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Too Much of a Good Thing

In my previous post, Leaning Redux, I wrote about the importance of leaning after engine start and prior to take-off, and I referred to an old article describing how to lean during run-up.  I mentioned that a Plus One pilot had abandoned a flight due to an incorrectly interpreted RPM drop during the mag check.  The result was a trip to the shop for an unnecessary expense and a clean bill of health for the plane.

Last week a student and instructor canceled their flight and squawked 1784R with the following description:

While at the runup with the mixture lean the right mag dropped 500 rpm and ultimately stopped the engine. After restart and doing preventative measures it continued this trend.

In a voicemail, the instructor reported that after trying the standard fouled plug clearing procedure, the RPM again dropped on a single mag and the engine mis-fired.

Again the mechanic checked the airplane (and sent a bill) but found nothing wrong.  The mechanic said that the problem was a too lean mixture and that the mis-firing was actually backfiring due to pre-ignition of the over-leaned fuel/air mixture.  He was able to reproduce the reported behavior by leaning until it occurred.

So, the lesson to be taken from these recent experiences is that while leaning is important, it is possible to overdo it.  It’s clear that properly operating an engine whose basic design could be 80 or more years old is more art than science, and requires more than just a superficial understanding of how it works.  The obvious response to a larger than acceptable RPM drop during mag check is to adjust the mixture.  The direction of adjustment of course depends where you started:  If you haven’t leaned very much, try a little more; if you leaned a fair bit already, try enriching the mixture.

Next time it could be that the problem wasn’t the mixture at all.  It truly could be fouled plugs, or indeed a magneto failure.  It could also be a problem that happens in the air instead of at the run-up.  One key to problem-solving is to maintain an open mind and avoid making assumptions.  Try to identify all the factors that could cause the conditions you are seeing. This approach possibly could save your life.