My 87 Nissan Pickup Xtra Cab 3.0 liter engine recently failed a CA Smog test due to the timing mark on the pulley could not be verified. Now the timing can’t be too far off cause the engine runs. If the pulley was put on the engine in the wrong position, can it just be pulled off and and put back on to get the mark put in the right place without screwing too much up?
I suppose you are talking about the crankshaft pulley, where the timing mark is. It only goes on one way, can’t be rotated to another position. The engine will run if the timing is off, but how good is another question.
It is possible the outer pulley of the balancer has shifted. This has been known to happen on older cars as the rubber isolator breaks down. You can check to see if this has happened by rotating the crankshaft until the timing mark is at TDC-(0 deg), and if the keyway on the crankshaft is pointed straight up. If the keyway is not, then the timing mark is wrong. A replacement balancer/pulley will fix that.
Thanks for the info. Now the hard part. I’m basically car dumb…what is the “keyway on the crankshaft” and where do I find it. 
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The pulley that is marked for setting the engines ignition timing can be fitted to the crankshaft in only one position. This one position only feature is achieved by the use of a keyway. Think of it as a “bump” on a otherwise smooth surface. The pulley has a recess cut in it to fit over this bump,hence the pulley is always in the correct position.
I move around a lot on IT Forums, I must comment on how tolerant we are when people ask such “really have no clue questions”. On the IT Forums the respondents "tear you a new one’ if you ask something even remotely (searching for a word) got it, “Out of touch”. Perhaps it has to do with the maturity of many of the people on IT Forums,absolutely no tolerance.