well, they are very lightly stressed engines. Producing well under 50hp/liter I doubt the piston crowns would get hot enough to burn the coke off.The engine is just a simple supercharged 2 cycle so you'd think out of all engines that it'd do well. Nope.
Pattern is staying steady on my car, drive gently and it cokes up in about 200 miles. Beat the piss outta it and it cleans right up (with some prodigious discharge of smoke, bet the chunks are not good for my turbines)