I got a old one guys...87 ranger 2wd and its been in my family since 87. All four kids learn to drive on it and its poor clutch. We are 18-28 now. So i decide to rebuild the faithful old truck. I do notice a lug from the motor on any gas other than high octane (93-97) Family says its always been like this. What causes this. NOTE: I am a mechanic for the army now. I figure heads or valves......but though i would do a poll. What do you guys suggest?