I agree with 'rjcarrasquel's' comment about the possibility of the engine wiring harness being the culprit of your miss-fire.
It is possible that insulation on one of the coil leads (3 in all) has crumbled as a result of handling them during the spark plug change.
This happened to me. The misfire at idle was a constant and quick 'thup,thup,thup,thup'. Not an arrhythmic stumble or cough like one associated with a bad lead shorting out in damp weather. I was able to confirm which coil lead it was by moving them a bit with a piece of wood while the motor was running. The misfire would come and go when this was done to the culprit lead. I then carefully sliced the black tape on the lead to expose the two wires - and sure enough - the wire's insulation had crumbled off allowing them to cross and thus causing the misfire.

If this is the case with your 104, you can try to re-insulate the exposed wires or replace the harness (I got mine through Phil at the parts shop). Be sure and get the correct part number where it runs in back of the battery.