[NSRCA-discussion] 120 AC issues UPDATE

Tue Jul 15 04:28:55 AKDT 2008

I am following this discussion with interest.  I have similar issues with a 140DZ.  Runs great on the bench, but quits about 50% of the time after the final spin in Masters.  It acts like you turned it off with a switch, will not respond to throttle and just kind of spins down.  The motor was used but ran well for the previous owner and was freshly factory rebuilt.

I tried new plugs, a header tank, finally a new fuel pump. Nothing worked.  In the end I replaced it with a 160, which has run like a clock in the same airframe.   I removed the header tank, it was not needed. 

I too suffered through the dreaded piston rock problem. That was "cured" in the 140's by either replacing the piston with a DZ piston or drilling 5 or so small gas ports from the top of the piston to the backside of the ring grove.  It is not hard, a drill press will do. I don't remember the exact drill size ( around a number 77 or so) but it is not all that critical. 

I am going to get back to that 140DZ after the NATS, it is a shame for such a beast to languish in a box.
If I come up with anything I'll report it here.

 Bob Kane
getterflash at yahoo.com

----- Original Message ----
From: Stuart Chale <schale at optonline.net>
To: General pattern discussion <nsrca-discussion at lists.nsrca.org>
Sent: Tuesday, July 15, 2008 8:02:46 AM
Subject: Re: [NSRCA-discussion] 120 AC issues UPDATE

Interesting email.
I guess in the betterment of science I should do one more test with a
different piston/ring in the offending engine before swapping it out
totally for another motor.
I am getting tired of pulling apart motors though.
What I really want to know is how to measure those gaps :)
I tried but the best I good get was somewhere between 0 and 1 mm with
my rulers :)

Stuart

Dave Harmon wrote: 

Stuart…..here
is an old email from MikeMcCormick that is interesting.

>>>>Because
the YS piston compresses in both the up and down
stroke, the ring is 
serving
double duty.  It must not only seal the mixture being
compressed, but 
it
must also separate the two compressed mixtures.  Once the
piston begins to 
rock
and the ring loses its seal, then surge begins.  If you
think about it, 
the
YS engines with the highest compression are the most
problematic (SC and 
the
L; the L is really just an SC version of the FZ).  Why do
the pistons 
rock? 
No side wall.  Plus (as Dick stated) the pistons are
made of soft 2017 
aluminum
and will wear quickly (especially if the engine is
run lean or fuel 
with
low oil content is used).  This piston design has been
in use since the 
AC
and always been a pain.  But in the 1.2's we could alway install an FS 
Piston
and cure the ill's. 
FYI, I once did a test on an AC
just to see how 
much
more boost the AC piston created vs the FS piston; it
was less than 1/4 
psi! 
I also noticed that any engine in which I installed a
FS piston ran 
better
than it did with the compound type piston.  I figured
this occured 
because
the FS piston ran "truer" in the bore; it
created less drag because 
it
was less prone to rock or cock in the bore (more side
wall).  But along 
comes
the 1.4 and we no longer have the FS piston to fall
back on,  Plus the 
1.4
piston was shorter and bigger in diameter,i.e, more
rocking force 
(around)
the wrist pin is created and there is less length to
stabilize the 
force. 
Things didn't get too bad until YS increased the
compression of the 
engine
(L version) and then the piston really started to get
beat up.  
Because
of the higher compression, the ring seal is even more
important and 
the
piston/ring fit has to be right for the engine to perform
optimally.  And 
the
optimal fit is within a narrow range.  I set the
piston/sleeve clearance 
at
.003 to .0035".  Max clearance is .005"  
.0035" is tight, and engines I 
set
up must be run rich for a long time to properly break
in.  I also set the 
ring
end gap as tight as I can get it, usually at
.0055", though I like it 
best
if I can get it at .003".  But when set up at these
clearances and run 
rich,
they will last 500 plus flights.
So
what is boils down to is the old piston is soft and has
very little side 
wall.
The forces on the piston are high and attack it from
both ends.  This 
creates
high side forces on the piston.  The minimum side
wall works hard to 
keep
the piston straight, but even in the best of conditions
(running the 
engine
rich and using lots of oil) it is a losing proposition
for the piston. 
 The
new piston has 4 times as much side wall area under the
ring groove 
(where
the old style piston experienced the greatest forces
and therefore the 
most
wear).  It runs very smoothly and I believe this is
because it too tends 
to
run very true in the bore.  I am sure it will last longer
because of the 
increased
side wall area.  
Sorry
for the length of this reply, and I hope it answers
your question.  But 
heck
while I am at it I might quickly address one other issue
I see a lot of 
problems
with.  Many flyers tend to set the mixtures too lean
on 1.4's.  
Because
of the different plumbing in these engines, the
regulators do not 
respond
as quickly as did those on 1.2's.  To set these, you
must do it 
slowly
and wait a few seconds for the regulator to equalize
with the change.  
If
you move the needle quickly, and not wait it is very
possible to get a 
false
reading on the mixture and usually it ends up too
lean.  Kills the 
engine
quickly.  

MikeMcCormick<<<<<

Regards

Dave Harmon
NSRCA 586
K6XYZ[at]sbcglobal[dot]net
Sperry, Ok.
-----Original
Message-----
From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Stuart Chale
Sent: Monday,
July 14, 200810:07
PM
To: adriancwong at earthlink.net; General
pattern discussion
Subject: Re:
[NSRCA-discussion]
120 AC issues UPDATE

I have
been mostly using
Cool Power 15%, but tried 30% heli today with the same results.  No
special YS blends available locally.
This plane is for my son to fly.  Sure makes my electric Beryll seem
real
easy :)
Stuart

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