Tillers

I have a Model 917.299391 Husqvarna front tine tiller purchased from Sears with their 3 year warranty. 3 different times now since purchased in 2011 I took it back for repair to fix it so it would start. All 3 times they have replaced the carburetor. Why does the carburetor go bad? I use only 100% gas and keep it fresh as I use it in my tractor and other outdoor equipment.
Is there something I cando to prevent the carb from going bad?

Thanks - Bob