No RPM Drop on Mag Check

[MartinPred]

Here's one for the hive mind.

Since I rebuilt my Lycoming O-360-A1D last year, with 2x freshly-rebuilt Bendix Mags, there's no drop in RPM, rise in EGT, or drop in CHT whenever I turn off the Left Mag. It's not a broken P-Lead or other grounding issue. Whenever I do a ground check and turn off the Left and then the Right mag, regardless of RPM, the engine quits like it should.

I've read that maybe bad mag timing could cause this. But I've triple-checked the timing and both mags open the points at 25 degrees BTDC like they should. The engine runs just fine on the right mag. And I have no problem making static RPM or producing good power all the way up to altitude.

I have noticed since I finished the rebuild that the right side of the engine (#1 and 3 cylinders) run significant hotter in both EGT and CHT than the left side--to the tune of 50-70 degrees CHT and 100-150 degrees EGT on both. It does that both sitting on the ground and in flight, so it's not air flow. I've replaced the carb and the induction tube flanges and gaskets (with the SDS ones) to no avail.

I also have an RF filter on the right mag, which I just replaced with a new one.

I'm stumped. Any other thoughts out there?

My next move is to remove that RF filter and see if that does it. But I don't see how that could be the problem.

Thanks,

-Matt
N402BD

[Flying8]

Maybe your ignition (key) switch is faulty and not grounding the left mag when the switch is moved to the RIGHT mag position. Even if it does ground both when switched to OFF.

I know you said there are no ground faults but to have zero RPM drop on a Mag indicates otherwise.

[Cumulo]

I am stumped as well. The A&P card in my pocket doesn't help either. All of the clues point to mag timing being too advanced. But you have triple checked that to 25 deg, right? The amount of smooth mag drop is usually a reliable indication of proper timing advance and little else.

Check that the piston TDC is consistent with your timing mark reference through the spark plug hole and re-buzz the mags.

The left-right temp difference is a puzzle.

Ron

[PCHunt]

Have you done all these checks and troubleshooting yourself?

Sometimes it's good to get a second set of eyes on the problem.

[Cumulo]

Originally Posted by Flying8

Maybe your ignition (key) switch is faulty and not grounding the left mag when the switch is moved to the RIGHT mag position. Even if it does ground both when switched to OFF.

I know you said there are no ground faults but to have zero RPM drop on a Mag indicates otherwise.

+1
Yeah, Zero drop. Key switches have had their probs with AD's, SB's. It would mean no ground on 2 positions.

The left-right temp is likely an indicator prob - grounds, etc. IMHO. Big temp diffs like that would give lots of other symptoms if real.

[lr172]

Originally Posted by Flying8

Maybe your ignition (key) switch is faulty and not grounding the left mag when the switch is moved to the RIGHT mag position. Even if it does ground both when switched to OFF.

I know you said there are no ground faults but to have zero RPM drop on a Mag indicates otherwise.

This is where I would start

[RV7A Flyer]

Thinking conversely, since you say there's no drop in RPM or change in EGT/CHT when you "turn off" the left mag, then perhaps it's never actually *on* (so turning it off has no effect at all).

[Jetmart]

Originally Posted by RV7A Flyer

Thinking conversely, since you say there's no drop in RPM or change in EGT/CHT when you "turn off" the left mag, then perhaps it's never actually *on* (so turning it off has no effect at all).

If this was the case than the engine would quit when the right mag was shut off.

[PCHunt]

One thing that you could try as a troubleshooting technique, is to take one P-Lead off so that the associated mag is always "hot". This mag can not be turned off with the switch. I would choose the right mag first.

Then start the plane, and run it. The assumption is that both mags are now working. Then turn the mag switch OFF. The right mag should continue working, and you can look for a mag drop this way.

You can do the same with the left mag, except you might have difficulty starting on the hot right mag.

Then shut down with the mixture, as normal.

[1001001]

Originally Posted by MartinPred

Here's one for the hive mind.

Since I rebuilt my Lycoming O-360-A1D last year, with 2x freshly-rebuilt Bendix Mags, there's no drop in RPM, rise in EGT, or drop in CHT whenever I turn off the Left Mag. It's not a broken P-Lead or other grounding issue. Whenever I do a ground check and turn off the Left and then the Right mag, regardless of RPM, the engine quits like it should.

I've read that maybe bad mag timing could cause this. But I've triple-checked the timing and both mags open the points at 25 degrees BTDC like they should. The engine runs just fine on the right mag. And I have no problem making static RPM or producing good power all the way up to altitude.

I have noticed since I finished the rebuild that the right side of the engine (#1 and 3 cylinders) run significant hotter in both EGT and CHT than the left side--to the tune of 50-70 degrees CHT and 100-150 degrees EGT on both. It does that both sitting on the ground and in flight, so it's not air flow. I've replaced the carb and the induction tube flanges and gaskets (with the SDS ones) to no avail.

I also have an RF filter on the right mag, which I just replaced with a new one.

I'm stumped. Any other thoughts out there?

My next move is to remove that RF filter and see if that does it. But I don't see how that could be the problem.

Thanks,

-Matt
N402BD

Forgive the question, but are you using a standard key switch ignition or independent grounding switches for the mags? If you are using the key switch, are you certain that the left and right mags are wired to the appropriate terminals?

Just some stuff to check to make sure which mag is really the issue, if not the switch.