Engine skipping in idle

[rangestang89]

i have an 89 4cl 5sp ranger it has a skip at idle and in the low rev range then at about 3500 to 4grand it makes a pop sound out of the intake i already changed plugs wires egr and pcv and no change i think timing or burnt valve not sure how to check timing on it i have a light thow but doing a compression test on it tomorrow any help appreciated thanx