Ardo,
You will need to order the Wolff new style hammer spring. They are the short springs for Sig pistols with the plastic mainspring seats. 21 lbs is the factory standard. A 18 lb spring should will work consistantly in your gun with no light primer strikes. You can order one from Top Gun Suppy. The link to the spring is below.
Wolff Spring
Jeff
Customer Service
Top Gun Supply