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Thread: #6 cylinder not firing

  1. #1
    Registered User BadDog's Avatar
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    Default #6 cylinder not firing

    '73, stock distributor, Pertronix Igniter, MSD 6A.
    Symptoms started one day last week after winding out 2nd gear. My tach jumped from around 4K up to around 6K, then back down to around 4K. After that, the engine was running roughly, not revving easily, and backfiring under anything more than light load. I thought maybe a plug was fouled because it definitely felt like it was down a cylinder.
    Here's what I've done:
    1) Changed the plugs
    2) Checked to make sure the distributor didn't move.
    3) checked the cam timing, thinking it was timing chain slip or the tensioner broke. It was dead on.
    4) Checked valve clearance, since it'd been about 2 years. No difference.
    5) At this point, it occurred to me to pull the plug wires one by one. This is when I found that #6 was not firing.
    6) I swapped the wires for positions 5 and 6. The problem stayed on 6.
    7) Swapped in a new rotor. No difference.
    8) Swapped in the last distributor cap I had. No difference.
    9) swapped in another new plug on #6 just in case I had a bad new plug.
    10) Propped the #6 wire and plug up on top of the value cover so I could start the car and watch for a spark. No sparks.

    At this point I'm thinking that something is wrong with my distributor, but I don't know what to do next. Help?
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

  2. #2
    Registered User Walter Moore's Avatar
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    Check the compression on the #6 cylinder. Except for item 10 of your list, everything else seems to eliminate ignition. The valve cover isn't necessarily a good ground, so you might not see a visible spark with the plug just touching the cover.
    '71 240Z, Because any fool can drive fast in a straight line.

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    Registered User BadDog's Avatar
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    Ground? I guess I never really thought that the plug, when installed, was grounded to the block. Maybe the plug isn't grounding for some reason? Because every other plug, I could hear the spark jumping from the wire to the plug if I wasn't quick enough hooking the wire back up...
    I guess I'll go pickup a compression tester, may come in handy later with my aging engine...
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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    SFZCC westpak's Avatar
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    Quote Originally Posted by Walter Moore View Post
    Check the compression on the #6 cylinder. Except for item 10 of your list, everything else seems to eliminate ignition. The valve cover isn't necessarily a good ground, so you might not see a visible spark with the plug just touching the cover.
    yeap I chased mine the same way until I found no compression on the faulty cylinder

  5. #5
    Z geek at large FastWoman's Avatar
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    Run your engine in the dark, and look for stray sparks. Your current might be shorting to ground prior to the plug.

    Check the connection to your #6 injector to make certain it's firing.

    Obviously check compression.

    Your problem is probably in there somewhere.

    FAIW, a weird problem one forum member had (argneist) was that the EGR port on his intake had eroded through to the manifold, bypassing the EGR valve. The leak was dumping lots of exhaust exhaust into his #4-#6 cylinders, causing them not to fire. There was possibly some blockage that prevented the exhaust gas from reaching #'s 1-3. I've not heard of this problem in any other car, but I thought I'd mention it.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Nova Scotia,Canada,Earth Blue's Avatar
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    you can pull the end of the wire from the plug and feel the sparks jump to the plug top as you pull it away a few mm. This works fine with bare hands and ngk and stock plug wires... bad wires or other brands may spark to your hand.
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    Registered User BadDog's Avatar
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    Thanks for the replies. For some reason, I'm not getting e-mails about responses. FastWoman: I have carbs, so no injectors to test; I'll have to try the "dark" trick soon. Blue, that's exactly what I was talking about, I could feel the spark jumping on all cylinders except 6.

    I just got done running the compression test. It seems to look surprisingly good and consistent:
    1: 155
    2: 155
    3: 160
    4: 155
    5: 155
    6: 160

    This was after a short warm-up, around 4 or 5 minutes.
    Last edited by BadDog; 05-16-2011 at 07:54 PM.
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

  8. #8
    Z geek at large FastWoman's Avatar
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    In trekkie alien voice:

    What IS... "carbs?"

    But seriously...

    Do you also have points? I'm just guessing here, but if the cam (correct term?) in your distributor is worn a bit more on the #6 than the other positions, OR if your distributor shaft is wobbling just right, AND if your points are adjusted so that they barely break as the distributor cam rotates, then you might be (barely) breaking on #'s 1-5 and not on #6.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Registered User BadDog's Avatar
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    yes, carbs. Points? No points. See the first line of my post: Pertronix Ignitor, MSD 6A. I've have the Pertronix for at least 5 years I think, and its been rock-solid. I've had the MSD box since last summer. I'm assuming they're OK, because from what I've read, if they had issues, I wouldn't have any spark at all.

    At this point I think I'm going to order a remanufactured distributor and install it. I seem to remember my manic Z mechanic mentioning something about a little bit of play in my distributor's shaft a year or two ago. I'm not sure how to check that, but there is a tiny bit of play if I try to wobble it around. I can also easily rotate it a little back and forth, which may be normal, I'm not sure.
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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    Z geek at large FastWoman's Avatar
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    D'oh! I can't read.

