Just overhauled 16 hp Briggs ans Stratton engine ?

I just over hauled the 16 hp v- twin engine on my 10,000 watt generator. New rings valve seals shaft seals, carb kit and gaskets. Now when I start in it fires right up but blows blue smoke for 2 min. Before it clears up. Only does this when cold. Start it while it's hot and no smoke. Put 8 hours on it and used 1 1/2 guarts of oil. Any suggestions?