GM-GMC: backfiring, mass air flow, open valve

Question
QUESTION: when my 1998 gmc k15oo with a 305 vortec gets warmed up . it starts to back firing . when i put it in gear it runs real rough like it trying to die but it doesn't . when cold it runs great . could it be the mass air flow or maybe even the cat clogged up . just replaced the plugs,rotor,disturber cap %26 wires not even a year ago .

ANSWER: if i was mine i would definatly look at the cat because maybe its tryin to clear its pipes. when it gets fully clogged the vehicle wont run anymore. theres a build up in the engine somewhere and i would think its probably running lean.

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QUESTION: just got done putting new spark plugs in my truck and changed the oil and it is still back firing . did notice that zylinder number 2 is not burning the fuel like it should do . plug is wet with gas . thinking that may have a sticking valve or even a open valve . was wondering what u may think it is

Answer
well you are gettin air in there somehow its running lean thats usually why you find wet plugs.

so yea it could be a sticky valve sticking open or always a lil open usually caused by the build up you need to blow some of the carbon out what i suggested to someone else was what i did to an old car

i took the plugs out for a while and sanded and dried them and put them back then i did that one day and took it to an open road and floored it blew all the carbon out dried the plugs right up