    Might still be in your distributor. You can try swapping BOTH your #5 and #6 wires AND plugs to see if the problem migrates to #5. If it doesn't, well, that's the entire ignition system after the distributor. I would think that would mean the problem is in the distributor.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Put a set of points back into the distributor and see what happens. It's quick and simple with the petronix setup. Just one cylinder not firing points to a bad plug, wire or cap externally. I'm thinking a magnet has moved in the petronix setup. They're only held in place with that green tape.
    Last edited by ezzzzzzz; 05-17-2011 at 07:48 AM.

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    f'ing slow internet made me think I hadn't gotten the last post out...nevermind.
    Last edited by ezzzzzzz; 05-17-2011 at 07:47 AM.

  13. #13
    Z geek at large FastWoman's Avatar
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    Green tape?! Are you serious?! Hmmm.... Yeah, that's where I'd look.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

  14. #14
    Registered User BadDog's Avatar
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    I took a quick look at the magnet ring before I left for work this morning. It looks like the tape has lifted a little and folded over in at least one spot. That may be the issue, or it may have happened when I tried to see how much play there was in rotating the rotor counter-clockwise and back, and the maybe rotor damaged the tape. I'll have to take a closer look at it tonight.
    I'll probably swap the old points back in tonight if I can find the old the ballast resistor. If points work OK, it's a Pertronix/magnet issue and I'll look into a replacement.
    I'll probably also go ahead and order a remanufactured dizzy, seeing as mine's probably pretty worn, and a new one will probably last close to another 38 years (and they seem to be in short supply!)
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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    Z geek at large FastWoman's Avatar
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    FAIW, Autozone sells a remanufactured dizzy for around $100 with a lifetime warranty. These things don't actually last anywhere near 38 years, due to a bad breaker plate design. It doesn't take long before the breaker plate freezes up and the vacuum advance sticks. The reman parts often aren't very good, but I figure if the R&R is an easy one, why not do it? So far my reman AZ dizzy is doing great. My original reman AZ alternator did start putting out slightly too high a voltage at one point, and AZ was very good about replacing it.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    The magnet simply triggers the sensor which fires the coil. If it's working on 5 cylinders then the sensor is fine. Some folks remove the tape and epoxy the magnets into place. I just installed a Petronix in my 71 and it is working fine (although I burned out the new coil by removing the ballast as directed in the instructions). I will probably pull the rotor assembly and glue those magnets into place before I make the 12 hour drive to Savannah for ZCON2011.

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    Registered User Zforce's Avatar
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    Definitely get rid of the green tape with epoxy.

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    Z geek at large FastWoman's Avatar
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    Ezzzzzzz, I don't know the specifics of the Pertronix rig, so I'm just making an educated guess:

    I would presume different magnetic lobes correspond to different cylinders. If there's anything marginal about the operation of the system (e.g. distance of sensor from lobes being incorrect or marginal sensitivity), and if there's any difference at all in the strength of the trigger for each cylinder (e.g. magnetic field strength generated by each magnetic lobe), then at least one cylinder will be prone to fail while the others are still (barely) operational.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Registered User Zforce's Avatar
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    Pretty sure Pertronix is a hall effect trigger.

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    For the L6, the Petronix unit is fixed at 28 degrees dwell. There are two mounting screws with no adjustment possible as delivered. It is a simple hall's effect sensor. There is a possibility of some failure due to a magnet strength but these things are so simple it's hard to be sure. Hopefully, he didn't follow the instructions. For what it is worth, the black lead from the distributor goes to the (-) of the coil. The red lead goes to the 12V side of the ballast resistor not the (+) of the coil as directed. The ballast should be used unless you are CERTAIN that the coil will operate on 12V. I burned up my 3 ohm coil following Petronix instructions of removing the ballast resistor. Swapping in points and resetting timing will point to the problem quickly.

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    Registered User BadDog's Avatar
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    3 words:
    Stupid
    Green
    Tape

    Yes, one of the magnets came out. The tape wasn't unwound or anything, the magnet just pulled itself through the tape! Superglued it back in place and drove 80 miles with no issues at all.

    Fastwoman - reman'd dizzies are functionally NLA from AutoZone, Parts America, and NAPA. All 3 said none available. Orderd mine from AZ before I even saw your post :-)

    ezzzzzzzzzzzzzzzzzzzzzzzzz - I've been running my Flamethrower coil (3 ohm) without the ballast resistor now for 4 years, no issues there. Plus, MSD says to remove it.

    I'm still going to put in my new dizzy when it comes. There's a little play in the shaft, the gears may be a little worn.

    Thanks to you all!
    Last edited by BadDog; 05-19-2011 at 02:01 PM.
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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    Z geek at large FastWoman's Avatar
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    Yikes, they're now NLA? That makes me wonder how they're going to honor the lifetime warranty. I'm sure more will become available, but I suppose the supply will be trickling away. I guess I got in towards the tail end of a good thing.

    Anyway, glad you found your problem!
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

  23. #23
    Registered User BadDog's Avatar
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    Yep, rock auto and NAPA both offer a rebuild service, but have no stock. NAPA said that rebuilds are only 80% successful. No guarantee the rebuild will be OK if you give your old one to them and wait 10 days
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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