Hi all,

Chased misfire on a plane.  After take off it would buck a couple times and be fine for a while.  Sometimes it would misfire a lot.  Here is what I checked. My suspicion was this was heat related.

  • Unhooked ground leads for ignition switch.  No Change.
  • Cleaned grounds No change.
  • Checked slides in Carbs.  Working free.
  • Checked carb bowls and cleaned both carbs.  No change.
  • Resistance check of pick up triggers.  Ok cold.  Added heat gun. Ok warm.
  • removed spark plug boots and checked resistance across coils.  OK at 6.5 k ohms.
  • Checked coils again after heating each coil while testing.  Found bad coil.   
  • Replace bad coil.  Re-sync carbs.
  • Retest.  Runs well.

The coil was breaking down only under load and only when warm.  By the time you landed, it was fine.  The heat gun was my magic tool for this problem.  It can be hard to find these problems, but remember you can mock up heat with a head gun and cold with some R134A in a pinch to help diagnose those temp related issues in your hangar.  Hope this helps.  

Here is a quick YouTube video of what the ohm meter did on the bad coil.




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