My big red 250es has bad piston rings so it smokes but more recently it started to sputter a lot and have a real hard time idling, It may be water in the gas i am not sure because it just started happening as it gets warmed up or im riding fast for a while then as i start to slow down it would go away, Now its constant it always wants to die and sputter even after a cold start. Could it be the piston rings?

 

Change the plug, it will be having a hard time in an engine burning oil, this may cure the symptoms for a while but they will return. I guess compression may have got so low that the machine won't run right but I think bad starting and poor running at low revs would be more likely. If a plug change doesn't work, why not try cleaning the carb out? If there is water present, more may be in the tank, but most likely some grit will have got into the jets.