chain drop when starting off in big ring

[colinjohnchapman]

i have just taken my summer rim brake bike of zwift trainer and put the rear wheel back on.
However i now have weird problem. the chain moves up from small front ring to big ring and back when on a bike stand.
But if i try and start riding in the big front ring the chain drops immediately.
But i can start in small front ring ok and i can change up to big ring when riding ok . Sora groupset which has been fine until now.
Have tried a different rear wheel (with same size cassette 11-28) made no difference. checked chain wear and it is not an issue.
any ideas?

 

[Ajax Bay]

Does the chain drop off to the outside (from the large chainring (obv))?
Does this happen irrespective of which rear sprocket you're in?
With the chain on the inner ring static, bike static and shift FD to outer , how far above the large chainring top tooth point is the corresponding FD cage outer plate? (max 2mm)

 

[oxoman]

Old bike mechanic I used to use, used to set the gap up using a 2p coin.