Hi Mastero,
the spring should provide all the tension the belt needs. Is the short shocker type thing maybe siezed?. You can release the tension to check it by removing the bolt that passes through the tensioner assy & a lug on the cyl.head, gently releasing it using a bar into the hole in the top of the tensioner. Are you sure you have the correct length of belt fitted?. The tensioner pivot can wear, allowing the roller to sit slightly skewed.
Regards Peter