Originally Posted by Flood
Big singles have the problem that with huge intake diameters, you will either have weak low-down response or not enough top-end power. A single carb might produce good power at full throttle, when gas velocity is high, but may stall at low rpms when you whack the throttle open. To overcome this, Honda designed the RFVC engine with radial valves, a centered spark plug and two carbs, opening one after another. The same concept was used on the early XRs and on Yamaha's XT600 (with a joined floatbowl, IIRC). Gilera's big single (Nordwest/Northcape) had the same system.
In short, it didn't really work. Comparing the RFVC-XL600 with the earlier XL500, the older had far more low-end power and there wasn't a lot of difference at full throttle.
Another solution is the use of pumper carbs, who squirt a small amount of fuel directly into the intake to avoid a lean mixture when the throttle is suddenly pulled open (basically, the inertia of fuel is higher than that of air, so opening the throttle fast will result in the fuel coming a bit "late"). Hope this is somehow understandable.
it didnt really work? wrong.. all the dual carbed bikes when tuned correctly will nearly run circles arround similar displacement single carb bikes. thats been my experience with them and Ive had several. if you try to match a huge carb to a certain displacement to get max power then you have a machine that wont idle and not smooth at low to mid revs, with the dual arrangement you can have cake and eat it too! what would otherwise be way overkill is now tamed and ridability doesnt suffer. Ive got a 250R thats a perfect example, lots of mods including 350 carbs. with stock gearing the 100mph mark is not out of reach, havnt been able to get there on any other 250s Ive had. and the induction roar the dual-carbed thumpers make is awsome too.