try seafoam.. i know it sounds like a quick fix, but it may help. sounds more like a lack of fuel, not extra fuel. when was the last time you cleaned the fuel system?
dieseling is igniting fuel without spark by using engine compression. To see if the plugs are firing correctly, pull one off, start it and watch to see if it sparks. remember,1. don't hold it with bare wet hands and 2. the plug needs to be grounded to spark. if all the plugs works fine it is a fuel issue. it doesn't seem like timing to me being as computers either work, or don't work. don't let it run too long w/out the plug in though, just enough to get a spark.
hope it helps
-reg