No rpm drop on left mag - the first time

BJohnson · #1 07-06-2015, 08:37 PM

What would cause the left mag rpm not to drop from the "Both" rpm until switching back to "Both" and then to the left mag again? With the second selection, the rpm drop for the left mag is back to normal. Is this an indication of a failing switch?

Toobuilder · #2 07-06-2015, 09:22 PM

The switch or the wiring connected to it would be the first place I'd look. Durring the "no mag drop" opperation, reach behind the panel and tug/shake the wires connected to the ignition switch. If you suddenly get your drop, then you probably have a loose connection.

Doug · #3 07-06-2015, 11:38 PM

If the engine quits when you move the switch to 'off' then the right mag is not working at all. If the engine continues to run (in the 'off' posn) then the right mag p-lead connections are suspect. (Do this test at idle rpm - and remember if you have a hot mag this will be dangerous for anyone touching the prop on the ground.)

vic syracuse · #4 07-07-2015, 06:32 AM

I just worked on an RV-4 that had NEVER had the magneto switch wired to ground. The P-Lead from the mag was wired to the switch and from there to the tach input, so flipping the switch to the OFF position gave an iindication of Zero on the tach and a false sense of a smooth running engine. I shuddered to think of how many people were very lucky over the years.

Yes, a grounding check would have confirmed.

Vic

DanBaier · #5 07-07-2015, 07:48 AM

What Doug said - consider that if you turn the switch to OFF and the engine dies, let it stop. (The temptation is to turn the switch back on and that can cause some problems.) Just be aware that you'll have some magneto grounding issues together with fuel/air mix in the cylinders. So I'd probably restart and then a normal shutdown with mixture.

rvbuilder2002 · #6 07-07-2015, 07:53 AM

Quote:

Originally Posted by DanBaier

What Doug said - consider that if you turn the switch to OFF and the engine dies, let it stop. (The temptation is to turn the switch back on and that can cause some problems.)

I am curious to learn what problems it can cause (as long as it is done at idle)?

DanBaier · #7 07-07-2015, 10:05 AM

I guess I'm being too conservative. One person's idle is excessive to another. You're absolutely right - at only 600 - 800 RPM, nothing bad is supposed to happen. In fact, I probably could find some service literature in a short search that suggests the on - off - on procedure.

I have, however, over the years had quite a few people ask, in varying similar situations, why the engine made such a loud noise (backfire). Doesn't sound like it is a big deal, but it could be.

Dan

BJohnson · #8 07-07-2015, 12:09 PM

Thanks for the replies. I will try and get to the airport tonight to tests these ideas. I used to rent from a flying club that required a brief mag off check following a lean idle run to clear the plugs to verify that the mags were working properly. But I have not continued that practice with my RV. The question I have about the loose wire is that the rpm drops on the second try with the left mag, repeatably. It seems like a loose wire would be less repeatable. That is an easy thing to check. I had the cowling of a few days ago for an oil change and all of the P-pleads were secure on the mags, so I will check the switch side.

Doug, I am not sure I understand:

Quote:

If the engine quits when you move the switch to 'off' then the right mag is not working at all.

Shouldn't that be the correct response when switching to "off"? Typo maybe?

LettersFromFlyoverCountry · #9 07-07-2015, 04:11 PM

Do you guys check EGT in addition to RPM drop to be sure it goes up when you shut off either mag (or Lightspeed). I would think that would be a dead giveaway if nothing's changing.

sailvi767 · #10 07-07-2015, 05:31 PM

I rely on EGT's exclusively on my aircraft with dual LS's. I have found that at 1700 rpm the engine is remarkably smooth running on 3 cylinders. You really have to pay attention to notice it. At idle it is however rough on 3. I watch and verify a EGT rise on each cylinder for the L and R side and then confirm a drop when going back to both.
G