Weil-McLain boiler. The thermostat has no control over the boiler. It gets up to temp setting and will not shut off. Old style Honeywell mercury thermostat. Any suggestions?

When you remove the grommet cover ring you should be able to see the mercury bubble, right. When there is a call for heat the bubble is tilted to one direction and the mercury closes a circuit, when it reaches temp, that bubble should be tilted in the opposite direction. Do you see that it has tilted to the opposite?