Did some playing around today, and got a proper tach installed on the D850.
It redlines at 3000rpm, which would explain my 65-68mph top speed. This is a hard limit and it hits this RPM even with nothing connected up to the engine.
I had a quick play and removed all the throttle bits, removed the plate, and pulled on the governer arm manually. This ruled out the actual lever stop causing the RPM limit.
It revved to the same RPM.
I turned out the fuel limiting screw 3 turns, same limit.
Any ideas how I can unlock the 600 more RPM?