I had the same issue on a new type 29 on my Vincent rapide. I lapped the needle, replaced gaskets, checked the jet block seating but it still leaked. Finally, I took the big base gasket out from the big nut that secures the jet block out from my old carb and put it in the new one…no leaks using the 70 year old jet block gasket. Cheers.
My original 1940 carb leaked badly from the same place, so I got a new carb and the new one did exactly the same!
Like mentioned here, it’s all about setting the carb up which I’m learning about.
It seems to be better now but not perfect.
