hobbsy,
Maybe the best thing we can do here on the beginners forum is tell them when not to step into it.

I can imagine the frustration a person new to the sport would experience trying all the various plug and fuel combinations on this engine. What a waste of time, money, and a major bruising to the ego when it still won't run well.

Incidently, a troublesome MDS 40 needs a smaller prop not a larger. They just flat out won't run on a larger one. Been there, done that.

Yes, all your MDSs run fine but there is a batch of stinkers out there which you have not had the misfortune to own. I think it would be great if someone sent you one to play with.