After installing new thermocoupler assembly,and new gas control valve, heater still won't stay lit. After liting pilot and turning to on then adjusting tthermostat to desired temp unit fires up and looks good ,then when boiler turns off so does the pilot light. Why ?

Sounds like you may have a combustion air problem. Plugged air inlet screen would be a good place to start. Try contacting State technical support and see what they can do for you.