Hi out there
I was going to try a smaller main jet and spoke to the folks at
www.nrp-carbs.co.uk who had supplied a new diaphram for my Kawasaki Z400. They advised that it was more likely to be a worn needle and needle jet. They suggested I look carefully for signs of discolouration or wear on the needle, particularly at its rest position with the slide at its lowest point.
On examination, with a magnifying glass, wear was evident.
To test the theory I lowered the needle with an additional spacer. This eliminated the smoking exhaust and just about stopped the missfire.
I have ordered a new jet and needle (3 weeks from Japan). Will keep you posted on outcome. Note NRP also supply parts to repair your leaking petcock.
Stu