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plug fouling?


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I have a 113 cu. inch S&S with a shorty G and today I noticed that the plug on the rear cylinder is much cleaner (whiter)than the plug for the front cylinder which is somewhat black except for the very tip which is tan. The ignition is I.S.T. so I presume this is not a timing problem. I believe the I.S.T. does not allow any timing modifications. Ignition was factory installed. Any idea what causes this? (Left plug in image is for front cylinder, right one is for rear)
Best regards


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It could be a number of things. You would have to go by process of elimination to sort it out.

Possibilities are an intake leak on the rear cylinder and a richer setting on the carb to get it to idle properly thus the cause for the rich plug in the front cylinder.

Possible a bad plug or wire will do it too. Too much resistance to fire properly.

Adjustable push rods? Are they adjusted properly?

Do a compression test and find out what kind of shape the cylinders are in. Is it oil fouled or gas fouled?