Badel02wrx

I recently bought a 92 FZR 600. It rode fine for a month and now 3 of the spark plugs keep going out. I have spark in the middle but not at the tip of the plugs; like a resistor went out. I changed them 2 and a half weeks ago then 3 days ago and they went out again today. last time i changed them the tips were black after only 2 weeks. Oil is new. battery is good just needs charged once every 2 weeks. No carb work done. Any help would be appreciated. The mount for the coils is cracked at the top but ok on the bottom. Another thing is it sometimes starts with a thumping sound from the exhaust then after i get on it, its sometimes fine. Thought it was fuel but thinking 2 of the plugs fouled out as it goesn't go over 6k rpm's when it sounds different. Spark plug #2 is the one that doesn't go out till last.

Badel02wrx

the plugs that went out after 2 weeks were black on the tip that sparks. Is the bike running to rich or the wrong oil and there getting to hot? The fuel filter has 3/8 inch inlet and outlet. When i look up the replacement it calls for 5/16.