Cold starting is very poor and I am running out of ideas

Evening all 

A bit of entertainment after the bonfire toffee.  

my 250 delux and my 400NB both run very well and start instantly when warm.  Both are low mileage original standard  examples.

the cold starting especially after a couple of days or more is not good to say the least. It takes ages  on the starter to get a glimmer of life. 

I have been through the carbs a few times and have ensured the choke operates  but I am now struggling for the nest steps.

when I eventually get them to fire it’s usually left cylinder first and then the right eventually decides joins in.  Once going they react to choke as expected and are fine to ride away.

they have been like it all summer and now I have had enough and need to fix the issue over winter.

the mixtures are set to the manual I.e. 1 1/2 and 2 turns respectively.

incidentally my 1982 cb750fd seems exactly the same.

I am currently looking at the synchronisation between the choke and fast idle cam would anyone know what the gap should be ?

any other thoughts are welcome.

thanks in advance 

JR