Runs Great With Breather Tube Disconnected

I have a Snapper Lawn Mower with a 11.5 B&S engine. It is a 92 model that I bought about 5 years ago. At the time the mower looked and ran like new and I got it for next to nothing. What I didn't notice until now is that the breather tube is not connected to the carb. The tube does not leak oil or smoke and the engine runs like a bear. I tried reattaching the tube and the engine ran really bad. I have the B&S shop manual for the engine so I checked and adjusted the carb to the correct settings. With the breather tube attached the engine still would not run correctly. I detached the tube and the engine ran perfectly. What do you think is going on here? Apparently the engine ran like this for the 5 years I have owned so it, so it must not hurt it to run it this way. I place a filter screen over the open tube line to the carb and one over the breather tube to keep out dirt and debris. Let me know what you think the problem could be. I was guessing that the carb may be starved for air and with the tube disconnected it gets what it needs. There is a new air filter on this machine, so it is not restricted because of that.