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!!!

A bit late to respond, but I just thought I should emphasize what the problem was, in case it's helpful to anyone in the future, as you "buried the lede" a bit in this post.

(Tell me if I'm mistaken here...)
The Slant Six has a fairly long-runner intake manifold, and because of this, has relatively little "common" area inside the intake manifold, unlike the average V8 manifold.
Its main vacuum tap is located in the #6 intake runner, which is a logical place from the perspective of an automaker, because it places it close to both the brake vacuum booster and the air conditioning equipment in the firewall, which are the two major users of manifold vacuum, when installed.

In normal use, the actual amount of manifold vacuum "bled off" through the accessories is minimal.
(I put the term "bled off" in quotes because the actual physics of the thing involve atmospheric pressure, rather than "suction," but that's not important to understanding it).

In the presence of a vacuum leak, though, especially in the brake booster, which can create quite a large leak, the loss of vacuum (filling the intake tract with air, causing the mixture to be too lean to burn) is concentrated on the #6 cylinder, rather than spread around to several cylinders, as it would be in a V8 intake, so rather than a generally lean mixture, you end up with a single "non-firing" cylinder.

This is a good piece of information to keep in mind for the rough-running engine that seems to have a bad #6 cylinder, though the first impulse when diagnosing any rough-running engine without an obvious explanation should be to pinch off any vacuum lines, if only because it's so easy to do.

Thank you for the update.

- Eric

Ok then question time.
I've heard about the location the main vacuum nipple causing problems with the/6 before.
I have a 5/8" thick egr block off plate on mine. Could a guy drill and tap for a nipple on the intake side of the EGR block off plate, that would be a better takeoff point for the brake booster vacuum instead of stealing it all from #6?
( Note I would have to take the threaded pipe plug out of the intake in the one carb barrel hole for vacuum to pass thru... This was something "I" did to my motor not a stock standard thing)

Well, first, as the last guy to post anything here, let me look uncomfortable, turn my head to the left, then to the right, then forward, and say, "Who? Me?"

Then, let me say that I cannot answer this question from experience.

Having said all of that, my answer would be that, yes, you should be able to do that, but I would question whether it would really make much difference in practice.

The brake booster only "draws" vacuum while it is operating, which is to say, as you depress or release the brake pedal.
If you are applying the brake, the odds are that your foot is off the gas and you are not accelerating, so the engine is either closed-throttle decelerating, or idling, both of which are high-vacuum conditions.
So, even if it is only "drawing" vacuum from a single runner, it is unlikely that the booster will significantly affect engine operation, and any tiny benefit obtained would probably not be worth the effort.

– Eric

That is not so.

With your foot off the brake, engine vacuum evacuates the booster. When you step on the brake, air is admitted to the rear chamber, behind the diaphragm(s), which causes the diaphragm(s) to move forward toward the still-evacuated front chamber, thus pushing the master cylinder pushrod—and it also closes the check valve between the vacuum hose and the booster, so as not to upset the fuel/air mixture in the intake tract. When you let go the brake pedal, the check valve opens and the booster is evacuated again. This is why you get one power-assisted stop after engine shutdown, and then a few decreasingly-assisted stops until the booster's internal pressure is atmospheric.
This is true and correct. There's nothing the matter with the booster being connected to № 6 runner. If there's a problem with the booster or its check valve, it'll make the engine run poorly no matter where the booster vacuum is sourced.

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

Y'know, I almost thought of looking it up to confirm this, but I relied on my memory, which in my case is never a good idea.

Thanks for the correction, Dan.

My key point, though, was that the booster is not leaking any non-combustible air into the manifold at any time except while you are using the brakes (if it's working properly). If your foot is away from the pedal, there is no vacuum "loss," and so there won't be any change in performance, no matter where the booster is connected.

- Eric