Q: I work on a IO-360-MIA Lycoming engine. My problem is that the right mag has a drop of 220 rpm. I have done internal timings, engine to mag timings, ignition leads check and cleaned the plugs. I have done nothing to the left mag, and now the left mag is dropping to 210 rpm.
ANGI LISA CHAMBERS, via email
A: Interesting question and hopefully somewhat easy to troubleshoot Angi Lisa. First, I have to ask a question to answer your question: What happens if you manually lean the mixture while doing a mag check? It may be just a slightly rich mixture causing the higher than normal drop-off.
Just so we’re all on the same page, I hope you are checking the magnetos with the engine at normal engine operating temperatures. For a proper magneto drop-off check on your engine, the rpm should be between 1800 and 2000 rpm.
Don’t overlook the possibility that the air filter may be dirty, reducing the intake air volume, which results in a richer mixture.
Even though you didn’t mention it, I’m curious to know what the fuel flow is at full throttle at takeoff power. I’m not certain if a partially plugged fuel nozzle could cause this problem, but if the fuel flow gauge needle goes to the top of the gauge, this could indicate a plugged nozzle.
I think you’ve done all of the things with regard to the mags and ignition harness, so let’s hope the above suggestions will put you on the right path to solve this situation.