Mag drop only 10 RPM

[NickG]

When performing the runup check, my right mag drops 60 RPM but he left one only 10RPM. Ovation w/IO-550G.

The "book" says 150RPM MAX drop on each magneto, 50 RPM MAX difference so it seems I'm OK but I'm new to the IO 550G ....

 

Is this normal?

[201Steve]

An even better indicator, what are your EGT temps doing on one mag? They should be rising. If one or more cylinders is dropping in EGT, that should be your focal point. 

[Ragsf15e]

1 hour ago, NickG said:

When performing the runup check, my right mag drops 60 RPM but he left one only 10RPM. Ovation w/IO-550G.

The "book" says 150RPM MAX drop on each magneto, 50 RPM MAX difference so it seems I'm OK but I'm new to the IO 550G ....

 

Is this normal?

As @201Steve said, make sure you get an egt rise on both.  If the timing is a little different between the two mags, it can act as you described.  If both are smooth, it’s fine.

[NickG]

Egt temps rising on both mags…..both are smooth…

[atpdave]

3 hours ago, NickG said:

When performing the runup check, my right mag drops 60 RPM but he left one only 10RPM. Ovation w/IO-550G.

The "book" says 150RPM MAX drop on each magneto, 50 RPM MAX difference so it seems I'm OK but I'm new to the IO 550G ....

 

Is this normal?

Last year I had this happen after the plane came out of an annual airworthiness inspection. Previously the mag drop was 20ish both mags. Had the timing checked and one mag was set 20d btdc and the other was at 24, both out of spec. Had the mags retimed to 22 and the problem went away.

[PT20J]

A split in rpm drop indicates that the mag timing is not the same for both mags. There are two timing parameters: internal timing (E-gap) and mag to engine timing. The problem is that unless you know the history of the mags (when overhauled, and how many times, and in what direction, the mags were "bumped" to change mag to engine timing) you don't really know which timing is incorrect. If the mags have less than 500 hours on them, you might just have your A&P check the timing and bump the mags as necessary to meet spec. If the internal timing is the source of the error, the worn mag will put out a weaker spark. I believe that the general rule is that mags can be bumped up to 5 degrees to compensate for internal timing being off. If the mags are approaching 500 hours I would probably just have them IRAN'd.