Modern Vespa

My Vespa has just started missing second gear. I pull away and when I change to 2nd it goes straight into 3rd. It will change down from 3rd to second with no issue. It either drops into 2nd/ neutral or straight into 3rd. Any suggestions would be appreciated.

what type of bike, what engine, etc?

service history?

any recent "events"?

Your Sprint Veloce "has just started" doing it...? Was the bike shifting nicely for a long time prior to this?

First step is adjusting the shift cables down at the shift box if it hasn't been done lately. Are you familiar with how to do this? If that's not it, might be your cruciform and/or even some gear shimming needed.

It's a 150 sprint veloce and I put new cables on it around 600 miles ago. It's been running fine up to recently. I have noticed that it tends to move from neutral to 1st without pulling the clutch lever when I'm moving the bike without the engine even running.

I will try adjusting the cables, any advise on doing this correctly.

Sounds like you may have them too tight.

Or maybe one too tight? It surprises me that this started after 600 miles, when I would guess the cables have stretched a little bit.

Any suggestions on how to set them at the best tension

Forum search for that one -- there are various strategies involving vise grips and bungee cords.

Unfortunately, it's not the cables, it's the cruciform. The problem will continue to get worse until you replace the cruciform.

While you're in there, check the gear stack clearance and see if you need a thicker spacer, as well as make sure you haven't damaged the lands on the gears, which will happen and require replacing gear cogs if you ignore it long enough.

You're dealing with a single-piece fly side bearing, so I wouldn't recommend trying to split the cases with the motor in the bike.