<VV> Re: Jet Question

[grant young]

<Should I start with 47 or 50? >
(As a rule of thumb, I generally use .052" main jets in carbs destined for 164 ci engines and .051" for the small blocks if the buyer does not know or specify. I would never use anything smaller than .049" in a primary.  Any or our engines can certainly do okay with .053" jets, but usually at the cost of 1-2 mpg. I have found some .054" mains in 1960's and .048" in later ones purported to be original. I have also found the best combination for a 140 to be a stock .051/.049 set up. This assumes, of course, that the primary carbs are correct with the largest available idle emulsion tubes and enrichment "jets". Late models have the enrichment jets which vary from about .025" to .033" (or .000" when they stick or plug), so it is important to keep them matched and cleaned. 
The Carbmeister