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Jaguar Repair/bypass valve adjustment


QUESTION: Hello Howard
77 xj6
What is the procedure to adjust the bypass valve?

ANSWER: I read that there was a method to adjust it but we just ran the adjusting screw in to hold the valve closed all the time and locked the little speed nut to hold it there. The small diaphragm attached to the valve would soon harden and it never worked anyway other then to cause the engine to coast down for a long time.


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

QUESTION: So Howard, you are stating that the temp compensator and the bypass valve are best rendered non-operational?
If so, how will this effect emissions? California requires a smog test...
Thank you

No, the temp compensator is ok and usually does not need any adjustment and it is a minor air bleed system. It makes a very minor adjustment to mixture between a cold engine and a warmed up engine so it is never is in effect when having an emission test.

The bypass valve with the little diaphragm is always a problem and it allows air to bypass the throttle plates when they close quickly and they never worked right so we would disable them by adjusting them closed all the time which will not show up on any emissions test. Unless you have the "L-1" emission testing which I understand was eliminated many years ago. I went to the Univ. of Colorado for a week to train on the "L-1" testing procedure. Then they eliminated it because no car could pass it and very few shops had the equipment to run the tests. They tested emissions while on a dyno.


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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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