Probably need a bit more info then, like what carb(s) manifold etc. Might be worth checking that you're getting vacuum to the rocker and if not if there's a blockage or fault somewhere. I fresh engine with a working PCV system should suck air at the breather. I rebuilt a 253 years ago and it would squeal (sucking air) through the oil filler cap until I put the right breather setup on it. Would suck your hand on when you put it over the hole.