Flabum is right; it's not made for idling...
(kidding... it should idle) That's a race engine though, really intended to zoom around at full throttle. When you can't get them to idle any more, it's possible that the bearings are shot, or the whole thing's just worn out. Better send it to me for proper Picco disposal..
Sadly, parts for that engine are no longer readily available. You can still get bearings, but piston & sleeve will likely have to be sourced from an older Picco R/C car engine, and possibly modified to the marine spec.
They really are/were a nice .21 engine.