Previously owned a 9N and an 8N now a new to me, Jubilee. Converting to 12volt and have started the work including new radiator. While it was uncowled I noticed the gas cap was very loose so bought a new one. Battery is disconnected, has not moved in a week or so. There was a strong smell of gas after a day with the new cap. Opened the cap and there was obviously a reasonable amount of pressure that had built up. Noticed the exterior of the carburetor was damp. My question is: whether the carb needs attention or not, where did the pressure or vacuum come from without the tractor being started or moved?