Slant Six Forum :: Misfire that JUST WON'T DIE!

Sounds like you are getting cross-firing across the terminals inside the cap due to improper phasing of the rotor and the reluctor. You could try to find a wide contact cap, or try shifting the reluctor phase by pulling it off and drilling/hogging another slot to let you shift it relative to the dist shaft.

The quick and dirty way to test would be to sacrifice your cap and widen the slot that locks it into the dist, then move the cap relative to the dist (first one way, then the other) and see where/if the problem goes away. If/when you find the right direction, then you can drill the reluctor to shift it permanently.

Lou

Any chance the vacuum advance is tapped to the wrong port? It shouldn't be giving advance at idle.

Anything over ten degrees base starts a misfire in #6

Forgot to check if someone "repaired" the distributor pickup pigtail and reversed the wires this morning, so I'll do that when I get home

I swapped the battery negative cable from the front of the block to one of the bellhousing bolts, just in case 6 somehow wasn't getting a good ground

All valves move as expected

Why would you be over 10° base? You should be shooting for about 5ish.

That's wayyyyyyy over yonder, about five or six miles on the other side of the fence that separates things that might be causing the problem from random nonsensical ideas our brain dishes up out of desperation.

How sure are you they're doing so at the right time? That is: are you sure the cam timing's right? And, are you sure the timing marks are accurate?

Just for good measure: what spark plugs are you running?

And also, I know you've been through caps and rotors, but that doesn't necessarily mean you've had good ones; read this long-but-worthwhile thread. But I think whatever contributions to the problem your cap and/or rotor might be making, they aren't the main thing.

Could the timing chain/set be very sloppy and had somehow jumped time?

I would also check the timing mark accuracy on the damper/tab against TDC with a borescope.
The damper may need to be refurbished. Send it to damper dudes if it does.

I think the root cause of your problem is likely not electrical.

Greg

Also, lining up dots and marks and stuff was almost kinda sorta mostly more or less an okeh way of doing it with factory '60s-'70s parts, but as replacement-part quality dropped over the years it stopped being even that good. You gotta-gotta-gotta dial in the cam properly, or the odds are against your getting the cam timing right.

And as John Jerome wrote in "Truck" (highly recommended), y'can't get the cam timing right until the cam timing's right.

Author:	Plymouth_Ruster [ Sat Oct 10, 2026 6:43 pm ]
Post subject:	Misfire that JUST WON'T DIE!
I'm back once again, for help with my '74/'75 Scamp. I'm going just a touch insane trying to get this thing to run properly. It starts misfiring horribly if I get too much spark advance - and by "too much," I mean less than 30 degrees total. Unplugging the vacuum advance fixes this, for the most part, at my current base timing of 5 degrees. It's a near bone-stock super six setup (very slightly higher compression - about 130psi. I'm running premium). It's got a tin top BBD that I've rebuilt and set to factory specs. If I have too much advance at idle, cylinder 6 starts missing entirely shortly after a cold start. This stays the same if I change plugs, wires, coil, distributor, ICM, you get the idea. Yes, I've swapped out the entire engine bay side of the ignition circuit - multiple times - minus the wiring. No change, except for the brand new ICM that died almost instantly. Go aftermarket, woohoo. I've inspected and cleaned every connection in the ignition circuit on both sides of the firewall, and everything is getting the correct voltage under load while running. It's definitely not mechanical - Compression test shows between 120-130psi on all cylinders, and vacuum is dead steady at a 650rpm idle with 16". I loosened the valves a bit just to be sure it wasn't that, all it did was make the valvetrain noisier. I probably forgot to write down a ton of stuff I did. I've been trying to solve this thing for weeks now and I'm going insane. Lemme know if you have any idea what I should do next.

Author:	Dart270 [ Sun Oct 11, 2026 4:46 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Sounds like you are getting cross-firing across the terminals inside the cap due to improper phasing of the rotor and the reluctor. You could try to find a wide contact cap, or try shifting the reluctor phase by pulling it off and drilling/hogging another slot to let you shift it relative to the dist shaft. The quick and dirty way to test would be to sacrifice your cap and widen the slot that locks it into the dist, then move the cap relative to the dist (first one way, then the other) and see where/if the problem goes away. If/when you find the right direction, then you can drill the reluctor to shift it permanently. Lou

Author:	Rick Covalt [ Sun Oct 11, 2026 5:22 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Sounds like he has already changed out the entire distributor?

Author:	MadScientistMatt [ Sun Oct 11, 2026 5:42 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Any chance the vacuum advance is tapped to the wrong port? It shouldn't be giving advance at idle.

