I happily have gotten my 1984 5.3 V12 xjs running and back on the road. It hadn't run since January and since developed a few bugs that I just finished working through. Compared to the way it used to feel though it feels down on power. When you used to floor it the kickdown kicked in and it pinned you in your seat. Now when you hit the kickdown it just sort of lazily accelerates.

Don't get me wrong it's still fast. Just not as fast. She idles and runs beautifully. No anomalies of any kind just a general lack of "umphhhh". Thought it might be the gas but I always put 91 octane in.

Thoughts?

 

Congratulations on getting her back on the road!

Sounds like it is not downshifting when you hit the kickdown (last little bit on the accelerator)?

If not, investigate the kickdown switch, they are known to fail through a variety of ways by sticking, disintegrating, etc. It is located on the throttle cable leading to the pedestal. Note there is a separate switch located on the pedestal itself commonly confused for the kickdown switch; that switch sends the ECU into open loop mode at WOT.

Cheers,

- Will

 

Another thought....

Have you checked your timing advance? The mechanical advance mechanism inside the distributor is notorious for sticking and the vacuum advance capsules are almost equally notorious for being inoperative.

Cheers
DD