In 60+ years, never had this problem before

[ratdog]

a while back in my “ new project” thread, I mentioned a problem that popped up on the earsplittingloudenboomer.  Suddenly it would start, run up to RPM for 30-60 feet, then it would cut out and coast to a stop.  I thought it was something to do with the fuel supply but also checked the valves.  Then my son, who is just learning this stuff, said “ why don’t we check the plug?”  Right!  The plug!  What does he know!  So he grabbed a plug wrench and pulled the plug





never have I seen a plug come apart like this!

so now I have to call him the motorcycle mechanic god, and I,’m his humble learner

[tortoise2]

Speculating you already know the cause . . but would be interesting to know the specific application and the plug used. Sometimes an improperly installed heli-coil (or similar) will not transfer heat properly.

[ratdog]

Walt is using a coil from a different bike ( like a v twin) to run greater then normal gaps. My guess is that it got easier to jump to the side ( lots of compression) and it blew apart the insulator. Not easy to see in the pic, but there are three clear spark paths and three clear cracks in the insulator. Just never had it happen before.

C7HSA, 205 psi compression, 50 mm overbore. Open intake(5” velocity stack)  open( reverse cone meg) 93 oct gas, jetted to tan plug

23 mm w 97 jet,  .035” gap with hotter coil from other source

the take out plug before it




BTW, the piston and head show no sign of Detonation.  Only issue is a dime size spot under the spark plug that is dull looking.  Maybe an electro plated plug electrode.

[ratdog]

Just thought I mention... we are using C8HSA plug at .035 now. I am working on some differentcams right now and hope to be replacing the “A9” cam with something with. A bit of preformance. Morecam will also cool the engine down little.