KLE running rough
Please make sure you have the right heat range plug in there. The 400 use an "8" range, and the 500 the "9" range.
Then have your carbs checked. A plug running black, might not be rich, as there is no more lead in the fuel, the burning color tends to be more black than grey.
Also check the technical thread on this, as there might be some input there.
I had the same problem ,and found out it was a dud set of plugs. A new set fixed the problem. Just be careful to put the plugs in correctly, you will find my sad tale under the technical thread.
If its stupid, but it works, it aint stupid.