All brains on deck - an unusual problem

[Zed Head]

Has anyone ever seen two adjacent cylinders, on two different carbs, foul in an identical manner and in the same amount of time?  While leaving the others clean (except for one that's half dirty)?  It's an odd thing that might mean something to someone.  It's from the poor-idle-when-hot problems on our Panama member's 204Z with ZTherapy SU's.  

This picture, apparently, comes from six brand new plugs installed, engine started and allowed to run for ten minutes, then plugs pulled.  Is there anything carb intake system related that could cause fuel-fouling like this?  Has anyone seen such a dramatic difference between cylinders like that?  So clean, next to so dirty, all in ten minutes.  If it was EFI we'd have all kinds of ideas.

Here's the picture, the currently active link, and a drawing from carpartsmanual for reference.  Just a new look at an old problem.

https://www.classiczcars.com/forums/topic/60072-misfiring-while-cruising/?page=7&tab=comments#comment-558478

 

[Captain Obvious]

Weird. No, I don't have an obvious answer, but here's some musings.

There's no guarantee that all three cylinders fed from the same intake manifold get exactly exactly the same percentage of input stream, but they've got to be close enough to not result in that. All those Z's on the road running carbs with the stock intake... I'm no expert, but I've not heard a contingent of people complaining that the intake manifolds were designed such that 3 and 4 run twice as rich as the rest.

Those clean plugs don't look like they've got anything at all on them. Like they're still brand new out of the box.  After ten minutes or so idling, I would expect to see something on all of the plugs. I can't see anything in the pic... Is there at least something on all of them?  Maybe the root problem is with the clean plugs, not the dirty ones?

I would confirm that all the cylinders are firing (probably by pulling spark plug wires while the engine is running or some other potentially dangerous method). Pull and ground each plug wire in turn and verify that the idle RPM goes down and gets rough? Don't get zapped, and don't burn out your ignition system by running OC.

I would also check the valve lash and run a compression test. Make a note not only of the final peak compression pressure, but also make note of (about) how many compression cycles it took to reach that pressure for each cylinder. If four cylinders take three revolutions to get to peak, but the other two take ten cycles, that could be a clue.

[siteunseen]

What about disabling each carb, let it run at idle and see what the plugs look like?

I can't stop thinking those two aren't getting fire like the rest. Maybe try a spark test in the dark or low light on each one? 

That petronix system is something I know nothing about so I'm throwing stuff out there.  Alabama game is boring.

[Zed Head]

I've never used one of those flashing spark checkers, the kind that fit between the plug and the wire.  Has anyone used one of those?  Would a misfire show up in the flashing even though it's fast?  Don't know if the human eye could catch it.

Here'a thought that just came to mind.  One of those old engine analyzers.  That would show misfires, if I recall right.  They might still be around down in Panama.  That would be a quick easy way to show if spark is good or bad on all six at the same time.  If I was down in Panama I'd try to find a shop with one of these I think.  Might make things clear pretty quickly.

http://www.enginebuildermag.com/2015/10/old-gold-vintage-engine-analyzers-and-distributor-testers/

 

 

[Mark Maras]

 Speaking of old engine analyzers, my Dad used to check his plug wires by waiting until dark and spraying water (spray bottle) on the plug wires while the engine was idling. It was my first of many light shows to come. @jalexquijano Give it a try. You never know, it might turn something up.

[siteunseen]

Yeah! Hold one at a time and see how much pain difference you get.

Jorge 

Edited September 29, 2018 by siteunseen

[siteunseen]

Is it possible the coolant line between the two manifolds could be the culprit?  That's the only thing that connects the two from what little I know. The spacers have more holes on the later 240s too. That slimy octopus is falling pretty quick off the wall now.

[Zed Head]

I have another wild arse guess, based on an old memory about the Pertronix trigger ring and the tape holding it on.  I believe that the magnetic ring is actually six magnets isn't it?  Or it's six bits of iron that pass a magnetic sensor.  Not sure which.

In the past, Jalex noted that the tape was peeling off of his, if I recall right.  If the trigger ring slipped, could this give two bad triggers out of six?  Do the bits inside the ring move separately?

I've never had a Pertronix igntion system so don't know all of the fine details about how they work.  But it might explain why the plug wires and plugs and cap all look good but there still seems to be some misfires.  Each magnet or iron piece triggers a certain cylinder.  Another reason to use an analyzer, I think, then work backwards to find the cause.

In the meantime maybe take a good close at how the Pertronix parts are fitting under the cap.

Edit - it's magnets.  Hall effect is the term I couldn't remember.

Edited September 29, 2018 by Zed Head

[Mark Maras]

 I like your wild arse guess. Now we need a Pertronix guru to walk us through a test procedure. Volunteers?

[madkaw]

Blown head gasket? 

[Dave WM]

regarding the color tune (clear plug). Have used one, you can slow down the video and really see what is going on.

 

 

[Dave WM]

power testing idea is 1st thing to check. pull one plug at a time, and see effect on idle. Please video while doing so we can all get a good chuckle and the zapping received. Or get some of those long purpose designed pullers that lets you grab the plug cable at the spark plug and pull with out the risk of getting zapped.

The problem with trying to diagnose over the internet is you always miss some crucial piece of the puzzle that the OP is just not relaying. The power test is the most obvious starting point, and perhaps it has been done, but if it was you would think there would be a huge imbalance just by looking at those plugs. what is thought to be loping due to a cam could really be loping due to a complete misfire. I have seen cars that sound like crap and the owner thinks its supposed to sound that way. So agree with capt O, I suspect the problem is not with the black plugs but the super clean ones not firing at all. the black is a result of trying to compensate for 4 others that are not working at all.