Harold Kestenholz – Hydronic Network

You seem to be describing a White Rogers mercury cycle piot ignition with the sensor gone bad. This sensor gets hot enough from the pilot to turn red, vaporizing mercuty inside the stainless steel sensor head to press a switch that is part of or plugs into the gas valve to complete a path to open the gas valve. They need replacement after many years. The spark seems weak or out of position to light the pilot also.