Pilot flame either too low or too far away from main burner gas jets. Result is gas from main burner is not igniting as soon as it leaves the burner- instead it only ignites after sufficient gas has filled the burner chamber to propagate a flame from the pilot to the main burner gas. The result as you have found out is a mini explosion. In severe cases this will snuff out the pilot and you have to go through the the 'start up procedure, which is not helpful when you come home to a cold house and a 'locked out' boiler.
Possible faults:
a) Pilot regulator turned too low or dirt in needle valve strangling the flow of gas to pilot jet.
b)Main burner gas regulator not allowing sufficient gas to main burner.
c)Pilot assembly could be out of alignment.
note:- For a smooth ignition transmission the pilot flame and main burner flame should be touching.
Geoff in Wales, October 2012
