I recently replaced the spark plugs on my 99 Nissan Sentra, it has about 114k on it. When I removed the plugs I noticed there was heavy carbon deposits on them. Any advice as to what it could be and how can I fix it?? Thanks in advace

buy new plugs

thats carbon build up that is present in the cylinder walls from the all fuel that is burnt and not sent out of the exhaust valve. you might need to a specail cleaner to remove all that build up of carbon in there.............