Ron Lee
Well Known Member
Equipment: RV-6A, O-360, Catto prop, Carb, Bendix mag on left side and Lightspeed on right.
Symptoms: Indicated RPM was starting to fluctuate a bit around 1700 RPM prior to the mag check. Not "excessive" but not quite normal. Engine sounds normal.
On a cross country flight, I thought there was a possible slight engine roughness so I did a mag check. There was a 600-900 RPM drop when I switch to the left mag. I thought at the time that the drop was real so back to both ignition systems and everything is fine but I did return home.
I could not replicate a REAL drop in RPM on the left mag on the ground back home but the "indicated" drop was still there.
Corrective actions:
1) Clean plugs fired by the Bendix mag. Used a plug tester and in the process elected to replace one that may not have been perfect. Runup was the same. Indicated drop of 600-900 at 1700 RPM when I switch to the mag only. No drop other than normal RPM drop.
2) Serviced the Bendix mag. Replaced and runup test was the same. Indicated drop of 600-900 at 1700 RPM when I switch to the mag only. No drop other than normal RPM drop.
3) Verified Bendix mag related spark plug wires were OK using a tester.
At this point I noticed that the indicated RPM was steady until some point just above 1330 RPM or so. Then the small fluctuations would begin and end below ~1300 RPM. By now it appears that the problem is in the wiring between the mag and the engine monitor. The P-lead goes to the LEFT tab on the starter switch and from there a capacitor is inline with the wire that goes to the engine monitor.
So I finally replaced that capacitor and the runup is now normal. No fluctuations above 1300 RPM and a normal drop during the mag check around 1700 RPM.
I had been told that the odds of the capacitor being bad were small yet that was the problem all along. Approximate time in service of those capacitors is 1900 hours if they are original (I am the third owner).
As far as the drop when I performed a mag check in flight...I now believe that it was never real. At the time I thought it was but with no recurrences it apparently was imagined. It was only an indicated drop. I missed the Petit Jean fly-in because of this but made the decision that I felt was most prudent at the time.
Symptoms: Indicated RPM was starting to fluctuate a bit around 1700 RPM prior to the mag check. Not "excessive" but not quite normal. Engine sounds normal.
On a cross country flight, I thought there was a possible slight engine roughness so I did a mag check. There was a 600-900 RPM drop when I switch to the left mag. I thought at the time that the drop was real so back to both ignition systems and everything is fine but I did return home.
I could not replicate a REAL drop in RPM on the left mag on the ground back home but the "indicated" drop was still there.
Corrective actions:
1) Clean plugs fired by the Bendix mag. Used a plug tester and in the process elected to replace one that may not have been perfect. Runup was the same. Indicated drop of 600-900 at 1700 RPM when I switch to the mag only. No drop other than normal RPM drop.
2) Serviced the Bendix mag. Replaced and runup test was the same. Indicated drop of 600-900 at 1700 RPM when I switch to the mag only. No drop other than normal RPM drop.
3) Verified Bendix mag related spark plug wires were OK using a tester.
At this point I noticed that the indicated RPM was steady until some point just above 1330 RPM or so. Then the small fluctuations would begin and end below ~1300 RPM. By now it appears that the problem is in the wiring between the mag and the engine monitor. The P-lead goes to the LEFT tab on the starter switch and from there a capacitor is inline with the wire that goes to the engine monitor.
So I finally replaced that capacitor and the runup is now normal. No fluctuations above 1300 RPM and a normal drop during the mag check around 1700 RPM.
I had been told that the odds of the capacitor being bad were small yet that was the problem all along. Approximate time in service of those capacitors is 1900 hours if they are original (I am the third owner).
As far as the drop when I performed a mag check in flight...I now believe that it was never real. At the time I thought it was but with no recurrences it apparently was imagined. It was only an indicated drop. I missed the Petit Jean fly-in because of this but made the decision that I felt was most prudent at the time.
Last edited:
