Now I am no expert but I have always been told to run ashless oil? 3W specifies ashless oil in their instruction manuals.

I did buy Frank Bowman rings for my DLE-30's that I have not installed yet. You may want to purchase a new ring for the engine and try using a different oil.

I am running a DLE-30, YDA-56 and a 3W-157 all on ashless oil and have had no issue with carbon build up.

Glad you found the problem. BTW: do not trust that the timing is right doe to being in the center of the adjustment. I had to grind the slots longer in mine to get enough travel to correct the timing,
Later!!
Anthony