The rear cylinder plug on my 03 Road King appears to have excessive carbon burn, based on the visual of the spark plug. I have checked the normal stuff, air filter, hoses from the air cleaner. spark plug wires and changed out the plugs for iridium's. I have read that it is possible that a Fuel Injector or wires to the injector could cause this issue. I do not have any experience with injectors or how they operate. Any help would be grateful. It also appears that i have lost some power on the throttle and the slip on pipes do not sound very good, more backfiring then usual.

Gooch

 

I'm gonna say start with the simple stuff first. Check to see if the bike threw any codes. Check for intake and exhaust leaks, loose O2 sensor wires and connectors, battery and ground wire connections.
Do you have a tuner?