Originally Posted by Zanshin
I still haven't changed the fuel (I haven't been using the dinghy very much recently), but if it were any of the jets, why does it only happen when I start a cold engine, once it warms up I see no issues at all - good idle, full power, clean and even burn.
But I think I'm getting very close to having to refill the tank with a correcltly measured mixture of oil and fuel, and can then see if the issue resolves itself.
Missed the "cold" part, however I think that a cold engine will not tolerate a lean condition as well as a warm one. Maybe the carb to manifold joint is a bit loose and as the engine warms up the heated metals expand enough to tighten the seal? That is probably a stretch, however. My guess is that there is a lean condition in the high speed system, possibly caused by a small blockage.
You might try running a 50% mix of gas & carb cleaner through it, let it set for a day or two & then give it a good long, hard run with regular mix, fresh fuel.
2 strokes always love new plugs, try the easy things first. Don't ask me how I know that.