Check the float level, if its high, itll cause rich running.
If you go to a hotter plug, youll just be dealing with the symtoms, and not the problem. Theres got to be a reason, its running rich, and youd be better off fixing that. Keep looking, youll figure it out.
How was the mixture screw, setting? Should be somewhere around 1 1/2 turns out.
The valves could be out of adjustment...that would cause hard starting, but not fouled plugs.
Let us know what you find out.