[Richard]

Michael has given good advice. Turn on everything, and plug in another coach or two. Seriously load it up, and run it for an hour to clear up the injectors and valves. Idling it, or running with a low load is responsible for fouling the combustion chamber.

If it doesn't clear up, then you can think about if you want to touch it. 20 hrs in 18 months isn't enough to justify much investment.