JD,
Your vibration at 1900 RPM seems to be a common denominator. That describes my vibration very well. I also have a nasty vibration like you from 800-1600 RPM that I believe is a combustion related event. I don't see how it could be otherwise, as everything is neutrally balanced. I have tested injectors, timed, set valve lash numerous times, but to no avail, I am down to delivery valves or the whole pump at this point.

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