How are you doing your plug check? With 380 mains you should be really rich. When you check your plugs are you running wide open for a long stretch and hitting the kill switch without letting off the throttle? Thats the way I've been told to check plugs. Otherwise your carbs may be gummed up and there isn't enough fuel getting through the jets. I was also told that you lose power with the larger air intake on the F5's.