congratulations on your find and welcome back. I have a 74 144 with the same setup as yours (B20A) single carb. Yes, we still had carburettors all the way up to the mid 80's here in Malaysia. I have been really tempted to make the switch to dual carbs, but comparing to other B20B Volvos in my area, mine has the smoothest and quietest idle. In fact, after a rebuild and a new cam two years back, I almost ruined the starter by trying to crank the motor while it was running! (it's that quiet, especially after some sound insulation I put on the firewall).
As for fuel economy, it is probably slightly better than the dual carb setup, but probably less power as well. On my 144, I gained a couple of miles to the gallon after the rebuilt, an addition of the M41, and electronic ignition from a 75 244.
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...and the bricks keep on rolling