If you have the AS Owners Manual then refer to Section I for complete furnace operating instructions (page I-24 for 1987 345 models). The figures show that the cover plate must be removed to gain access to the gas valve behind it. For start up, set the thermostat and gas valve to "OFF" and wait 5 minutes. Turn gas valve to "ON", turn thermostat to "ON" and set to desired temperature. Allow 15 to 30 seconds for the burner to ignite. If it fails to lite then set the thermostat to "OFF", wait 5 minutes and then set back to "ON". After 3 failed attempts shut down the whole system and troubleshoot! I can provide photocopies of the pages in handy PDF format, and copies of the Duo-Therm 901 Series Owner's Manual as well... PM me if you need them.
"Underway is the only way!"