Mag Drop Question

[INA201]

I'm just curious here.  I had my mags recently rebuilt due to 500 hour.  The engine still starts and runs great. I'm just curious as to why my mag drop is now 75-100 rpm where it was a a very low mag drop prior maybe 25 RPM or so and almost unnoticeable.  I will say that CHTS seem to be a little lower but that's the only change. 

[DanM20C]

23 minutes ago, INA201 said:

I'm just curious here.  I had my mags recently rebuilt due to 500 hour.  The engine still starts and runs great. I'm just curious as to why my mag drop is now 75-100 rpm where it was a a very low mag drop prior maybe 25 RPM or so and almost unnoticeable.  I will say that CHTS seem to be a little lower but that's the only change. 

It’s probably just due to the timing not being exactly where it was before.  Lower CHTs would be indicative of timing being slightly retarded from previous. Generally you should see slightly higher EGTs too. 

 The greater mag drop could also be a difference in timing between the two mags. 

Cheers, 

Dan

[kortopates]

What you want to see is near the same identical RPM drop on both mags, and more importantly a healthy EGT rise (50-75F+) on each plug; or more if done lean. An almost unnoticeable drop is not healthy.   

[INA201]

49 minutes ago, kortopates said:

What you want to see is near the same identical RPM drop on both mags, and more importantly a healthy EGT rise (50-75F+) on each plug; or more if done lean. An almost unnoticeable drop is not healthy.   

Yes, both mags drop equally along with corresponding EGT rise.   I was thinking that the low mag drop prior to overhaul was abnormal.  My oil consumption may be less too but its only been 12 hours now so not enough data on that.  It's interesting how many things can be affected by timing and other seemingly small adjustments.  Thank you guys for the feedback.

[NotarPilot]

On 11/4/2018 at 5:27 PM, kortopates said:

What you want to see is near the same identical RPM drop on both mags, and more importantly a healthy EGT rise (50-75F+) on each plug; or more if done lean. An almost unnoticeable drop is not healthy.   

Could you expand on that Paul? What is an almost unnoticeable drop indicative of?

[Raptor05121]

I believe that would be failure to ground. OP, sounds to me your timing is now more correct than before. My mag drop is around 75-100 rpm.

[Guest]

9 hours ago, NotarPilot said:

Could you expand on that Paul? What is an almost unnoticeable drop indicative of?

Improper internal timing and timing to the engine.

Clarence

[kortopates]

9 hours ago, NotarPilot said:

Could you expand on that Paul? What is an almost unnoticeable drop indicative of?

@Raptor05121 nailed the "no drop" issue, which would be indicative of the selected mag not grounding from either a broken p-lead or ignition switch fault. And @M20Doc explained the sources for why or how the Mags can create an unbalanced mag drop. You can add weak plugs to that as well. But after doing my mag timing, typically after annual, my mags will both result in a 60 rpm drop on my engine with 0 rpm differential between mags. (simply because of anal about getting each mag timed identically to spec with a digital leveler and all the plugs evenly gaped). Remember each mag (conventionally) fires 1/2 of the bottoms and 1/2 on the tops on the opposite side from the bottoms, so seeing balanced mag drop shows you each mag with its plugs is putting out an equivalent strength in ignition spark. Typically the OEM limits the unbalance to 50 rpm maximum since its indicates one of the mags is weak* compared to the other. (*but weak could really be just split timing too). But at the same time the max mag drop can be as much as 150-175 rpm depending on the OEM which is too large in my opinion to tell if there is really an issue or not and the drop is really going to vary based on mixture too. But for the last few decades we've had a much better way to determine ignition health during the run-up using an engine monitor to see the EGT rise of every individual plug when isolated to a mag; which is highlighted by putting the engine monitor in normalize mode after EGTs stabilize at run up RPM before you start selecting individual mags. It provides far more valuable information that the rpm drop does; especially when done lean.

[INA201]

6 hours ago, Raptor05121 said:

I believe that would be failure to ground. OP, sounds to me your timing is now more correct than before. My mag drop is around 75-100 rpm.

Makes sense on the grounding too.  BTW I went to an MSC that got all this straight.  I think the timing is now correct too.

[bradp]

Also when timing mag's I've gone to this setup:

Laser cross hair mounted to a small octopus leg cross hair.  Setup sits on top of the case without interfering / knocking into anything.

Shoots a deadly accurate sight line right down the case split.  I am using the timing marks rather than flower pot method.

Got a 75 rpm drop with zero difference between left and right with this. 

 

 

 

[Guest]

Lycoming engines use fixed timing marks on the front of the ring gear support along the a mark on the starter nose case or alternatively the marks on the back of the ring gear support and the split line in the crankcase.

The timing disk and TDC locator is used on TCM engines.

Both methods are only as precise as the maintainer performing the task.

Clarence