Seen this happen many times. I have a friend that is a in house tech for a company and he had a fleet of lifts with the Mitsu engines. Seemed his woes came on about the 2000 hr mark. They also smoked on idle. He used one of the units to try and trouble shoot why they were smoking. He had the head reworked, new fuel system and ignition system. He even tried that restore for engine stuff. Never got worse and never got better. Some visibly smoked and some smelled as though they were burning oil It is possible that the cylinders are egg shaping and ring grooves are aligning as they are getting wear on them but thats just a thought. BTW my friend never found the cause.