Stupid question but here goes

[Joe Larussa]

Been having rough engine on left mag issues. Installed new plugs and no change. We started checking the P-leads with engine running. When moving around

the P-lead for the right mag when running on the right mag engine practically dies, yet it's been the left mag that's been running rough. Could there be

some possible connection? Moving around the P-lead on the left mag makes no difference. I hope this makes sense.

[RLCarter]

1 hour ago, Joe Larussa said:

Been having rough engine on left mag issues. Installed new plugs and no change. We started checking the P-leads with engine running. When moving around

the P-lead for the right mag when running on the right mag engine practically dies, yet it's been the left mag that's been running rough. Could there be

some possible connection? Moving around the P-lead on the left mag makes no difference. I hope this makes sense.

Ohm out the P-leads, they could be reversed (happened on a 172 I fly, left mag was dead when in reality it was the right mag that was bad)

[Joe Larussa]

2 hours ago, RLCarter said:

Ohm out the P-leads, they could be reversed (happened on a 172 I fly, left mag was dead when in reality it was the right mag that was bad)

Checked continuity and things are as they should be. Real head scratcher.

[carusoam]

Moving back further from the P-leads… that big expensive ignition switch is probably next on the list to look at….

They have been known to wear out after 5k hours and 40 years….   

There is a rebuild kit for them…. Have a look at the rebuild kit to get a feel for what typically wears… then look at your switch…

 

That could give you the opportunity to get a fancy push button start switch….  
 

PP thoughts only… not a mechanic…
 

Best regards,

-a-

[Guest]

A failing condenser in the magneto?

Clarence

[Hank]

I had a (left) magneto failure away from home, sent it out for OH. During my 3 hr. flight home, that one failed, too . . . 

[Joe Larussa]

So we discovered something interesting yesterday. We changed the plugs yesterday and there was no improvement.

So we started just letting the engine idle while moving around the ends of the P-lead at the back of the mag.

Moving around the right P-lead made the engine run rough and almost die. Looks like the end was grounding out.

I'm not an A&P but hear me out. When you select the left mag, you're grounding out the right to run only on the left, yes?

So while checking left mag the right really is coming on and off intermittently. Can this be the issue?

[PT20J]

Sometimes there is more than one issue and it can get really confusing, so I find it best when I find something like your funky p-lead to stop and fix that before continuing troubleshooting.

[EricJ]

2 hours ago, Joe Larussa said:

So we discovered something interesting yesterday. We changed the plugs yesterday and there was no improvement.

So we started just letting the engine idle while moving around the ends of the P-lead at the back of the mag.

Moving around the right P-lead made the engine run rough and almost die. Looks like the end was grounding out.

I'm not an A&P but hear me out. When you select the left mag, you're grounding out the right to run only on the left, yes?

So while checking left mag the right really is coming on and off intermittently. Can this be the issue?

Sort of.  If it is running okay on the left mag the right one being intermittent shouldn't make the engine run rough, since it should run okay on the left mag.

Does it run okay on the right mag if the p-lead is disconnected completely?    If so, it's the lead.   Many of them are shielded and if the shield is grounding to the conductor somewhere it can make it intermittent.