Engine low power

did for both sides and it is correct. I am having trouble understanding why it will run smoothly if you disable either set of plugs. I thought there might be some cross firing from wires running
adjacent to each other, but have the new wires spread out so that should not be an issue.

That is exactly why I said to re-check the firing order.

re-checked firing order and it is correct for o both intake & exhaust sides. when running rough it is producing black exhaust - lots of carbon - and smells almost like raw fuel- not oily.When running smooth the exhaust is clean.

83 nissan 4x4, engine cranks and runs but seems like it has no power when you try to go. Its not bogging down it just seems weak.

It has a lot of miles on it. check engine compression. It may be getting weak. Also, check to see if the catalytic converter is plugging.

Engine Performance problem
1987 Nissan Truck 4 cyl Two Wheel Drive Manual 140000 miles

truck was running fine and all of a sudden started running real bad.I got a compression test done and the two back cylinders are reading under 25.The guy said I need a new engine.

HI there , more than likely you have a blown head gasket between the cylinders, if this is the case it will be the head off re machine and crack test and a gasket set, far from a new engine and a good deal of your money in his pocket i feel..

Mark (mhpautos)