No rpm drop on left mag - the first time

[Doug]

Yes not a helpful way to express it - point taken. What I was trying to say is that if you have an open p-lead ground you cannot be certain that both mags are indeed working.

If say the right magneto is not working then the engine will quit in the 'off' position as normally happens only if the left magneto p-lead is connected correctly, also the engine will also quit when 'right' is selected.

Further, if the left magneto p-lead is not being grounded, the engine will run in 'left', 'right' and 'both' (and 'off') on the left magneto only, you will have no knowledge about the condition of the right magneto.

Key switch 'off' - both the right and left magneto p-leads grounded - the engine quits. If it continues to run then either one or both p-leads is not being grounded. (obviously)

Seeing as you do note a drop on the 'right' position, both mags are 'probably' working.

[Jesse]

We are missing some information. If there is an rpm drop on the right position, then both mags have to be working, at least to some extent. I would be suspicious of the switch in this case, as the OP mentioned. Again, if there is a mag drop in any situation, then both mags are firing.

What is your run up procedure? The correct procedure, as I see it, is to switch from both to left, note EGT rise on all cylinders, return to both, note EGT drop on all cylinders, then from both to eight, note EGT rise on all cylinders, return to both.

I would suggest a bad switch, but this is easy to test. Disconnect the p-leads from the mag switch and start the engine. Touch each lead to ground (a screw head or other metal part. If both give a mag drop, then both are working. Hook the switch back up and see if there let returns. If so, then suspect the mag switch. Safety note, don't leave the p-leads disconnected, as a turn of the prop can be catastrophic if either of the mags are hot.

Another possibility is that one mag is retarded more than the other such that switching off the retarded mag does not give you a significant rpm drop, while switching off the advanced mag will give a significant rpm drop.

As I mentioned above, and as others have mentioned, I like to use EGT's as a mag test. I listen for an rpm drop, but don't really care how big or small (within reason), as long as all of the EGT's go up on each single mag.

Worth noting here is that a lot of planes have an AD to do the switch off test to verify that the off position kills the engine. This means that switches can fail and that you should check that at least periodically (and at idle). Your intermittent problem sounds like a failing switch to me, although if a certain procedure gets the same result every time sounds a little less like a failing switch.

[BJohnson]

I got a chance to swing by the hanger to run some tests. None of the wires are loose. I checked them before starting the plane, and during run up when just on the left mag, with no change.

I leaned it out at idle and turned the mags off and the engine started to die.

But again, at run-up, when the rpm dropped and egts raised when switching first to right mag, returning to both raised rpm and lowered egts, then switching to left mag, no rpm drop and no change in egt. Back to both, then over to left a second time and I got the normal rpm drop and rising egt. I switched back to both and then left about five other times and each time rpm dropped and egts rose. So there is NO rpm drop only on the first selection of left mag.

My normal run up is 1800rpm on both, then right, both, left, both. I have looked mostly at rpm drop. Why does egt raise when running on just one mag?

The engine runs on either right or left mag with reduced rpm (second try on left side), and the engine quits when off is selected. So the mags must be working and grounded. But for some reason, only on the first selection of left mag, does the rpm not drop. I forgot to mention that both mags are impulse coupled and the switch is not jumpered out for only left mag impulse coupled. Curious

[Jesse]

That really does sound like a switch issue to me. You could try disconnecting the right mag p-lead and touch it to ground when running to make sure it gets an rpm drop EVERY time. The other option is to install another switch and see if that fixes the issue.

[aerhed]

I've done what Jesse is talking about. If you can, disconnect both p-leads at the switch and airframe ground them one at a time during runup.