I had a similar problem with one mixture screw - adjustment having no affect on running. I resorted to setting it to recommended position followed by only a little tweaking.
Re. starting problem: There is an o-ring behind the choke rotary valve too. Take care that the choke paper gasket is good; Check rotary valve is fitted the correct way round. Also that left and right are not interchangeable (You probably know that).
Maybe an air leak? Inspect inlet rubbers for cracks/perishing.
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