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Jaguar Repair/Fuel Mixture screw on 1984 Xj6


Mixture screw
Mixture screw  

Idle Speed Screw ?
Idle Speed Screw ?  

  I wanted to adjust the idle speed of my XJ6 (it was a bit high at 1100 rpms) but I misread my manual and instead adjusted the allen key screw on the top edge of the Airflow meter which is actually the mixture screw.  I turned it back and forth but didn't notice any real change to my idle speed.  I didn't notice how much I turned it so now I don't know what it was set to before.  I turned it all the way down but now when the engine is cold I notice it's hard to start and stumbles when I give it some gas.  Once it's warm it seems ok.  So I think I have it too lean now ?  I read somewhere a good starting point is to screw it all the way down then back off 4 revolutions ?  Which way is lean or rich ?  I have a simple CO meter so I could try to match the CO % on the emissions sticker - I think it was 0.5 - 1.5 %.  I also read you can open up the airflow meter and adjust it based on parts inside the meter...  What would you suggest I do to get it properly set ?  I've attached a photo of the screw I adjusted, I think the real speed screw is just below closer to the engine and at an angle to get to, I've attached a photo of that too.



Hi Steve,

First you need to confirm that the throttle plate stop screw is backed off so the throttle plate is closed all the way, then turn the throttle stop screw in just to touch but not open the throttle. This is to take the load off of the throttle butterfly but not open it. That is the function of the throttle stop screw.

You can set the mixture screw out about 2 0r 3 turns. Then warm the engine up and set the idle down as far as you can and still have the engine run and set the timing (very important).

Now with the engine warmed up and the timing set and the mixture screw set to 2 or 3 turns out and the idle speed screw (Allen screw) set to as low an RPM as you can get it without it dying. Now turn the mixture screw in each direction to see which way makes the engine speed up. As soon as you get the RPM to increase reset the Allen idle speed screw so that you lower the RPM again to as low as you can get it to run. Now again adjust the mixture screw which ever way will make it speed up. Repeat this process and you will quickly arrive at a point that it is idling very slowly and if you turn the mixture screw in either direction the engine will die.

Now set the idle speed screw (Allen screw) up to 600 0r 700 RPM which ever you like and check your CO. It should be correct.


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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.

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