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Help interpreting a LOP mag check

00Dan

Well Known Member
I went up and did some GAMI sweeps and LOP mag checks yesterday. The mag checks are approximately 19 minutes in; I did three 30 second checks for each -right magneto off, both, then left magneto off. The checks were performed at WOT at 9500’ PA.

https://apps.savvyaviation.com/flights/4924354/e136f83e-aab4-4c7e-9084-9d83c40157b9

I’ll admit I’m still a bit unsure on what to look for here. Qualitatively, the plane ran worse on the left magneto than the right, which I’m sure shows in the RPM and fuel flow.

Some stats:

O-320-A, 7:1 CR.
Bendix S4LN magnetos, impulse coupling on the left.
Tempest UREM38S fine wire plugs.
 
When you go from two spark events to one, you should expect to see a good EGT rise on each cylinder, along with a small RPM drop. If the EGT rise and RPM drop is not reasonably consistent between the two mags, there is a good chance that the timing is not consistent between the two mags. Variation in advance can cause differences in the single spark behavior.

I would not expect identical behavior, as the flame front generated by the two plugs may be slightly different, however, I would expect a similar change in engine sound for both the left and right. Not getting that further points to timing variances.

With both mags in good health with the same timing, you should see similar RPM drops, EGT rises, and similar sound changes. Don't forget that when LOP, it is harder to light the mixture and will more easily show weakness in the ingition system. Maybe wise to do the same test when ROP as well and compare both test results.

Spark energy created by a mag is highly dependant upon proper E gap setting and this could cause good behavior ROP but poor behavior LOP.

Larry
 
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I cleaned, gapped, and rotated my plugs then flew again.

https://apps.savvyaviation.com/flights/4928221/8f02426a-b563-4561-87f3-d20e8681794f

The series of three mag checks starting at 18 minutes were ROP, then one more set at 24:30 LOP.

The #2 EGT trace stands out to me during the LOP check. The plugs all tested good so I’m not sure if I need to be checking my ignition harness or my magnetos. The mags have about 450 hours on them. Although none of the plugs drop off the rise has some oscillations in it on all cylinders.

Maybe I’m just over analyzing it.
 
Looks like you're getting less of a raise when running on the right magneto, but only in cylinders 1&3. Not sure how to interpret this but since 2&4 look ok, timing should not be a problem.
 
Looks like you're getting less of a raise when running on the right magneto, but only in cylinders 1&3. Not sure how to interpret this but since 2&4 look ok, timing should not be a problem.

I was under the impression the EGT delta for a side between mags is attributable to top vs bottom plugs firing, and is normal. If anything, I’d say 2&4 are what look odd since they don’t have that pattern.
 
Looks about right. might be helpfull to leave the mags on and off for a longer time to get more readable results.

You should see similar behavior with top and bottom plugs. Maybe slight differences but not significant. Gravity doesn't play a role here and the cyl head is symmetrical in shape with flat top pistons. If this is a carb'ed engine, you will see some differences cyl to cyl due to uneven fuel distribution.

When you have big differences ROP to LOP, weak spark energy could be a culprit. Either too much resistance on the plug or wires or e gap out of spec.
 
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