I've got a 93 Civic CX.

I recently dropped in a JDM ZC motor (D16A6).

The car runs great and my fuel economy still seems normal but over the last 3 or 4 days I've noticed a small puff of black smoke at high rpms or under strain and my tailpipe is collecting a pretty fair amount of soot.

In the last 5000 miles its had fresh NGK plugs and NGK wires, new cap and rotor, and a new fuel filter.

I haven't pulled the plugs to check them yet but as of 2 weeks ago they looked like they were burning perfectly.

The car is running on the factory P05 ECU and has the one-wire o2 sensor.

I'm not getting a CEL and the ECU isn't storing any codes.

I haven't taken it for emissions yet this year so I'm not sure what my levels are.

What are some things that could be causing this?