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Thread: Excessive*carbon buildup: R100RT '91

  1. #1
    Registered User
    Join Date
    Mar 2012
    Kansas City, MO

    Excessive*carbon buildup: R100RT '91

    Replacing leaky cylinder o-rings, repairing stripped cylinder studs, etc. It is is a good time to do this.

    I was just about to put the spark plugs back in after all of the above mentioned repairs and was going to start it up when I noticed that the left spark plug had a small chunk missing off of the insulator. I had the heads re-done about 10,000 miles ago and both combustion chambers were clean, no carbon. Rather than hope that whatever caused this problem would fix itself, I decided I had to at least make sure that there wasn't foreign debris bouncing around in the left cylinder. I know that cracked or damaged insulators can be caused by detonation, high heat differentials, etc. also carbon hot spots.

    When I removed the left head I could not specifically find the piece of insulator but it appeared to me to have greater than normal carbon deposits. (I just pulled the head on the right side and although the carbon is marginally heavy it is not as bad as the left side.) The carbon on the left side was (for lack of a better description) chunky and built up. It seems to me that it could be quite possible that that excessive carbon was causing heat issues that resulted in a cracked and broken insulator.

    However, I have NEVER seen this engine smoke on start up after sitting on the sidestand and I have NEVER heard this engine knock, ping or make any bad noises. It has ALWAYS ran perfect (even with a cracked spark plug insulator). Although it has leaked oil due to bad o-rings, it has never had an issue with burning oil or oil in the combustion chamber.

    So......I don't want to have to go into this motor every 10,000 mile to de-carbonize so what in your esteemed opinion could be causing such a thing and how should I be working toward preventing it in the future.

    As always, your opinions are appreciated very much!

  2. #2
    Registered User jad01's Avatar
    Join Date
    Dec 2006
    Houston, TX or Portland, OR
    I would recommend applying a dose of Techron in the fuel a tank (or two) before you are ready for an oil change to keep it at bay. Maybe it's a symptom of the rings on that side not being fully broken in, or perhaps one or more is sticking?
    '78 R80/7 and '84 R100RS (Blues Brothers), '86 K75C (Icy Hot)
    '90 and '93 Mazda Miatas (Jelly Bean and Red Hot), '97 Nissan XE PU (Mighty Mouse)
    '96 Giant Upland (big Kendas, baby!)

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