weak yellow flame-should be sharp and blue=blockage in pilot assembly or;
faulty thermocouple(metal pin the pilot is supposed to heat)requires replacing or;
fault on main gas valve-the other end of thermocouple connects here or;
worst case you have a o.d.d sensor(oxygen depletion device) indicating a possible problem with the flue or main burner!
Dont play with gas... call an engineer
seriously!, November 2008