the original weber instructions call for the original vapour separator to be left in place, so I would do that and see how you go. some find the carb starts to suffer from fuel starvation which is usually because the return restrictor has gone missing. you can push a welding tip up the hose to replicate (this would probably also work with the Y fitting if need be if you delete the separator) was the rubber base flange replaced at the same time? those can split with age. plus if the front steady bracket or rubber bobbin mount to the airbox is missing the unit will flap about and eventually tear the flange
