Blowing sparks out the stack

We did it again. Carboned up an exhaust valve, it won't close all the way, and we are blowing sparks out the stack. Did this last year, and paid $300 for a valve job. This year I'm cleaning the valves myself. This comes from running straight V100 without a (bio)diesel startup/shutdown. I ran out of diesel, had no biodiesel made up, and was running straight veggie full time without switching. We tell people not to do this, Oops......