[wrench]

Part of the noise is caused when you slam the throttle shut on decel, the resultant high rpm, high vacuum condition causes the engine to suck a lot of fuel through the carbs, putting a lot of raw gas in the exhaust, a lot of oem carbs have a dashpot on them to stop the throttle from closing completely on decel, giving time (1 or 2 seconds). for the high vacuum to decrease before fully closing the throttle. dont know what you could do about it on webers, though.