RE: 3W 50i problem.
A 3W should never be used as a sinker. they are first class engines as you know from your other one!
I have a 55 US Competition version which has a side carb. On mine the carb diaphram vent is connected to the velocity stack of the carb. Worked so well that I vented my other non 3W engines the same way. There seems to be no difference with or without the cowl.