You get a similar thing to that pottering around with big petrol engines in cars. If I have just been running round town for a week or two in the Discovery without seeing many revs, if you give it a bootful of throttle to say 4000rpm in the first couple of gears, you get quite a noticeable amount of smoke as the crud burns off the plugs then it is fine after that. I think they call it an "Italian service"
My Johnson goes lumpy and coughs and farts and jumps around under about 2000rpm after a very short time idling, as soon as you open it up over about 2500rpm again it clears in a few seconds, so I put it down to gunge on the plugs. It just won't run smoothly at idle/low revs for any time at all. I just sort of assumed it was a "2 stroke thing"....?