I posted a topic a while back concerning a Yamaha XS 650 coil problem. I have since replaced the suspect coil and found the problem to be a spark plug. As stated in the previous post, I felt the ignition system, up to the coils, was working fine since I could switch the "good" coil over to the other side and get fire. It turns out that when I did that, I used the firing plug also. Not suspecting a plug problem at the time. Now back to the beginning...... I started out pulling the old mis-matched plugs to put in two of the same number. NGK B8ES. I had one brand new one and one with about 10 snowmobile miles on it. I had suspected a coil problem with the snowmobile as it would cut out on one side after a short time riding. Always the same side.I'd pull the plug and it would look fine(no fouling). Put in a new one and it would start and run fine for a while again. I suspected a weak coil that would quit when warmed. I kept the plugs as they were nearly new, not suspecting a plug problem. I used 3 of these on the one side of the XS after installing the new coil to no avail. I then took the plug from the other side and when I switched it to the "bad" side it fired fine. So, after this long winded post, I guess the question is: what would make these plugs go bad? Is the snowmobile somehow ruining them or are they bad to start with? Since the old firing plug was a B7ES, I bought 2 new ones and installed them and the bike runs great now. Anyone have any idea why these went bad?
Orignal From: Spark plugs
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