[NSRCA-discussion] OS 120AX header?

[Jay Marshall]

I believe that the stock silencer for the 120AX has quite a bit of back
pressure.  In my Venus I moved the fuel tank to the most reward possible
position and tried it w/o a pump. No difference in performance from the
forward position even on long verticals.

 

Jay Marshall 

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Bob Richards
Sent: Saturday, October 06, 2007 9:22 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] OS 120AX header?

 

Jim,

 

I have not run my 120AX enough to conclude anything yet, but the little bit
that I have run it left me thinking there was not enough muffler pressure
either. I tried with/without the internal baffle, but I think the best combo
would be to leave the internal baffle out and try restricting the tailpipe
as you say.

 

There are a couple of guys in our club that have the 120AX running the Jerry
Smith mousse can mufflers and get MUCH better performance, and more reliable
running as well since there is considerably more muffler pressure. They also
fly several Outlaw delta planes with .32 size engines and do incredible
manuevers, using the same type muffler setup. The only way they can pull the
Gs they do and still run is because of the much higher backpressure.

 

With a large volume muffler (the larger the better) the more pressure you
can run in the muffler and the engine still run fine. In fact, you will burn
less fuel as well since less excess fuel/air mixture blown out of the
exhaust during the exhaust scavenging.

 

I did a lot of muffler/prop experimenting (sport planes) many years ago. :-)
I learned a LOT.

 

Bob R.

 


J N Hiller <jnhiller at earthlink.net> wrote:

16 x 10 should be no problem. I have run one all season with stock muffler.
I didn't like how many RPM's the 16 x 8 turned in horizontal flight.

This may be a little off subject but may be of interest to other 120AX
users.

I fought a rich midrange (poor transition) from day one in order to keep
from running too lean on a long vertical pull, which seamed to get worse in
cool air. I have two motors that have different carburetors and focused on
this as a problem even though both acted similar eventually concluding that
there wasn't enough muffler pressure. I have never gotten along well with
pumps so I tried restricting the muffler outlet using a short exhaust tube
(3/8" ID) with a brass tube insert. I tried a couple different sizes
settling on 13/32 OD, which is pinched on the end from cutting with a tubing
cutter (.350" ID). Flaring the other end keeps it in the exhaust tube
without clamps. I also tried an OS-"F" plug which I thing may be better than
the #8. It will be 9 months before I can run it in hot weather but I was
able to run leaner without starving in the verticals and the transition is
immediate.

Jim Hiller

Spokane

 

 

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