Seaward Oven Thermostate Adjustment/Repair

[zstine]

My oven temp is way too low. It's a Seaward model 31323 that's probably 20+ years old. The oven burner doesn't turn on until set at 400F and reaches only 140F at that setting. At a setting of 550F, the oven only gets to about 250F. I looked at the thermostat valve, but didn't see an obvious adjustment. Is there an adjustment / calibration? If not, what would be the suspect part needing repair or replacement?
Thanks!