Author:	Plymouth_Ruster [ Sun Oct 11, 2026 10:56 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Quote: Sounds like you are getting cross-firing across the terminals inside the cap due to improper phasing of the rotor and the reluctor. You could try to find a wide contact cap, or try shifting the reluctor phase by pulling it off and drilling/hogging another slot to let you shift it relative to the dist shaft. The quick and dirty way to test would be to sacrifice your cap and widen the slot that locks it into the dist, then move the cap relative to the dist (first one way, then the other) and see where/if the problem goes away. If/when you find the right direction, then you can drill the reluctor to shift it permanently. Lou As Rick says, I've been through every combination of three distributors, two caps, and two rotors. Quote: Any chance the vacuum advance is tapped to the wrong port? It shouldn't be giving advance at idle. It's not. Anything over ten degrees base starts a misfire in #6, and it's pretty clear I've got weak spark all around as it tends to stumble on the others when I do that. It seems that anything 5 degrees and under that for base timing, without the vac advance, is okay - much over that or adding vac advance causes it to miss right off idle, getting worse as you go up the rpm range. Edit: I realized I haven't been 100% clear on the behavior so far, so to clarify; With the vacuum advance unplugged, anything over 5 degrees of base timing makes cylinder 6 misfire at idle. It's always 6, I've swapped plugs and wires with no change. It also tends to stumble on a few other cylinders like this. Vacuum advance is running off ported vacuum (no advance at idle) which I've been lead to believe is correct. Under 5 degrees it'll run okay from the mechanical advance, but obviously it's pretty sluggish even for a 225. When I get up to 10 degrees base timing, 6 isn't hitting at all at idle but will if I rev it up (although after a second, it'll start randomly missing on EVERY cylinder). Vacuum advance has no effect on the idle, but anything more than 5 degrees base plus mechanical advance causes the horrible misfiring, which gets worse as timing increases. It always revs clean, though, which led me to believe it was a fuel issue, but no - more or less fuel than it already has makes it run worse. I've adjusted the metering rods richer and leaner to little effect. I've been through two ICMs - the original to the car, which i assumed was the problem initially because it sure as hell LOOKS like it'd be the problem, and then a brand new one which stopped firing the coil at all in START (which is what caused me to tear every connection apart and clean it) after which I swapped back to the apparently immortal original. No change between the two, exact same symptoms. The three distributors were the beat to hell original, which i assumed was the problem because the vacuum advance made the reluctor smack into the pickup, a brand new Chinese one with the same issue, and now I'm running a NOS OE unit that is basically the best you could hope for in every sense. No change between any of them. Two coils, again the original to the car and a new Denso unit (also an external resistor type). Both measure healthy resistances. Swapped out the ballast resistor, nothing wrong with the original but didn't hurt to try. I unplugged the voltage regulator and both field leads from the alternator to see if it was somehow backfeeding into the ignition circuit, no change except that it somehow killed the voltage regulator. There was a splice in the coil negative wire, so I stripped both ends down to clean wire and gave it a weather-sealed crimp connector. No change. There's only about a .2v drop between the battery and the ballast resistor input under load, which i though was pretty good given than I started out with over 2v drop. Has a healthy 8.5v output whuch does not drop at all on its path to the coil. Again, nothing changed. The spark plug wires are brand new except for the #2 wire, which I had to steal from another engine after the new #2 wire ripped in half the first time i removed it from the plug. I have another set on order. I expect no change. Forgot to check if someone "repaired" the distributor pickup pigtail and reversed the wires this morning, so I'll do that when I get home. Oh, and I swapped the battery negative cable from the front of the block to one of the bellhousing bolts, just in case 6 somehow wasn't getting a good ground. My "ground strap" is actually just a second battery cable, from the front of the motor to the firewall. This problem has also persisted between two cylinder heads - the original hydraulic head (engine had previously been swapped to solid cam/lifters/valvetrain) and the fresh, ported and 340 sprung head that's on there now. All valves move as expected. I think that's it. Hopefully that's enough to explain everything, not usually something I'm good at.

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Author:	SlantSixDan [ Sun Oct 11, 2026 3:14 pm ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Quote: Anything over ten degrees base starts a misfire in #6 Why would you be over 10° base? You should be shooting for about 5ish. Quote: Forgot to check if someone "repaired" the distributor pickup pigtail and reversed the wires this morning, so I'll do that when I get home Good idea. Quote: I swapped the battery negative cable from the front of the block to one of the bellhousing bolts, just in case 6 somehow wasn't getting a good ground That's wayyyyyyy over yonder, about five or six miles on the other side of the fence that separates things that might be causing the problem from random nonsensical ideas our brain dishes up out of desperation. Quote: All valves move as expected Good. How sure are you they're doing so at the right time? That is: are you sure the cam timing's right? And, are you sure the timing marks are accurate? Just for good measure: what spark plugs are you running? And also, I know you've been through caps and rotors, but that doesn't necessarily mean you've had good ones; read this long-but-worthwhile thread. But I think whatever contributions to the problem your cap and/or rotor might be making, they aren't the main thing.

