351c backfiring problem

well i put new sparkys and verry expensive ones i might add and the same thing is hapening is there any way to tell if tweaked a valve without takeing the head off its weird because i feel like i still have compresion if there is a way without taking the head off can somone tell me how

hi u doin guy well i ended up just opening the whole engine and tearing it down and yall were right i threw the cam and it was pretty nasty i thank all of yall for ur replies now i got to buy a new cam and i got decision to make wat would u all choose for my set up