Progressively Greater RPM Drops During Mag Check

[Oklahoma Mooney]

Appreciate advice on this. I have an O-360  (M20D) that normal runs great. During the Annual (25hrs ago) I had both mags (Bendix) inspected and IRAN'd. The first 18 hours the mag checks were great, dropping about 50 - 70 RPM on either mag, smooth. With my most recent flights the best I can manage is about 110 drop on each mag and a slight occasional miss / stumble on either mag, When flying It seems like it just doesn't run quite as smooth anymore. Still making power /speed numbers. I've checked the mags in the air and same drops as above. 

I have chrome cylinders so they are more prone to oil fouling but normally things can be cleaned up with typical procedures etc. This is not the case anymore. I pulled the plugs today (they were in pretty good condition) and switched to Tempest fine wires, hoping that would solve the issue, but there was exactly ZERO change during initial runup.

It may be worth noting that after flying and taxi in on previous flights, mag checks before shutdown are good and smooth with around 70 per side drop.

My thought is to have the timing checked and wring out the ignition harness, but any other input would be great. 

Thanks

[EricJ]

3 hours ago, Oklahoma Mooney said:

It may be worth noting that after flying and taxi in on previous flights, mag checks before shutdown are good and smooth with around 70 per side drop.

My thought is to have the timing checked and wring out the ignition harness, but any other input would be great. 

Thanks

If the points were replaced it seems like some of them like to wear in a bit and the timing changes a little as they do.   You sometimes see recommendations to re-check the timing after ten hours or something, and I think this is why.    I had to do that on mine after the dual mag was IRAN'ed (essentially an overhaul) and it made a difference.

[Oklahoma Mooney]

Thank you for the information, that makes sense. I'll have my A&P check it. 

[Ragsf15e]

23 minutes ago, Oklahoma Mooney said:

Thank you for the information, that makes sense. I'll have my A&P check it. 

I had a similar timing change as @EricJ described after having my mag oh.  Recheck timing helped.

One other thought is that (maybe) it’s hotter now than when you did this earlier (25 hours ago) and the density altitude is different.  Try leaning a little, slightly less than full rich, for your mag check and see if it smooths out.  Impossible to hurt anything at that low power setting as long as you don’t run it so long it gets warm chts, so you can play with it a little.

[Oklahoma Mooney]

Thank you for the input, I will try that. 

[Oklahoma Mooney]

UPDATE: Wringing out the ignition harness we did find 2 leads that failed the test. We checked mag timing and the left mag was 1/2 degree less than right so we adjusted it to match at 25 degrees. 

I ordered a Kelly harness with the elbow but was disappointed to find that the elbows are NOT low profile, and have a second nut after the bend that will not work with my cooling doghouse. 

My current harness (Aero-Lite Skytronics) have the 90 degree elbows (brass I think) to be able to fit in the doghouse. Maggie harnesses are available with elbows but would a standard harness work and make the bends?

[N201MKTurbo]

25 minutes ago, Oklahoma Mooney said:

UPDATE: Wringing out the ignition harness we did find 2 leads that failed the test. We checked mag timing and the left mag was 1/2 degree less than right so we adjusted it to match at 25 degrees. 

I ordered a Kelly harness with the elbow but was disappointed to find that the elbows are NOT low profile, and have a second nut after the bend that will not work with my cooling doghouse. 

My current harness (Aero-Lite Skytronics) have the 90 degree elbows (brass I think) to be able to fit in the doghouse. Maggie harnesses are available with elbows but would a standard harness work and make the bends?

I would get the Maggie.

 

[Nico1]

+1 Maggie, they are great

[Oklahoma Mooney]

Thank you for the reply. I spoke to the folks at New Horizons Ignition (Maggie Harnesses) and I sent them my old harness to be sure that the new one will be built to the right specs. They are great to work with. Be advised their website is being overhauled so it makes it seem like they are out of business but they are very much still active and the website should be up soon.