Thats highly unusual for the carbon to come off in large quantities that easily. I have have a small amount come off doing a rear main on a 308 engine with heads on but i only noticed by a tiny amount of carbon made its way to a port where i could see it. It never actually effected the engine turning over. Is there definately no way and cleaner product or anything of the like could have made its way into the engine during what you have been doing?