W123, Diesel Engine Repair

I have a mint '82 4-speed with 120k actual miles (3k mile oil changes since new) that has a heavy metallic knock at idle (all cylinders) and less power than it should (max speed 80 mph). Items I've done: new injectors, valves carefully adjusted, compression check (330~360 psi all), new filters (all), 2 cans diesel purge...all with no effect on noise and low power. I'm thinking perhaps injection timing might be the problem but know nothing about how to check/adjust. Also, the car does not smoke much and has excellent emissions numbers. Any thoughts? Thanks so much for your input! Kurt