On a Craftsman rider I have a B&S 331877-0805-E1 Code 070323ZD. The Breather/ Exhaust tube #405450 blew off. When trying to put it back on I felt and saw a black disk just small enough to fit inside the rubber tube. I put the tube back on the elbow and it blew off again. This time the small black disk was no where to be found. After remounting the exhaust tube I clamped it to the elbow. When I nstarted mowing it really started blowing smoke. I saw that the elbow connector tube had a small hole in the center of a blank inside the elbow. All I could figure was that that disk that is now missing was a check valve or at least something to trstrict the amount of oil that could be pulled into the carb. Anyone know what the story is?