i've yet to see a twin weber manifold with provision for a pcv valve,maybe some factory setups,but nothing aftermarket.the pcv lets fumes from the crankcase enter the inlet manifold,so blocking the pcv isn't going to raise the pressure in the crankcase unless you block
off the pipe coming out of the block.this pipe is usually vented to a catch tank,along with the breather off the rocker cover.BUT,i always thought the pcv valve was required for road registration.so,buy a new one or get a good one from a wreckers & replace the one you've got in the car.i once traced the dreaded rattling coming from the engine in one of my 1200's to a dud pcv valve,it quickly got an aerial trip to the bin.