You could have high crankcase pressure and oil could also be gettin passed the breather filter and is then being sucked into the metering head via the breather hose that goes from rocker cover into metering unit,

Take the oil cap off when its running and see if it makes any difference..
If it runs lumpy there is a prob....

I've used to run one with that hose venting to atmosphere and would plug the hole on the metering head, that helps it idle better as plugging that hole makes it fuel more also as the flap is sucked down more.....