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Old 08-06-2007, 02:49 AM   #39
Flood's Avatar
Joined: Oct 2005
Location: Austria
Oddometer: 10,161
Originally Posted by kktos
friend of mine bought a XLRM (noticed the M ;) ) for less than 300euro.
he's rebuilding it coz for this price it isnt shiny.....
I saw the double carb yesterday eve and I was wondering the purpose of it.
As we dont do it anymore.
any idea ?
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.

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