sounds like "either"--> the needle-&-seats (float-levels) differ from one carb to the other

Or

there is a slight air leak in the carb that`s not rich

or both of the above

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.