That fork definitely won't be suited to your weight, you will need a firmer spring if you can get one.
With some low end Suntour forks you can't get firmer springs.
Contact Suntour North America and see if you can get a new spring that'll suit your weight:
http://www.srsuntourna.com/consumer/contact/
I read a post on another forum where the 250 lb owner of a new Trek X-Caliber 4 was only getting 24mm of travel from his Suntour fork with 100mm of travel, he had the preload set at hard as it could go.
He contacted Suntour about getting a extra firm spring, they said the spring in his fork was only rated for a 180 lb rider and they didn't make any other springs for the fork he had, they basically said the fork he had wasn't suited for his weight and he'd have to upgrade to a new fork that could handle his weight.