Hi (btw, excuses in advance for my spanglish).

B18A and B20A engines uses one carb. Cars with this engines was not sold on the US market. The one Zenith Stromberg 175CD2S on B18A produce only 85 HP, and two SU carbs on B18B produce 115 Hp (a lot more). The manifolds on B18A and B18B is different.

Earlier B18A (the B18A-1) uses a Zenith V36 old carb, low hp. Later B18A-2 uses the Zenith Stromberg carb. Both models are underpower (the 2-carb B18B engine was nicknamed "the american engine" due its superior power, and was oriented to sports and american markets).

On the tech side, I read (on Ditlew's "Essential Volvo 120 and P1800" book) the following: B18A 9.5-11.9 lt/100km (fuel consumption) and 140-146 kph (max speed), B18B 12.4-13.0 lt/100 km and 164-167 kph (of course, it was measured on new engines, our cars will be higher lt/100 km and lower kph by now).

Just reading: the B18D engine (1961-68 on US) uses 2 carbs, its power was 95-115 hp, 8.9-12.4 lt/100 km and 151-156 kph (the best of both worlds).

Hope this data helps,

Joaquin

PD: If you like, some images of my one-carb engine available on http://www.volvoadventures.com/121spec.html