you will see that when you light piliot and main burner comes on the main flame goes all floppy and goes out this is due to main heat exchanger external fines being blocked up not allowing burnt fumes to pass through if the is case heat exchanger requires removing and power jetting clean
or
if pilot jet to small causingforce of main burner to pull flame from thermocouple remady clean pilot jet not with pin but with compressed air can or remove and blow water through
tony
April 2008