Cold start idle issue

[Hoopla77]

Hi all, i have an issue with cold starting on my zs180. Every time i start it the engine starts first time but as the revs go up then quickly back down it dies out stright away.

If i hold the throttle down for a brief second upon starting and let it out gently it idles fine. However it has to really watm up before it doesnt die out when the revs drop quickly, such as reversing out of the drive and coming to a stop/coming to the end of my road.

It does run on lpg but that shouldnt affect it as it starts on petrol until warm.

Any thoughts appriciated!