I had 2 of these Mazda motors, with high hours, have the problem with lots of carbon on the valves. They were hard to start and were miss firing. The carbon was this thick, black, sticky, sooty stuff built up under the heads of the valve. Pieces were starting to break off and getting stuck under where the valve seats. I've never seen a carbon problem that bad. They were LP powered and didn't show any signs of smoking. I'm not sure what caused it, motor oil or bad LPG. Anyway, cleaning and a valve job made everything right again.