Not firing on all cylinders

I just swapped in an '93-'94 Grand Cherokee engine into a '91 XJ. The old engine ran OK, no problem with that but the "new" doesn't run on all cylinders, maybe just 4. All cylinders do have compression and the spark plugs are new. The ignition is the same as on the old engine, almost new rotor and cap. What could possibly be the problem, maybe injectors? How can I diagnose bad injector?
Thanks!

you first need to find out if all the cylinders have spark. if they do, then you probably have a fuel problem. and then work from there.