I had the same problem with my new to me bike. The threaded tip of the spark was corroded (rusted) where it connects into the coil. The temperary fix was to clean up the end with some sandpaper and cover it with some dielectric grease. Then I bought new plugs and a used coil off Ebay. Of course I did all the carb stuff before I pulled the plugs.
2004 KTM 950 ADV , 2006 DR-Z400E plated , 2001 YZ250N 2T woods bike , 1979 Suzuki RM400 VMX Race bike, 1986 Yamaha FJ1200