Author:	Greg Ondayko [ Sun Oct 11, 2026 5:09 pm ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Could the timing chain/set be very sloppy and had somehow jumped time? I would also check the timing mark accuracy on the damper/tab against TDC with a borescope. The damper may need to be refurbished. Send it to damper dudes if it does. I think the root cause of your problem is likely not electrical. Greg

Author:	Plymouth_Ruster [ Sun Oct 11, 2026 5:49 pm ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Quote: Why would you be over 10° base? You should be shooting for about 5ish. Well, every other engine I've worked on has used ten to twelve degrees as a base. Most don't even complain about it until 20 or so base. I suppose the fender tag does state to set the 225 AT TDC, though. Quote: That's wayyyyyyy over yonder, about five or six miles on the other side of the fence that separates things that might be causing the problem from random nonsensical ideas our brain dishes up out of desperation. Then you know where I'm coming from! Quote: How sure are you they're doing so at the right time? That is: are you sure the cam timing's right? And, are you sure the timing marks are accurate? I don't have many pictures of the reseal, but from what i remember the timing was bang on and it had a little slop but not so much that I bothered to replace the set. The good compression numbers and dead steady vacuum give me confidence that there's no issues there, but I'll see if I can't find the pictures to be sure. Quote: Just for good measure: what spark plugs are you running? I was running some copper NGK V-powers, but i swapped to a set of old ACs out of another engine that ran pretty well. Quote: And also, I know you've been through caps and rotors, but that doesn't necessarily mean you've had good ones; read this long-but-worthwhile thread. But I think whatever contributions to the problem your cap and/or rotor might be making, they aren't the main thing. Agreed. I'll check that thread out when I have a bit of time.

Author:	Plymouth_Ruster [ Sun Oct 11, 2026 6:05 pm ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Let's see if i remember how to share an image on this forum... This is the only picture of the timing set. Crank gear is half a turn from the key pointing straight up and the cam gear is a quarter turn from the mark pointing straight down. Seems right to me. Edit: SUNOVABITCH. The timing mark isn't AT the keyway, it's twenty degrees off! The whole damn crank is off by 45 degrees! How the hell didn't I notice that?!

Author:	SlantSixDan [ Sun Oct 11, 2026 7:30 pm ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Also, lining up dots and marks and stuff was almost kinda sorta mostly more or less an okeh way of doing it with factory '60s-'70s parts, but as replacement-part quality dropped over the years it stopped being even that good. You gotta-gotta-gotta dial in the cam properly, or the odds are against your getting the cam timing right. And as John Jerome wrote in "Truck" (highly recommended), y'can't get the cam timing right until the cam timing's right.

Author:	Plymouth_Ruster [ Mon Oct 12, 2026 6:32 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Quote: Also, lining up dots and marks and stuff was almost kinda sorta mostly more or less an okeh way of doing it with factory '60s-'70s parts, but as replacement-part quality dropped over the years it stopped being even that good. You gotta-gotta-gotta dial in the cam properly, or the odds are against your getting the cam timing right. And as John Jerome wrote in "Truck" (highly recommended), y'can't get the cam timing right until the cam timing's right. That's all well and good, but this is not the final engine for this car and my current goal is to make it less worse in the short term. Perfection would be nice, but "less terrible than 22.5 degrees retarded valvetrain" is what I'm shooting for here. If I lose a little power to it being off a degree or four in one direction or another but it still runs well enough to drive, that's fine by me. If it were up to me I'd be ripping the timing cover off right this second, but I finally remembered I shredded my harmonic puller last time I used it and the three-jaw wasn't cutting it, so the goal is to hit oreilly's for a new one right as they open and, with any luck, have the thing back together by the time I leave for work.

Author:	mr.norm [ Mon Oct 12, 2026 8:59 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Years ago I had a very similar problem with a /6 in a 64 Valiant. I had convinced myself it was ignition. Ran through a bunch of ignition parts with no change. Long story short, I had a vacuum leak caused by a cracked intake manifold. Worth a look. Good luck!!

Author:	Plymouth_Ruster [ Mon Oct 12, 2026 9:41 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
Okay, so it only really super looked like it was out of time in that photo. So that was a huge waste of time, but at least now the oil pan will leak.

Author:	Rick Covalt [ Mon Oct 12, 2026 11:17 am ]
Post subject:	Re: Misfire that JUST WON'T DIE!
The marks mean almost nothing. They can be stamped wrong and it could still be way out of time. You have to use a degree wheel. Checking for an air leak is a great thing to do also.

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