Huh I've never run nitrous or turbo but to date I can count on 1 hand how many champion plugs I've had trouble with over the years. And when I have for what they cost no big deal. I had a set once that must have been dropped in shipping and handling once but that was about it.

That is all my race car has. That is high for a stock motor for sure.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Compression results. Screw in style (#of cranks to get the value)

#1 125 (3)
#2 125 (3)
#3 130 (3)
#4 125 (3)
#5 120 (4)
#6 120 (6)

Also, a big find, #6 is missing at idle. Its not "firing" at all.
I do have spark. Cap to wire, good. Wire to plug, good.
Can an overused #6 vacuum port cause this? I just finished the Compression test, I will plug it up just cause.

The cylinder should be getting fuel. I've had a temporary vacuum gauge on the #6 port this whole time and I'm around 18-20.
Could the intake valve be plugged up, passage clogged, or otherwise inoperable and my vacuum signal is being produced by the other cylinders?

I think I'm getting closer but this is weird. Its seeming like I got unlucky in that something went wrong as I replaced the ignition.

Compression looks Ok, but a leak down test would tell you for sure how much leakage you have and where.Yes plug your vacuum port. Eliminate as many variables as you can.

Did you replace the plugs? I think I remember you did.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Forgive me, but,

If cylinder #6 has spark and good compression, how do you know it's not firing?

– Eric

It wasn't "firing" purposefully in quotes due to the running condition not changing while holding the wire and watching the spark go to the cap or plug depending on the test.

BUT that doesn't matter because I found it!!!

Taking off the #6 vacuum line and plugging smoothed it out.
The brake booster let go. I cant believe that happened at the same time I changed the ignition.

I need a beer.

Thanks everyone.

And there you have it! Congratulation!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Good sleuthing—could've taken a whole lot more tailchasing.

Reminds me of a food processor I rebuilt recently, a 1985 KitchenAid, made in France by Robot Coupe. It ran briefly, then tripped the thermal cutout. I expected to find burnt (or at least heat-discoloured) motor windings, but no, they looked like new. The motor's run capacitor measured out right on spec, but that's a no-load test; it was faulty. I installed a new one and did some other refurbishments, went to do a final test before reassemblyand it ran well, but the switch was wonky. Weird, because it was perfectly fine before. I had a new one and installed it, but still weird. Chalk it up to the perversity of inanimate objects.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yep, I can't say it enough, don't let coincidence be Your nemesis. Having worked in shops for 36+ years, I've seen that happen more times than I can count. Techs get locked into "it must be what I just changed/disturbed/adjusted", many times it is, but coincidental fails are not uncommon. Glad You found it before catching Your tail, lol!!!