Mag Troubles Take 2

[GAHorn]

Here's what a spark plug might look like if it's been shorting inside the shell through a cracked insulator. See the dark lines about 7 o'clock? That's where the spark has been leaking/tracking to the shell, causing the plug to misfire. This results in a magneto drop or a rough engine when running on one magneto.

sparkarc.jpg (8.35 KiB) Viewed 1786 times

This can result from dropping the spark plug during work. If you ever drop a spark plug....throw it away! Deliberately damage the threads so it can never be installed and toss it into the trash.

[Jr.CubBuilder]

SO now my question is what about the spark plugs that I previously had in make it run like that during mag checks? When I swapped plugs top to bottom, why didn't it exhibit the same problem on the other mag? Why did the bench check on the plugs check out fine? I'm skeptical that the new plugs fixed the problem but the engine checked fine both pre and post flight so I have no other reason not to think so.

Whatcha all think?

My guess is you had several bad plugs, when you swapped top with bottom you just ended up with a couple more bad plugs running off the same mag. The mixture that the plugs see inside the combustion chamber isn't the same at both the top and bottom plugs.

I had a hot rod once that drove me to tears because of this odd lurching sputter that I could not eradicate no matter how much I tinkered with the timing or the accelerator pump. Then one night I was fiddling with the throttle under the hood and noticed sparks shorting to ground on the outside of the plugs when I blipped the throttle. Long and short of it is a richer mixture from the accelerator pump would increase the resistance seen across the plug gap just enough to cause the spark to short to ground along the side of the plugs.