My 5252 started running rough when the weather turned much colder. I took the air filter off to check it and noticed that there was a small hose that goes in the carb throat. I took off the air filter mount and saw that the hose was a crankcase vent. When I started the engine there were small droplets of water coming out of the hose. This was going straight into the carb and causing the rough engine. I repositioned the hose to the outside of the air filter. No more rough running engine. I guess this would only affect tractors that are stored in unheated garages.