Message-ID: <36139011.29A1@ibm.net>
Date: Thu, 01 Oct 1998 09:22:09 -0500
From: Snowbird <snbird@ibm.net>
Reply-To: snbird@ibm.net
X-Mailer: Mozilla 3.0 (Win95; I)
MIME-Version: 1.0
Newsgroups: rec.aviation.student
Subject: Re: Sacred Art of Mixture Control (Re: Sudent using KATANA?)
References: <36122B11.1EC0@ibm.net> <19980930114347.05884.00004033@ng-fd1.aol.com>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
NNTP-Posting-Host: 32.101.53.214
X-Trace: 1 Oct 1998 14:55:29 GMT, 32.101.53.214
Organization: IBM.NET
Lines: 61
X-Notice: Items posted that violate the IBM.NET Acceptable Use Policy
X-Notice: should be reported to postmaster@ibm.net
X-Complaints-To: postmaster@ibm.net
Path: news.jprc.com!dca1-feed2.news.digex.net!digex!news1.radix.net!tor-nx1.netcom.ca!news.maxwell.syr.edu!newsfeed.cwix.com!165.87.194.242!newsm2.ibm.net!ibm.net!news1.ibm.net!32.101.53.214
Xref: news.jprc.com rec.aviation.student:34030

HLAviation wrote:

> >I don't know about your plane, but back on the same topic, the
> >point stands that if the plane is equipped with a carburator which
> >has a seperate idler circuit, it's apparently debatable whether
> >adjusting the manual mixture control has much effect on the mixture
> >when taxiing about at low rpms.

> Huh? How do you shut down your engine?  In a plane w/ mixture control > its done by pulling the mixture controll out and it cuts out from > being too lean to run.
> If this doesn't exemplify the effect of the mixture control at 
> low RPM I don't know what could.

Well, no, from an engineering standpoint that doesn't necessarily
follow at all.  It simply demonstrates that at its limit, the manual
mixture control cuts off fuel flow through both the idle jet and
the main venturi.  I can think of several ways to design this.

It's clear that there is a separate idle fuel nozzle, at least in
the Marvel-Schebler carb in my plane and many common engines.  It's
clear that the fuel flow through this jet has a separate mixture
control, a screw on the bottom of the carb.  Exactly how this 
interacts with the manual mixture is not quite clear to me (except
that at the limit, it's cut out).  As rpms increase, the idle fuel 
flow changes (flows through different places) and flow through the 
main venturi increases.  It's not entirely clear to me how much of 
the total fuel flow is through the main venturi (which is definately
affected by mixture) at different rpms, except that at the limit
of full throttle/cruise throttle, it's all through the main venturi.

I know I was told by my mechanic that if my plugs are fouling during
low-power taxi and ground run, he should adjust my idle mixture because
that has more effect on low power mixture than the manual mixture does.
I taxi at medium power (about 1000 rpm) so that I can steer largely
with rudder, so I lean during taxi as well.  I lean aggressively, so
far out that the control is just short of idle cutoff (the engine
seems quite insensitive to manual mixture settings which would make
a huge difference at cruise, which IMO supports what my mechanic told
me).  We have arguments about my ground leaning, he doesn't feel it's
necessary and feels it's poor practice.

My bottom line is, I've gone well beyond what I'm comfortable talking
about.  I'm no mechanic and only know a little bit about what
the carburetor in my own plane looks like.  So that this exchange
has some potential educational value for anyone interested, there
is an excellent post on the subject by Rod Farlee at:
http://x4.dejanews.com/getdoc.xp?AN=382583127&CONTEXT=907252106.1323172357&hitnum=4
(copy the whole thing and paste, I don't think clicking will work)
And articles by John Schwaner at:
http://www.sierra.net/skyranch/marvsh.htm
http://www.sierra.net/skyranch/glean.htm

Other than that, I'm outta here except to note again that so far 
as I can tell, ground leaning is a topic for debate not a clear 
cut "this is good practice, this is bad practice" issue, and maybe
it's worth a conversation with a trusted local mechanic.

Even a fish wouldn't get into trouble if he kept his mouth shut,
and I'm going to take the same precept to heart for my fingers here.

Snowbird

