What does the plug look like?
This could be carbs way out of synch, it could be a clogged idle jet, could be stuck float, could be points, could be coil voltage, could be a bad ground in the ignition circuit...
Is this repeatable? Are there any circumstances that this doesn't happen?
Accurate diagnosis via internet is virtually impossible, as we can never have enough contextual information.
(doesn't mean we aren't willing to try, however)
While each system is fairly simple, they ways they interact and affect each other can be complex.
In general, the best approach I've found is to patiently go through each system and assure that it is as correct as you can make it.
I suggest Electrical first, as it is fairly simple to meter out. (although bad grounds and failing coils can lead to a lot of hair loss)
Compression test to rule out valve/ring issues
Go through the carbs and get them so clean you could eat off of any surface you choose.
Check ignition timing carefully, probably replace condenser(s) as a safety.
Any order is fine, as in the end it will likely be the last place you check...
