I just bought a 83, xlx 1000 the problem I am having is, the bike runs great on new plugs, but only after 20 to 30 miles the bike starts to pop and seems to run on 1 cylinder, and then dies all together, after I take a look at the plugs they are covered in black carbon. I can put in new plugs and the bike will fire and run great for 20 to 30 miles, before it spits and sputters. Any idea what could be causing this. Thanks Marc.