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Iridium spark plug warning

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I recently found myself chasing my tail trying to troubleshoot a poorly running engine thinking that I had carburetor issues. I had pulled the carb and cleaned it prior to an upcoming race and afterwards it ran like crap. After pulling the carb a couple times trying to get it fixed without success, I ran out of time and had to adjust it the best I could to run in my race. I got the carb adjusted so it started easily and ran decent when it was hot but it did not like to run when cold and I really had to warm it up well in advance to ride it. I ended up running my race and a couple of track days on the bike in this condition putting roughly 3.5 hrs on the engine before I decided to pull the spark plug to see what it looked like. To my surprise, this is what I found when I pulled my spark plug.



Notice the missing ground strap. To be honest, I was shocked that I just pulled this plug out of an engine that was running. Notice the soot build up over where the ground strap broke, yep ... the engine will run without the ground strap.  :jawdrop:


A tear down and leak test shows that I got lucky and the ground strap exited the engine without causing any damage. Looking at the piston and head with the naked eye didn't reveal any damage what so ever. With a 10x eye loop I could only spot 2 or 3 minor indentations in the piston & head. After cleaning some carbon off of the spark plug relief, cut into the piston, I could see where the piston made contact with the spark plug ground strap.

NGK makes a spark plug numbered CR8EHIX-9 which is the Iridium version of the standard CR8EH-9 spark plug that the 150R calls for. When I cleaned my carb at the beginning of this fiasco, I also changed out the spark plug and I decided to try out the NGK Iridium spark plug, this is the plug that broke. I took some time to compare a new Iridium and standard spark plug and also find out just how much clearance I had while I had things torn apart. Here's what I discovered. The piston (JE 12.5:1) has a relief for the spark plug for a good reason, the tolerances are tight only measuring .032" with a standard plug (my cylinder is is shaved by .009"). The Iridium plug extends .018" deeper than the standard plug leaving me only .013 clearance. So if you do decide to try the Iridium plug, make sure you know you have the clearance because it could be very close.


TL/DR: The Iridium plug is slightly longer than the standard spark plug, make sure you have the additional clearance.

Edited by KLX_331_Rider
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