All replies are appreciated. My son's jeep blew a head gasket and he replaced it. He has put the engine back together but it back fires out of the throttle body when he tries to accelerate. It acts like the timing is a little off but there is no adjustment there and it looks like its in time when we put the piston on TDC. All of the vaccum line seem to be in the right places. can I get suggestions on what he might have messed up please?

 

Spark plug gap ? I know my Jeep did that when the gap was wrong in one of the plugs lol.. Im not a expert so thats all i could think of. I know my Jeep also had missfire issues and a shaky engine when the #3 valve spring broke so maybe you could have a broken valve spring ?

 

The base timing sounds like it is off, or there is a firing order issue. The base timing has to be set even though the PCM adjusts the timing.