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Spark plugs 3 and 4 fouling at idle on my 240z

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I recently carried out a compression test on my 240z engine and the result was perfect. Nevertheless, last sunday i left the car idling with the Hood open for 10 to 12 minutes and the engine started to idle roughly. I then decided to drive the car and the engine started missing at 2nd and 3rd gear. Surprise, when i got home i pulled spark plug 3 and 4 and both were black fouled. The other ones looks okay.  If i already changed the spark plug wires, cap and rotor and the compression is good what could be causing this? Should i use some nose pliers to tighten the terminals?

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Dual SUs? Or side drafts?


Side drafts the middle carb float is stuck or for what ever reason it's flooding, or jet backed off/fell out or has the wrong jets in it.


Dual Hitachi SUs probably the intake valve guide seals are bad and burning oil.

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