Read most of the forums here on PB after a few searches on how to reference the P/V in a blown application. Found diagrams, some people cut new channels in the base plate, others do it via the metering block etc.... All depends what type of carb you have i guess...
I went a different way which i just wanted to share.
This carb is a 4150 Mighty Demon 850cfm.
First i used a pipe cutter and cut a small piece of 3/16 steel line and flared the end in anticipation of a external vacuum line.
I decided to drill directly from the outside of the main block, directly into the P/V vacuum chamber. I marked out my tradjectory and my planned exit point onto the P/V chamber for the dill bit!
I decided to drill through the center of the fitting that hangs off the side of the body of the carb. Not sure what they are used for, i assume they are a generic casting and they are tapped for screwing on attachments of somesort?? Anyway, i thought it was the perfect place for my vacuum point.
I used a 9/64 drill bit for the hole. I emerged into the P/V chamber as planned! Bit scary, didnt know what to expect while i was drilling, just did it slowly, took my time. Had to drill on a slight angle, and tilt to get into the chamber from the fitting on the outside.
After i had drilled through, i used a larger drill bit for the 1/2" or so to allow fitament of my steel line.
I then used a 2 part epoxy to seal the steel line into place.
I will block the existing vacuum pickups in the bottom of the P/V chamber from the underside of the body with either some lead shot or tap the hole and install a small screw in brass plug.
I then decided to give the carb some protection with a fuel / temp resistant metallic paint, the vacuum fitting looks almost factory due to the location i installed it. (Having second thoughts about the colour though.... Looks too blingy... Might strip it off yet...) Now i can run an external vacuum line directly down underneath the blower to the manifold and the P/V will be boost referenced. I will cap the vacuum port seen coming from the base plate. (Bowls are wrong way round for the photo... was only a mock up. Putting a kit through the carb before re-assembly)
I know many people say running a P/V on a blown deal in a boat is a waste of time, but my Bahner will not be a race boat, just a tough blown lake cruiser. Atleast with my modification i can run it with a P/V and see how it goes. I can always remove the P/V and block it, and just cap my new port if i change my mind. Im only running a P/V in the primaries, secondary P/V location is blocked and will run a larger jet size. Now i have to do the other carb!