[patrick_777]

Maybe you're actually getting too much fuel into the cylinder and you're drowning out the spark. What was your main jet set to when you had the carb off? Did you change it? Are your mixture screws drilled out where you can alter them and lean it out? Just spitballing here (mostly what I do), but if you've tried other known good plugs and they're all getting saturated with gas when you try and fire it, then it's taking way too much gas into the cylinder and literally drowning the spark out of the plug from the first crank.