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Jaguar Repair/tuning ZS cd175 water chokes


Hello Howard
1977 Jaguar XJ6L
Got them back on the car.
The by pass valves are neutralized and the idle fine tune screws all the way in...this is how I found them.
What is the procedure for tuning the carbs?
Thank you

Hi David,

First back off all the throttle stop screws and the fast idle stop screw. then slowly open the throttle by hand and note that both throttles open exactly at the same time. If not adjust the linkage on the throttle shafts so they do open exactly the same time when opened by hand.

Now run the idle stop screws down until they almost contact the throttles but not quite. Now take a thin feeler gauge or a piece of cellophane to use as a feeler gauge to set the stop screws so they barely contact the feeler gauge or cellophane. Now turn both stop screws down exactly 1 and 1/2 turns. This is just a starting point.

Remove the top pots and examine the needles at the shoulder and see if they are both the same amount recessed up in the piston. If not even them up with the 3mm Allen wrench down through the oil reservoir tube. Check the diaphragm for any holes and replace if any are found.

The float level can not be checked with the carbs on the engine. But there is a method to confirm that it is close. Put your face close to the open carburetor and "Puff" a quick blow down at the jet and watch to see if fuel comes up out of the jet a little. If you see the fuel it is close. If not you have to remove the carb and float chamber and check the float level.

With it all back together, start the engine and warm it up. Then set the idle with the idle stop screws but "BE"SURE" to turn BOTH screws the same amount to get the idle to about 700 to 900 RPM. Now if you have a Unisin gauge use it to put a fine adjustment on the throttle stop screws. If you don't have the gauge use a piece of vacuum hose and put one end in the front of the carb and the other end in your ear and listen to the "Hiss" sound and you can get the balance correct by making the "Hiss" about the same with the throttle stop screws.

Once you have completed that lift the spring loaded pin under the lip of the top pot and note the engine response. If the Engine dies then turn the 3mm Allen adjusting screw down in the OTHER carburetor. (which makes it richer. adjust it no more then 1/2 turn and recheck. If the engine speeds up Adjust the OTHER carb 3mm Allen screw up (counter clockwise) to lean out the mixture. When it is correct the engine will slow down and sound like an old tractor engine idling.

After adjusting the mixture you may need to readjust the idle speed.


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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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