The weber book has what you're looking -- how to choose the correct carburetor size.
Here's the chart from the book. I recommend you buy the book for the explanations.
Carburetor SizingQuote:
For a single carburetor barrel (bore) per cylinder, rather than multiple carbs or bore feeding all cylinder via a plenum, use this chart:
* 6000-RPM A12: dual 40s are a bit large, but in reality seem to work OK if you fit the smallest venturis.
* 8000 RPM A12, calling for 36mm venturis per the chart, you could use either 40s or 45s.
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