We hear a lot about lean misfire.  (As it is commonly referred to, I  really
don't think it exists.)  Would you be so kind as to define  what you mean by
lean misfire?  What is the phenomenon?  Also, do you  think it's the same
phenomenon as rich misfire?

Walter,

 

Misfire occurs when a component is missing from the fuel, air and spark trinity.  There are a broad range of F/A ratios that are ignitable.  Assuming an ignitable range, there are several reasons that the spark component may fail in aircraft engines.  With magnetos and fixed timing, plug spark failures can occur from a bad plug, bad harness, bad mag, arcing in the mag (non-pressurized at high altitude), bad points or distribution of the spark, and a weak spark (there are probably others).  In electronic systems (EI), timing may be an additional contributor although a weak spark seldom is.  Since aircraft engines have two plugs, each fired by a different ignition system, it is infrequent that both fail to ignite the mixture.  The failure of one may result in the burn being later than optimal.  If the single failure is constant, it sometimes can be seen by a cooler than normal CHT and a hotter than normal EGT.

 

Lean misfire can occur when the F/A is ignitable but lean enough to be subject to the delivery of a weak or mistimed spark.  Weak from a magneto where the spark can't make the gap under compression or, from a timing point of view in the case of an EI that changes timing, the spark occurs too early before the mix is fully made or compressed enough.  In this case, the mixture may ignite in the hot exhaust rather than the cylinder and the chance that neither ignition may start the burn could be increased.  There is a CAFE article on EIs that discusses lean misfire and that article is not at my finger tips.

 

A rich misfire (F/A still in ignitable range), hmmmmm, could be pretty much the same except for the amount of fuel available.  

 

What do I know?  I fly normally aspirated engine equipped with moped iridium plugs and a good dual EI where the fuel flow amongst the cylinders is within .3 gph at peak.  I like 4-cylinders because it is easier to get them to work together than if I had 6.  Smooth LOP operation can be accomplished with the EGT at 50-60F LOP of the leanest cylinder, timing 29-30 DBTDC (depending on MAP/RPM), although 20-30F LOP is the best economy cruise condition that would be used.  Rough operation does occur when a cylinder reaches lean misfire (a bad A/F ratio).  I am never rough at best power although that may not be what you meant by conditions for "rich misfire."  Hmmmm, I am not rough at rich rich settings such as takeoff power and full rich (not to be confused with proper mixture settings at high altitude airports).

 

My conclusion - I dunno.  Just threw out lean misfire as a possibility.  Turbos confuse me anyway - just another F/A variable and a hot one to boot.  I try not to get too high anyway (altitude, altitude for those with kinky minds).