The Magneto Check

You’re in the runup area, looking forward to the flight ahead. By now, the preflight checks have become somewhat second nature and rote to you: Throttle to 1700RPM. Mag check left. Check right. Back to both. And then?

All of a sudden you notice something’s not quite normal. One mag is dropping more than the normal POH limits. What’s going on?

The most common reason for this is a spark plug fouled with soot or lead. You could have prevented this, possibly, by leaning the mix during taxi. But that was then, and this is now.

The recommended solution is to operate the engine at a higher temperature and lean mixture to burn off the offending gunk from the plug.

Area In Front of Plane: Clear of gravel, sand, debris, etc.
Magneto Switch: Both
Throttle: More than normal runup RPM, but NOT more than 75% power
Mixture: Lean until engine runs rough, then enrichen until just smooth
Monitor Engine: for 30 seconds. CHT or oil temp should increase
Throttle: Reduce to normal runup RPM
Mixture: Set for takeoff

Then, repeat your mag check as usual. It’s possible you may have to repeat this two or three times to clear the fouled plug. If you still have no success, taxi back and shut down. More troubleshooting is needed before the plane can be returned to flight.

For more information, see this AOPA article.

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