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Jaguar Repair/FAST IDLE '72 JAG XKE V12


QUESTION: AFTER WARM UP, JAG WANTS TO IDLE 1100-1200 RPM. Carbs have been rebuilt and adjusted. Idle trim screws are backed out. Intake hiss is equal on all carbs.
Distributor has been modified with a Pertronix Ignitor electronic timing rotor and pick up module. This eliminates the amplifier unit.
Set up timing as outlined in shop manual,12 deg BTDC, electronic timing rotor magnet in center of pick up module. Vac advance unhooked. When the car is cold or hot it starts up perfectly. With choke full on it starts out idling at 800-850 rpm.
Any ideas ??

Thanks Richard

ANSWER: Hi Richard,

When you say "Idle Trim Screws", Are you talking about the large brass screws directly in the side of each carb on the right side? If so they were used by the factory and when the cars were received by the dealer, they were supose to be screwed all the way in and laocked down and not used.

Did your carbs have the spring loaded valve in the throtle plates (butterflies)? If so you need to either solder them shut or replace the throttle plates with plain ones.

If none of that applies you may have an intake leak somewhere. Start pinching off vacuum hoses at the fast idle to see if you get any change and spray around the intake and carbs. No engine can run at 1100 to 1200 RPM without extra air going into the intake. Simple as that.

Let me know what you find.


---------- FOLLOW-UP ----------

Thanks for leading me in the right direction. It was a vacuum leak. The small ports on the intake manifold where the air clear box vacuum hose hook up where the problem. Plugged both of those and hooked up the vacuum advance and the idle came down.
What type of fluid should be used in the manual transmission and rear end ??

Once again thanks for your help,


Thanks for the feed back and glad I could aim you in the correct direction.

The oils recomended in your owners manual usually are oils in England and sometimes not applicable in the US. However, You can get the oils at any auto parts store and just get hypoid gear oil 90W for the differential that is for a limited slip. Most auto parts stores that I have gotten the oil from have a standard hypoid gear oil and they add an additive for limited slip. All E-types were limited slip. As were the XJ-S. The XJ-6 was normally non-limited slip but could be ordered with limited slip.

We used engine oil in the standard gearboxes. (don't put any additives in the gearbox)


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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