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Date: Mon, 21 Dec 1998 00:01:11 -0600
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Subject: Re: Lean mixture during run-up
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Rod Farlee wrote:

> O-360s don't have much problem with lead fouling of the spark plugs.

I'll be sure to explain this to a couple of O-360 owners of my
acquaintance, who are forever fouling their plugs *grin*. 

> However, low-compression (6.5 to 7.0:1) engines certainly don't.
> These include the small Continentals (C-65 through O-300) and
> Lycomings (O-235, O-320) in most trainers.  They were not designed
> for 100LL fuel.  If these engines aren't leaned in cruise, and leaned > on the ground in a training environment (lots of landings and taxi 
> time), they will have lead fouling problems.  

Thing is, Rod, where I learned to fly, "lean during taxi" was not
on the menu in these small trainers.  Now they may have cleaned
the plugs routinely at 50 hrs, I don't know; they may also have
used hotter plugs which are less prone to fouling, or adjusted
the mixture seasonally (I doubt the latter, the way the temps ping-pong
about here).

So while I lean on the ground myself for various reasons (in my 
O-360 which doesn't need it *grin*), and think it's a fine thing
to do, I wonder if John's FBO owner may have a point that maint.
can be done so it's really not necessary (John, did you learn to 
fly in that Archer? or in something smaller?  and did he want you
to lean it?)

> Regarding the "mis-adjusted idle mixture setting": mixture depends
> on density altitude.

Well, of course, but I'm not sure that the difference in DA is that
significant at idle rpms?  At least, I haven't noticed problems
with throttle response as the wx is getting colder, yet my plugs
don't foul in summer...my idle mixture is set for a very minimal
rise in rpm, as described in John Schwaner's article.   So I do
think there is something to my mechanic's claim that proper idle 
mixture is the real key to not fouling plugs...and he's maintained
a few trainers in his time.  Maybe if I moved to MN or switched
from 9000 ft DA to sea level regularly?

> I think it is misleading to say that leaning during taxi and runup is > in conflict with Lycoming's recommendations.  Lycoming says nothing,
> pro or con, about this in Service Letter 256, the Operator's Manual,
> or Lycoming "Flyer" key reprints.  They don't conflict, they simply 
> say nothing either way.

Good point; it would be equally misleading to claim it's a practice
recommended by the mfr. or POH.

Snowbird


