Ford Repair: Oil fouling plugs., valve seals, power stroke

Question
I have a 91 f 150 with a 302. I am getting slight amounts of oil in no# 3 and 4 plugs. The truck runs rough, no pick up, seems to idle fine except when under load i get a popping as if it were carbeurated and backfiring....it will pop and studder. I was told valve seals. Are these hard to change and when i do get in there is there anything i should be aware of for wear and tear on the valves besides the obvious bent or broken, pieces or anything. Have you ever heard of the popping? Any ideas or thoughts would be great....Thanks for your time...

Answer
Hi Brian thanks for asking. This sounds like you have a mechanical problem. Valve seats are not replacable, the heads will have to be removed and sent to a machine shop to have the valves and seats ground down as well as having the head mcahined. There is a poping noise because the valves arnt completly  closed when that cylinder gets to the power stroke.
good luck
Chris P