[ambmech1]

1970 ys 400. Mild cam, Headers, quadrajet with cast iron intake.
Just before putting the car away this past fall I had an issue after starting it. Ran very rough like missing a few cylinders. Pulled the plugs and found 1,7,4, and 6 with the ground electrode bent over and touching the center electrode. No mechanical damage noticed. Other 4 plugs were fine and burning normal. Light tan color. Put new plugs in and it's fine. No noise and running strong and smooth. A little research and it may be a detonation issue? Being a dual plane intake is one side of my carb malfunctioning? Anyone ever run into this? Seems to run fine now but I'm worried it could happen again with more serious consequences.
Thanks in advance,
Joe.

[STEELCITYFIREBIRD]

Pictures of plugs?

[ambmech1]

Here's 3. 1st pic shows one of the good plugs.

[ambmech1]

Running stock HEI. Gapped at 45.

[STEELCITYFIREBIRD]

I can't think of anything doing that except mechanical contact.

[jonmachota78]

1746 would be one side of a dual plane. Hmmmm
The good plug gap looks kinda wide.

Ok so if something like a small nut fell in one side of the manifold, it could possibly bouce around and go between those 4 cylinder hitting those plugs before shooting out the exhaust. Sounds unlikely but possible?

[unruhjonny]

I hope there's nothing floating around in those bores...

My knee-jerk reaction was that this would have to be a mechanical thing... but I have been wrong many times before...

[1968GTO421]

Keep us posted on how the new plugs are doing while we all scratch our heads over this. Reminds me of 25 years ago and timing belt breakage on a certain 4 cylinder car (not ours) that I worked on, except the valves and plugs were flattened and pistons cracked.

[STEELCITYFIREBIRD]

My best guess is that you opened up the gap and the electrode hit the plug hole threads while screwing them in, closing the gap a good bit causing them to eventually foul. Then when you wound them out they got shut completely from the deposits.

[Joel Koontz]

Are there any thread marks on the back side of the electrodes that are bent shut?

[ambmech1]

Would a lean condition cause the ground electrode to get super hot and bend? I just find it weird that it coincides with the upper plenum of the intake manifold. Not Pontiac but on searching the internet there is reference of detonation overheating the tip of the plug and basically hammering it. I would think if it was detonation it would affect all cylinders.

[ambmech1]

Originally Posted by Joel Koontz

Are there any thread marks on the back side of the electrodes that are bent shut?

That's what's baffling there are no marks on the tip of the plug. Almost like it's melted and just dropped down.

[Joel Koontz]

Originally Posted by ambmech1

That's what's baffling there are no marks on the tip of the plug. Almost like it's melted and just dropped down.

I am not referring to the tip, but the part that is directly above the threads.

If the bend of the electrode was such that it stuck out past the threads, screwing the plug in would push the electrode in/down.

[tom s]

something had to hit them.Tom

[dmac]

Gremlins, I've had lots of them messing around in my cars.

[AdamIsAdam]

Subscribed because I love a good mind teaser!

[STEELCITYFIREBIRD]

Did my neighbor put the plugs in?

[jonmachota78]

1746. what are the chances. thats one side of a dual plane manifold. flying debrie will be issolated to one side of the manifold because of the divider. seems freaky, BUT 1746. that's too much of a coinidence. if you're not sure what im talking about, one side of a dual plane is 1746, the other is 2358. coincidence?

[STEELCITYFIREBIRD]

Correlation to the intake feed split can't be a coincidence, I agree.

[AdamIsAdam]

carb parts falling into the engine? How's it running now?