Magneto with no drop?

Anyone ever have a mag that didn’t have a noticeable drop in rpm when doing a mag check during run-up?

I double checked my wiring and tested to see if both mags shut down when I put the ignition to off. Engine shut down.

Thanks,
D

Check that when switch is on L, the right mag P lead is grounded, and vice-versa. If that?s okay, check the mag timing.

At 1800 RPM's I always had a drop in each magneto. Could be that the mag that is dropping of needs to be retimed. Check timing on both mags, make sure they are in sync. Also, pull plugs on the weak mag and clean them. In addition, check your plug wires make sure they are not chaffed or damaged. You should have a drop when the aircraft is on one magneto, an average of 50 RPM.

The quickest way to verify function with no noticable drop is an immediate rise in EGT when running on one ignition. If you have no RPM drop AND no associated EGT rise, you're not shutting one ignition down.

Hot Mag Check

Do you perform a "hot mag" check on each flight?

Just before I shutdown, I briefly turn both mags off, and then back on. Leave them off about one full second, long enough to be sure the engine would shut down with both mags off. If you turn both mags off, and the engine continues running, you have a "hot mag", hence the name of the test.

Usual cause is a broken "P" lead, or a bad switch.

If it's a key switch & not individual switches, it's possible for the switch to ground both mags in the 'off' position, but *not* ground one or the other when in Left or Right positions.

Just a thought...

If you use toggle switches, there is no common "off" to test, unlike a rotating key switch, right?

I don't know the risk of turning the engine switches back on to catch the engine, I was taught if you shut it off with mags, let it die. That may have been more for muffler protection on production planes.

I also don't know if key switches are better than old blade switches- I hand propped a plane on the second of 4 planned priming rotations. If I had not treated each blade pull properly I might have been struck. Metal 2 blade prop on a wheezy 65hp gave lots of clearance but it was eye opening.

The pilot in the plane is a bud, and we had done it right. The Bendix blade switch was off. He motioned me over (go slow and around the prop, as much as you will have an interest in seeing the switch/s) and she was "off" and idling nicely.

One swipe cycle off-both-off killed the engine. Corrosion and wear broke the "off" path to ground just enough while it sat from last flight.

I have an acs switch which I did hot mag test and when it set it to ?off? the engine shut down. I?m thinking it?s poor timing on one of he mags.

Did you read post #6?

BobTurner said:

Check that when switch is on L, the right mag P lead is grounded, and vice-versa. If that?s okay, check the mag timing.

Click to expand...

+1

Toobuilder said:

The quickest way to verify function with no noticable drop is an immediate rise in EGT when running on one ignition. If you have no RPM drop AND no associated EGT rise, you're not shutting one ignition down.

Click to expand...

+1

Oops, that is +2!?