As Jagboi64 mentioned there are a few things that could be off, that you can verify..

Check distributor timing at 3000 RPM with accurate RPM meter; should be 33* BTDC with vacuum line connected. At that RPM there will be no vacuum advance.
Clean and lube mechanical advance mechanism shaft and pivot points in distributor.
Check transmission to throttle cable for proper adjustment. Early shifting to 3rd would give weak performance below 3000 RPM

Rgds
David

 

David --
Thanks.. I always like it when I need my son to do something on the car. Its the best.
Didn't know we can lube the mechanical advance. on the dizzy. That's an easy one.
Transmission to throttle cable thought is also something I never thought about - though I perceive the shifting isn't my problem. Which is why the low power in that range is so noticeable. The shape of my power curve -- or perceived power curve - might be absolutely hideous.

 

Verify that number for your particular car, the timing changed considerably through the Series III production depending on year, market and emission specification. I have seen the mechanical weights range from 8 degrees to 17 degrees max advance depending on the year the distributor came from, with corresponding changes to the vacuum capsule as well.

Another thought: is the fuel filter clean? A restricted fuel filter can cause low performance.

 

Thanks! And I'll check the specs for my VIN. I think I'm in the last group of the real XJ6s that had the bigger advance spec.

And yep, changed both the fuel pump (which had been getting noisy) and the filter - I was absolutely certain it was one or both of those.
Interesting aside - it's my dd here in San Francisco. It's really noticeable to have a drop in torque here. Also holds true for humans.

 

Few more long shots that you can check:
Plugged air intake filter,
Missing air horn P/N EAC2512
Plugged Catalytic converter (other will have to describe how, I can't find my notes. I put a pressure gauge on exhaust manifold and checked pressure at several RPMs)
If you look on under hood you will have Decal for CA approved timing (17* at 800 RPM, vacuum removed and plugged): this should give you 33* at 3000 RPM with vacuum line hooked up.
Rgds
David