[embee]

Most likely reason is running too lean, very often due to air leaks. Do what the others advise, plus check the front intake vac stub pipe has a cap on it. It's buried down below the front carb, a bit of a pain to get to but you should be able to see it reasonably easily.

Someone in teh past may have fitted a piece of hose to it in order to make carb balancing a lot easier, a common thing to do, and if this is the case then check that the hose is capped off.

Check carbs are seated in the intake rubbers properly, that they are not perished/cracked (bearing in mind curveys are getting on a bit now), and the clips are reasonably tight.