One thing to do is to check the jets after pushing the choke off to see if they are all the way up. If not, sometimes it is because the return springs are not strong enough, either because they are old or just too weak. I have found that springs from the different major suppliers all have markedly different tensions.
Also, sometimes you need to work the jets up and down a bit if the carbs have been rebuilt, just in case the cork gaskets are too stiff. Also, if the corks were not left in an oil bath for about 24 hours before installing them, they could be too stiff and aare keeping the jet from returning fully up.
When your engine was rebuilt, were the carbs rebuilt at the same time?
Quite often when these carbs are rebuilt, the person working on them fails to remove the old cork seal at the very top of the jet assembly (#92 in the illustration on p. 24 of the Moss Catalog). They cram another seal up there thinking that it is clear, and the added thickness keeps the jet from fully rising to the top. This is probably not your problem, but it is worth considering if the problem is with the jet not returning fully.
Just a thought...