I had the same problem as you. First the rollers in the clutch basket were the wrong size, original spring was not straight. Bought a NOS one and had the same problem...

Then I bought this:

https://www.thb-support.nl/product/bsa-m20-koppeling-centrale-veer-vervangings-set-15-4311-15/

Works perfect! I can pull the clutchlever with one finger.

Or get a complete kit with modern kevlar plates:

https://www.thb-support.nl/product/bsa-m20-complete-kevlar-koppelingsset/