Oil being blown out the rocker cover breather generally means that there's blow-by (ie loss of compression due to piston rings, causing the crankcase to become pressurised, which gets forced through the PCV valve into the rocker cover). Normally, that breather gets re-routed back into the intake so the blowby gets burnt again in the combustion process, but since you've got aftermarket carbies in there they probably don't have a fitting for it. So, main options would be to get rid of the filter on the breather, and install a full catch-can setup instead. You'll still have blowby, but the catch-can should be baffled to stop oil being sprayed out the filter on the catch-can (these are illegal by the way in most if not all states, but doesn't stop a lot of people). If you want to be really anal, you'll need to get a fitting of some sort put in the intake to plumb either the breather line back in, or the output line from the catch-can. Oh, and might as well test for compression too. Only takes a few minutes once you know how to do it, and definitely worthwhile in determining the health of your engine