i did think of that but wasnt sure it would work without using the vacuum port on it, i thought about plumbing it in to the piston side of a bov with a spring rated to the correct pressure, then when it exceeded that pressure it would exhaust through the normal inlet side in a sort of back to front manner, when you say it will lean the mixture, how will it make the mixture to lean?
sorry by fuelling i meant tuning, high idle speed etc due to this bypass valve stuff
just researched r5 carb and it looks like i can use the dizzy vac lines to open the bov happy days!