Misfire Sparkplugs are black? - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 7 (permalink) Old 01-05-2017, 01:29 PM Thread Starter
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Misfire Sparkplugs are black?

Hello I am new to the Z world. I just got a 1971 240z with an L28 in it with original 240 carbs.

I thought I had a misfire so pulled all of the plugs and discovered that my first three look pretty good about cardboard brown, however my 4-6 plugs where really black and oily. I'm guessing that it is a carburetor issue but not exactly sure if any one can point me in the right direction that would be greatly appreciated.

71' 240z
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post #2 of 7 (permalink) Old 01-05-2017, 04:49 PM
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Rear carb may be running rich. Lean it out by twisting the adjustment nut CCW when looking from the top of the carb. There are lots of posts on how to adjust your carbs.

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post #3 of 7 (permalink) Old 01-08-2017, 03:16 PM
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Unfortunately, a rich mixture doesn't produce oil though, just a dry black carbon fouling and it would be on all cylinders, not just two of them. You can clean and lean, but if the plugs come out oily again either your valve seals are leaking or the oil control rings are bad on those two cylinders. I made a Youtube video on replacing the valve stem seals. If you see smoke when taking off from a stoplight, it is probably the rings though. Compression and leak down tests can still show that the cylinders have good compression too if just the oil rings are bad, but it wouldn't hurt to do a compression test. It's an easy and low cost engine to rebuild though.

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post #4 of 7 (permalink) Old 01-08-2017, 06:19 PM
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Sounds like your rear carburetor is running really rich. Probably not oily just wet from too much fuel. There is a tool called color tune available on Amazon etc. Very easy to use. It is a spark plug type tool that has a glass eye you can see the combustion chamber while the vehicle is running. Just adjust the carburetor until you see a blue flame inside. Very easy to use and very accurate.
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post #5 of 7 (permalink) Old 01-08-2017, 08:12 PM
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first check the choke linkage to make sure the rear carb isn't running partially choked! (It's not really "choked", ...just operate the choke cable from under the hood and you'll see how the linkage works, make sure the rear isn't stuck down a little bit)

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post #6 of 7 (permalink) Old 01-09-2017, 03:19 PM
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Adjust the carb, check plugs in a few thousand miles. If black, do a wet/dry compression test to see if compression is down. I suspect some oil usage, so check that dipstick often.

One other thing - when I had a '70 240Z, I learned to ONLY run NGK spark plugs, and to change them every 10K miles. Seemed like I'd get a miss by 11K miles if I kept running them. Fortunately changing plugs is a 20 minute job.
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post #7 of 7 (permalink) Old 01-09-2017, 08:43 PM
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Ghosttanker has the first step. Don't adjust ANYTHING until you have determined the jet has gone all the way back up into the carb body and is not binding/sticking when the STARTER circuit is used....

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