Tim,

It sounds like very uneven fuel distribution. To test your theory about cowl pressure you could try this experiment.

Set up the power setting you want, then climb steeply to minimum comfortable speed. Since dynamic pressure is proportional to velocity squared this reduces the pressure differential across the engine to near zero. Lean to slight trace of misfire. If it is lean of peak you are on the right track. Accelerate back to normal speed and see if it gets rough.

Is the incoming air blowing at high velocity on the front injectors, if so you might rig some temporary shields to protect them.

Are there any fuel stains around the injectors indicating reverse flow.

Let us know what you find.

From: Tim Olson <Tim@5000feet.com>
Subject: [LML] Plenum pressure and Injector performance LOP
To: lml@lancaironline.net
Date: Monday, August 24, 2009, 1:40 PM

Hey guys, I am remembering a long time ago reading about
how the air pressure surrounding the injector can affect how
well the injector works.  If I'm not mistaken, turbocharged
engines even use some pressure manifold around the injector
to accommodate for the increased Manifold Pressure.

What gets me curious is that I'm having a discussion with some
folks right now who are all having problems running Lean of Peak
and their engines stumble before they get to peak EGT. Between
the group, there are a variety of ignitions such as 2 mags
or 1 lightspeed and one mag, and varieties such as 8.5:1,
9:1, and 10:1 compression ratios, but the one constant is that
they all have what is purported to be a more effective cowl...
the Sam James cowl with a plenum.  So I'm wondering if the
knowledge base some of you have would provide any insight
to the issue. I originally was worried that the 10:1 pistons
might make LOP operation harder to accomplish, but perhaps
there is just too much air pressure on the outside of the
injector?  The cowl is known to often have too little exit
area, so it could be that the pressure differential is
even too high between the top and bottom half of the
cowl.

Any insight you can give?
Thanks,
Tim

--
For archives and unsub http://mail.lancaironline.net:81/lists/lml/List.html