I have a 1989 Dodge D250 with 140,000 miles, 5.9 L 360 CID. It has an emission system on it that looks like a California truck. About 6 weeks ago it started running very badly, it had a stumble/hesitation like a backfire with no noise. It caused the truck to jerk and lurch very hard. I replaced the fuel filter which was very badly clogged. The timing mark would fluctuate in about a 10 degree space. I replaced the distributor ( electric, not vacuum ) and the timing mark was steady at 10 degrees BTDC. This helped but did not eliminate the problem. I then replaced the spark plugs & the wires which helped a little more but still did not stop the problem completely. I then took the truck for inspection and emission for licensing where it failed the HC(ppm) readings. The standard for both idle and high speed is 220, The truck idle reading was 274 and the high speed was 492. I took the truck home and replaced the oxygen sensor because it was running so rich. I then drove the truck to see how it was and I could still notice a small jerk/lurch and the idle is not as smooth as it used to be. I have 9 more days to repeat the emission test but would like to make sure it will pass. Is this as simple as replacing vacuum lines or is something wrong that I have not completely eliminated ?