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Spark Plug Muncher

Discussion in 'Supersport (1974-2007)' started by countcoupe, May 31, 2020.

  1. Hey, team.
    Why is my 1993 900SS destroying the vertical cylinder spark plugs?
    As previously documented I changed the plugs, about 40 miles ago. Since then I've had to change to the spark plug in the vertical cylinder twice!
    For the last year, when i have had the opportunity to ride it, the bike has been a bit lumpy up to about 1/2 throttle. This prompted me to change the plugs and clean the carbs.
    I know I need to give the keihin carbs a proper service. I'm pretty sure at the very least the float valves need replacing and probably the emulsion tubes.
    But that said it wouldn't be the fueling causing the spark plug to fail. Would it?
    Is it more likely to to be the coils and why? DSC_0061.JPG
    • Like Like x 2
  2. I reckon you might have the carb running weak and the plug is getting too hot.

    Or, maybe it isn't the plugs (have you tried a failed one in the other head?) and the cap or lead is faulty but every time you disturb them to fit a new plug, they are alright for a while.
    • Agree Agree x 1
  3. Are they genuine plugs? Lots of good fakes around.
  4. As Chris says, some pictures of the failed plugs would be very helpful here.
  5. I've always considered that my bike runs rich rather than lean. Each spark plug has done such few miles It seems odd that poor fueling would kill the plugs. I'll post a photo of the plug tonight.
  6. So after a bit of Internet searching, I've found a possible issue! Ca cycleworks suggest using ngk DR9EA for their up graded coils. These two failed plugs are ngk DPR8EA9.
    I've attached a pic but it's poor quality. DSC_1288.JPG
    • Funny Funny x 1
  7. From the photo it looks like a fuelling not spark plug issue, bike running rich and causing dry fouling.
  8. Could you rotate the spark plug so we can see less of the business end, soften the foreground focus and increase the depth of field.
    Increase the background lighting and make more of a shadow... perhaps shake the plug during the shot.

    Ps I was actually frightened to enter this thread given the title and responders, but you live and learn.
    Spark plugs aint rugs.
    • Like Like x 1
  9. Plugs need sockets
    • Agree Agree x 1
  10. what is dry fouling?
    The vertical spark plugs are definitely failing, whether it is due to poor fueling or not.
  11. Ahh, just googled it. Now I get your drift!!
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  12. This is what I would do to begin with, remove both plugs from the bike, clean the soot from the plug you have pictured , check the gap, place the cleaned plug in the HT cap for the vertical cylinder and hold it against the cylinder, ensuring the the threads of the plug are contacting a fin on the cylinder (not too close to the spark plug hole), don’t touch the metal part of the plug with your fingers, hold the HT cap, get someone to press the starter button whilst you look for a spark, then take the other plug , put it in the vert cylinder HT cap and repeat this process, any difference in the results?;

    Sooted/cleaned plug doesn’t spark but the other plug does, replace failed plug with a new one, turn mixture screw in on vert cylinder carb til it seats and then turn out 2.5 turns, go for a ride, see if it happens again.

    No difference in sparks- refit both plugs, turn the mixture screw on the vert cylinder carb until it seats, then turn it out 2.5 turns, go for a ride, see if it happens again.

    If it rides OK take the plugs out, your looking for them to be dark grey/black on the circular face and for the electrode to be light tan or light grey (not white!).
    #13 Mark9, Jun 2, 2020
    Last edited: Jun 2, 2020
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  13. In addition to what Chris says/asks, does the engine still run, run roughly or just running on one cylinder ?
    Looking at the picture of the plug you have posted it is definately running on the rich side as it is not the "light brown" colour of a good mixture.
    What plug gaps are you using ?

    As an "aside", my '94 750SS runs DPR9EA-9 plugs with the CCW coils. I'm sure the -9 relates to a larger than "normal" plug gap, in my case 0.9mm I think it is, (but could be wrong).
    My bobber uses NGK plugs which are -11's, but this engine does run a magneto ignition.

    Have you tried swopping the coils over ?
    #14 CAT3, Jun 2, 2020
    Last edited: Jun 2, 2020
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  14. Thanks for your help.
    Yep, done all that minus the fuel screw.
    The duff spark plugs wouldn't produce a spark on either lead. Lovely blue flash on the horizontal plug on either lead. Not sure of the plug gap, but equal on both plugs.
    The bike has run a bit rich for thousands of miles. It's all part of the fcr fun.
    As I have said at the beginning of the thread, bike is really lumpy at upto half throttle. I just bought some bits to service the carbs.
    It will have to be a process of elimination involving coil swaps, carb refurb, etc etc.
  16. Don't know if the 900's are the same as the 750's, (I'm guessing they are) but you could also try swopping over the ignition units that are fitted next to the coils.
    It would be a quick task to perform & see if it trasfers the fault to the other cylinder.
  17. The spark plugs have done about 20 miles from new to fail. I cant believe poor fueling would ruin a spark plug after 20 miles. Previous to these recent pair of failed plugs used in the vertical cylinder, I've never had a failed plug and I haven't made any other adjustments to the bike!
  18. Fouling can occur within low miles. Another factor could be cylinder compression. A compression test will negate or confirm.
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