It's a pre-mix thing.

The pump delivers something silly like 100:1 at idle, so it's no surprise that if it's coming in at 30:1 ish then it'll probably smoke.

If I'm sitting at the lights idling it'll start to smoke, but once I'm away and have hooked a couple of cogs it'll clear up.

Take a look at the GP bikes. During the race there's not a wisp of smoke as they're so finely set up, but on the slow down lap quite a few will be puffing smoke.