It's called incomplete combustion. When the fuel and air enter the cylinder, most of it should be burned up during the 'power' stroke in a 4 stroke engine. However, in your case, there is still unburned fuel (much more than normal) leaving the engine. This wastes fuel and pollutes the environment at the same time. One culprit might be the spark plugs. Check them and replace them if they are a couple years old and/or have black stuff on them. (Look through my answers for the procedure to change spark plugs). If they are not the culprit then the fuel could be igniting at the wrong time, when it is not fully compressed, and therefore you'd lose power, hear a 'knocking' noise, and have unburned fuel (in some cases) leave the cylinder. To solve this problem, higher octane fuel is required. Another (hopefully this isn't it) culprit could be the piston rings. If they are not holding a seal in the cylinder, then fuel & air could be escaping into the oil, and the piston would not be able to adequately compress the mixture, resulting in incomplete combustion. One way to check this without dismantling the engine is to drain the oil and see if there's any fuel in it (it will be shiny and multi-coloured). If the oil is clean, chances are this is not the problem. The problem is that with such an old truck, if it's not maintained then several components can start to go, leading to sometimes costly repairs. I know other ABers will think of other possbilitis but I just got up and that's all that I can think of right now. Hope this helps, Good luck