I bought a 87 nissan pickup and I find the timing is set fully retard. I thought that is why it backfires and runs badly. Its hard to start cold and has little torque. I got a timeing light and set the timing where it is supposed to be, but then it really wouldnt run at all. The more I backed the timeing up the better it ran, until I reached fully retard again, and it runs like it did before, bad, but it runs. Any ideas of why a car/truck would run better like this? Obviously the previous owner set it like this on purpose.

I'd guess that the mechanical timing is off if the ignition timing has to be set wrong to get it to run at all.