What did you do with the flywheel?
There would have to be a huge problem in a combustion chamber for it to rattle under fire. Like one cylinder has almost no compression. That would feel like a miss.
Is the vibration over the entire operating range of RPM or does it have a harmonic problem like vibrates at 2500 to 2700 then is smooth everywhere in between?
When you washed the engine, was it hot? Could you have fractured a weight in the flywheel or balancer that was flung off when it was started?
Just a few questions to get the thought process moving.
