If you look close at yours somwhere on it it shoudl say at what temp it opens (its stamped into it) Better you test it tho first, Boil some water and sling it in. It shoudl open up a good 10mm if all is well. If not then like Jim said, tiem for a new one.

If it is ok different ones can be got, most common ones open at 80deg but many 2 stokes will open fully at 60 deg, You'll just have to find one with a lower opening temp than yours.