+1 on polish the jets. All John's other suggestions are good, too, but dull jets seems to be the most common problem. Best I can make out, the originals were supplied slick, but modern replacements are supplied with a more matte surface that really drags against the glands. Any fine abrasive will do to polish them, I've even used white toothpaste. Obviously clean away all traces of whatever you use before reinstalling.
Note that the linkage goes together in a non-obvious way. The two clevises on the link between the carbs don't straddle the arms, but instead hang out on one side. Seems crazy, but does work better that way.
The joints all have to have a small amount of slop in them. If by chance you (or the DPO) has substituted bolts for any of the pins, they have to be left loose.
Choke cable needs to be the type with a stranded inner core.
If all else fails, disassemble the jet housings and check that the cup washers are turned the right way (the cupped side goes towards the seal). I would also replace the springs if I didn't know their history, since it's not uncommon to find that someone has stretched them trying to get the glands to seal.
Same goes for the return springs; the next step after stretching the gland springs is to cut the return springs shorter to get the jets to return reliably