XK 140 Idle Tune Help

XK
1

Good Morning,

I have owned my XK 140 MC for about 10 years. My H6 carbs have never adjusted according to the guide and it is always a lingering concern. Everything to my knowledge has been adjusted correctly, needles aligned, correct dashpot oil, Joe Curto replaced my throttle shafts and butterflys, plugs wires distributor are all correct and working. I have done everything I can think of and its never changed.

My Question. According to the book when I raise the damper by 1/4 of a inch or so the engine should increase speed for a short amount of time and then return to standard idle. In my case no matter where the jet is adjusted, when raising the damper the engine wants to stall and die. From what I’ve read this is a sign that carbs(Both of them) are running lean. Has anyone else experienced the standard W02 needle on the stock h6 carburetors not allowing enough fuel? Or is something else I am missing completely incorrect? What Idle should be normal on a 140 with the ctype head and h6, I have tried idle speed of 500 but the engine shakes pretty bad, up to about 750/800.

Thanks so much for any knowledge or help.

2

You did not mention adjusting the float levels. They are critical for setting mixture.
Take out the dampers and look down the jets, and you should see the fuel meniscus down about 1/8" below the top of the jet.
Lift out the floats with a bent paper clip and check them to see that they have no fuel inside. A half sunk float will also throw off your mixture setting.

Three times I have run across the sunk float syndrome.
The first time was with my 120 at a British car show near my house and I had some silicone with me, which lasted just long enough to get home before it was sunk again.
The second time was on Interstate 94 about 100 miles from home, but I had a spare float with me, so I changed it at a truck stop with a bunch of truckers watching me.
The third time was at the British car show, the call came over the PA system, “anybody got an SU float please bring it to the Triumph area”. I drove over holding the float out the window and got a big cheer from a crowd of TR fans around a sad looking guy and his TR3.

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3

Hi,

what Rob says is correct.

the WO2 is nearly the same as the SM I use with the pancake filters. Don’t rely on the described test. I check our Jaguar engines with an CO Tester and to make things a bit better, I use an old BOSCH engine tester I bought for €100 some years ago. Our Jags therefore performe fantastic power by very moderate consumption. I really recommend the CO-test.

Best regards
Lindi

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Thanks so much for the reply’s.

Both floats are working and the float needle valve was set to the 7/16th bar in the SU carb kit from XK/s unlimited. I should say the car runs and drives perfectly. The carburetors have just never adjusted to match the description of every SU guide or book that I’ve read, and I more or less wondered if there is an issue with that.

Whats a normal idle for a 140?

Thanks

5

Hi Scaperd,

With respect, you are not performing the pin lifting test properly.

Instead of raising the damper by 1/4", try raising the damper by only about 1/32 of an inch, and keep it raised.

If, while the damper is raised the 1/32", the idle speed increases, this indicates the mixture strength of that carburetor is too rich.

If, while the damper is raised the 1/32" the idle speed immediately decreases, this indicates the mixture strength of that carb is too lean.

I did not make this up ! It’s all explained on page 46 of the Jaguar XK140 Operating, Maintenance and Service Handbook (the owners manual).

Try this method and please let us know how it works !

1 Like
6

Thank you, thank you, thank you.

This is exactly what I was looking for. I spent an hour out there and you are 100% correct. I was raising the damper by 1/4 inch or more until the engine responded from me raising it.

I did was you said and just barely raised it and held it in place for 5 to 10 seconds, listening to the engine. Moving the jet by 1 flat at a time in the appropriate direction and it made a noticeable difference. Both exhaust notes are now perfectly even. The engine is beautifully responsive and no shutter/shake at idle. Sadly if it wasn’t raining I would take it out for a nice long drive. But after 10 years of playing with it I finally understand the last piece of the puzzle.

Thanks so much for all your help, in the end its the smallest details that make the biggest difference.

7

Scaperd,

First, I’m pleased to read that you made progress using the 1/32" lift pin approach. I have had success with both the 1/32" lift, and the 1/4" lift described in the Service Manual. The CRITICAL difference is that whereas the 1/32" lift approach applies to the mixture of the carb who’s pin you are lifting, the 1/4" lift approach applies to the mixture of the carb you are NOT lifting - i.e. the other one. The two methods work in a different way (which has been discussed at length in prior threads you should be able to find in the archives, so I won’t go into it here), and the 1/4" method is rather counter intuitive, in that you apply the action to one carb to determine the setting of the OTHER carb. I confess that though I’ve had some success with both, I find the 1/4" method easier, as controlling the lift pin to as little as 1/32" or so is beyond my ability unless I’ve had just the right amount of coffee prior to the exercise…:slight_smile:

-David