderson at gartner.com>
> To: <discussion at nsrca.org>
> Sent: Thursday, January 29, 2004 11:21 AM
> Subject: RE: YS FZ91 Problem
>
>
> These are always tough to answer and, although I have two of them, I don't
> really feel that I have the knowledge base that I have with the 1.20-1.40.
>
> The servo and fuel line sound like good suspects. Also look for tank
> pressure after the run is over.
>
> I did have some idle problems. I believed it was the diaphragm but did not
> have a spare for the 91. In the end I fitted a complete regulator assembly
> from a YS 1.20/1.40. It worked. Looking at the 91 diaphragm it is
different
> in that it is a one piece gasket and diaphragm combo. Two of mine were
> pretty distorted. Central Hobbies has the spares.
>
> You can test the engine on the ground. Use a minimum of 20% nitro and a
> fresh OS "F" plug. It should idle for at least a full five minutes. Pick
up
> the plane and hold the nose up. It should keep running and maybe speed up
a
> fraction due to the leaner feed. Point the nose down at about 20 degrees
to
> level. Should still idle but a little slower. If it does all of these
things
> right the idle is right. If it quits with nose up it is too lean. If it
> quits with nose slightly down it is too rich.
>
> So how do you set the idle. That screw is more of a transition screw than
an
> idle adjuster. It need to have a reasonably correct fuel flow to work. The
> YS at idle will actually draw fuel without pressure. The regulator
prevents
> too much fuel reaching the engine at idle. I set my idle with the
regulator.
>
> Start with Main needle at approx. normal position. Tank fully pressurized.
> Engine warmed up. See if it will hold an idle. Turn in for lean and out
for
> rich, about a 1/16" of turn at a time. If idle is good, reset main needle.
> May be leaner or richer depending upon what you do to the regulator. Then
> you can set the transition mixture.
>
>
> Good luck,
>
> Eric.
>
> P.S. Where are you??
>
>
> -----Original Message-----
> From: discussion-request at nsrca.org
> [mailto:discussion-request at nsrca.org]On Behalf Of Ed White
> Sent: Thursday, January 29, 2004 12:11 AM
> To: discussion at nsrca.org
> 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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