I have 86 xjs that from day 1 has a idle surge from 900 to 1700 rpm only if i go short trips a few miles if i go longer say upwards of 20 miles it does not happen when its in park.

 

could be the AAV,pretty common symptom,try clean and lube with WD40 and maybe some carb cleaner.It's covered in 'the book' by Kirby Palm free download if you don't have it already.It's a lifesaver!!
Les

 

thanks i will give that a try

 

If you want to experiment remove the air filter from the left side and block off the 1" hole you see in the back of the air filter housing. That's the air intake to the AAV. If the surging goes away, there's your sign.

Some guys just leave blocked off. I did.

Cheers
DD

 

What's the AAV? I have a bit of a surge and this could be the problem.
EDIT - I found the info on the AAV (auxiliary air valve) on page 265 of Kirby's Book.

Thx