I suspect that the carb is gummed up. You may need to clean the carb and check any adjustments that can be made to the carb. Of course, you could just convert to a Weber DGV carb and gain so much over the stock Hitachi carb. There are also sidedraft Weber carb manifolds of the Nissan L4.
Anyway, as suggested above by dimefury the gas could be bad and you should drain out all the old gas. I think there should be a drain plug in the bottom of the gas tank, but you can also use a small electric fuel pump and come vinal hose. Stick the hose down the gas filler and connect the pump to 12v. That is how I do it.
OK, after you drain out the gas, put in something like BG 44K or 2 or 3 bottles of any fuel system cleaner and then put some good gas (you do not need anything above regular low octane gas, premium would be a waste in the standard low compression L20B engine).
NOW, run the car at idle (or in your case just keep it alive) for about 30 min or a little longer. Then stop the truck and leave it for about 18-24 hours. After that, see if it is any better. Even if it is not, drive it around for the day and it may start doing better.
No mater if it is better or not, you should take it to high RPM on the highway and do a "Ferrari tune-up" for a few miles. Don't redline it, but keep the RPMs high. This will help to blow out carbon that has built up in the engine.
Once you have done all of this, and if it is still not right, you may need to rebuild/replace the carb.
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PARTS FOR SALE