Rick, I would only strip the engine down as a absolute last resort, this is a much sought after steam plant and not to be messed with unless you are fully knowledgeable about its operation , never had a max 4 but i have several max 2 engines as with most steam engines left overtime as Derek said the problem most certain is gumming up with dried steam oil ,my way and others probably might not agree is to simply pop the engine in a dish of some sort with some turps/white spirit brush allover with a soft small paint brush on top of the cylinder heads as well ,notice the small holes in the cylinder heads they must be clear they a mitt a small amount of wet steam when running also put some in the steam entry and turn over by hand several times you should feel a difference when you have done that dry off and put some very light oil in the steam entry and all moving parts remember if running on air you must periodically put some light oil in the air line as i say this is what i have done withe most engines i have got and never experienced any problems after ,as i say a great steam plant so much so one went on ebay only this year for approximately £2500.00 if my suggestion dose not help i recommend seeking professional advice, best wishes Allan