I have a 2004 turf tracer. For the last few weeks it has been carboning up the plugs, running terribly. It gets to the point were it just quits, after sitting you can start it up but only idle it. It also blows a good bit of black smoke. If you change the plugs it runs well for about 5 minutes. Do you think this could be a coil? I had a guy tell me it may be a valve thats carboned up. I put high test in it with new plugs and it didnt make a difference. Any advice??