This deposit of carbon caused a big knock that we figured was a rod bearing or broken piston skirt. Took the engine out and apart to find the reason for the knock. It ran for less then two minutes total trouble shooting the sound before I was afraid to start it anymore. It had normal oil pressure, Pulling the plug wires one at a time made no difference. Tried in gear to see if it was a converter - flex plate problem. The bearings are undamaged. Oil comsumption was around 500 miles per quart.We had run 800 miles home the day before. This happened in the driveway when it was started to go wash it and put it away. The oil was Quaker State 30 wt.