2000 XJ 900 S Diversion: REV COUNTER JUMPS TO 10000 REVS...

What's the color of the smoke? white, white with a blueish color, or black?

If the engine is revving up then i'd say you likely have a vacuum leak, or the cars' high-idle setting is stuck on. If this is just the rev counter and the engine idles like normal, that I would guess that the rev counter is getting a bad electrical signal from somewhere and would require checking a wiring diagram at the least.

check fuses

check the condition and tension of the drive belt. Check that the battery terminals and posts are shiny clean and tight. Have the battery load tested for faulty cells. Have the alternator tested for output

sounds like an engine sensor try the crankshaft sensor as the rev counter gets its info from that , might be helpful to get the OBD read by a suitable mechanic

yes rev counter has a problem,. remove rev counter from speedo cluster and take to instrument place to get circuit checked and rectified, give clear discription of symtoms under 3000rpm fluctuates etc.

sounds like the idle speed sensor,if your rpm's are idling up and down

Fuel filter or vacuum leak good place to start. Good luck and have a good day.

Try it with the fuel tank on reserve