[Grady Clay]

With dramatic differences among cylinders I think some careful diagnosis is in order. Black can also mean oil.

Since it runs properly, go do some spirited highway driving with fresh oil and fuel.

First do a cylinder leak test and a cranking compression test. If there is any leak, note if it is intake valve, exhaust valve, ring, or head gasket. There are plenty of posts on this subject.

Next measure, not adjust, the valve clearance. You want to see if there is a difference between #2 and #3 compared to the other four. Inspect the rocker arms and cam surfaces with a mirror. Make sure the rocker arm shafts are properly in place.

Take a “light on a stick” past the fan and see if the #2 and #3 cylinders and heads have anything restricting the air flow. Same for the other cylinders.

If you still have the heat exchangers off the engine, rotate the engine and inspect the exhaust valve seating surfaces best you can.

Check the cap, wires, and plug connectors for high voltage arc tracks. If in doubt, replace them later – these are normal service items.

Please report what you find.

Best,
Grady