Subscriber question:
"What's the most likely cause of a large RPM drop during a runup magneto check and what should be done about it?" — Dennis B.
Bob:
“It might be a fouled spark plug or perhaps something more serious. I will describe my procedure for dealing with this.
Start with the fouled spark plug possibility. Try clearing the plugs by running your engine up to 2000 RPM on both mags leaning to about 50 RPM lean of peak (continue to lean until you maximize RPM, then lean further until you lose about 50 RPM). Let the engine run for about 30 seconds and then try another normal run-up (at the normal run-up RPM) to see if the problem cleared up. If not, try the same procedure again.
If after the 3rd try it doesn’t clear up, get some maintenance help to correct the problem before flying the airplane.
This type of problem (fouled plugs) could be caused by not properly leaning the mixture. An overly rich mixture will burn incompletely, leaving a coating of deposits on the spark plugs.
Additionally, overly rich mixtures waste gas and reduce the performance of the airplane in cruise in terms of range and cruise speed.”
Learn more about leaning during taxi in this short excerpt from our “Airplane Engines” Manual.
Is your procedure for clearing a fouled plug the same as Bob or different?