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Jaguar Repair/Idle mixture adjustment on e-type V-12



 I wanted to lean out my 73 e-type stromberg carburettors to get it through our local emissions test.  I have a gas tester that measures Carbon Monoxide and have been able to get my Spitfire and TR7 to pass in the past by adjusting the main needle using the stromberg tool placed in the dashpot.  There is no mention of this in my e-type manual and when I tried to use the tool in the dashpot I could not get any of the needles on any of the 4 carbs to move.  I checked under the float bowls, there are brass plugs but no slot to indicate that the seat can be adjusted.  They are Stromberb 175 CD-2 - are the needles fixed or glued in place and if so how do you adjust the mixture ?



ANSWER: Hi Steve,

Over the years in dealerships on MG, Triumph and Jaguar I have worked on a lot of Stromberg carburetors and I never found any that had the 3mm Allen adjusting screw that were not adjustable. Now you may have all four adjusting screws stuck or turned all the way and locked up but if it has the 3mm Allen in the top pot it is an adjustable needle.

Turn the piston over and look closely at the sleve that supports the needle and see if it is recessed up into the piston. If it is, empty the oil out and clean the tube and put a penitrating oil like WD-40 in it and try to turn the adjusting screw counter clockwise to loosen the screw adjustment. Also try removing the set screw in the side of the piston close to the bottom as the peg on the end of the screw could be corroded.

Here in the US we had some Strombergs that were sealed with a alloy plug but you could not insert a 3mm Allen wrench in those.

I have run into several that were difficult to turn but none that were locked up.


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

QUESTION: Thanks Howard, I'll pull the pistons and try to free up the needle adjustment.  Maybe I'll find some locktite there from a previous owner or shop....  I'll let you know what I find.

Thanks again,


ANSWER: Here is a cut-away of the piston so you can see how it is inside. The "Blue" is the needle retainer and the "Red" is the adjusting screw for the 3mm Allen wrench. The adjusting screw has an "O" ring on it to hold the oil up above. There is a Speed clip that holds the screw down and the "Blue" needle retainer has a set screw in a groove in it's side so as not to rotate when turning the adjusting screw, however the set screw end peg only fits into a groove in the needle retainer and does not pinch it. That is why I said to remove that screw first so as to be sure it is not jamed up against the needle retainer.

When I rebuild these carburetors I remove that screw and turn the whole unit up side down and put it on a block of wood and then I take a tool I made (a 4 inch piece of straight brake line and slip it over the needle and with a small hammer I drive the whole needle retainer and the adjusting screw with it's speed clip out the top. Then I clean up all pieces and replace the "O" ring and if the needle is badly worn I replace it too.


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


   I pulled the pistons and while holding them firmly was able to turn the needle adjusters with some WD30.  I noticed the needles were about 1 whole turn rich and recessed about a millimeter into the piston bottom.  I started with them all flush with the bottom of the piston and ended up about 1/4 turn rich from there.  I don't know if my gastester was designed for a v12 but it sure gave much higher CO readings than my other cars.  Anyway I was happy when I passed the emissions test on the 3rd try !  

  Reading my manual it appears that the needle adjustment was set at the factory and probably for emissions reasons there is nothing in my manual about adjusting the mixture - only balancing the air flow and idle on the diagonal carbs - the front left and back right then syncing the others with it partner on each side.  To fine tune the mixture, do you use the same method as other cars - lift the piston slightly to see how rpm reacts or is there another way when you have 4 carbs to adjust ?  



Hi Steve,

Here in the US we get a paper when doing emission tests and they give us the readings on HC and CO and CO-2. Here in TN we only need to pass the HC and CO readings.

On adjusting the carbs it is difficult for me to use the "Lift the pin" method on multi (more then 2) carburetor cars like the V-12 and the 3 carb 6 cylinder jags with SUs. When you lift the pin you are in effect disabling that carb and depending on the other carbs to keep the engine running. This in effect makes the other carb support the running of the engine and thus the power of the other carb becomes important. The reason the engine speeds up and stays fast when that laboring carb is too rich is because you have disabled the fuel supply of the lifted carb and allowed excess air in and that mixed with the over rich mixture of the running carb makes the engine speed up.

So when you have 4 carburetors and you disable only 1 of them you are still running on 3 carbs. So it becomes a puzzle to use the lift pin method. I use to try it on the 3 carb jags but even then it was a puzzle because if you lift only one pin you are testing the other 2 and you can't lift 2 as it will not run on only one. So I would just count the turns of the jet adjustment screws and depend on the exhaust analyzer and the drivability on a road test.

I was lucky to have a scope and an exhaust analyzer in the dealerships I worked in so I did what you just did by counting the number of turns out and just made sure I turned each out the same number of turns. This works well as long as each cylinder has about the same compression and vacuum as the others and that you synchronize the throttle plates correctly.

I found that at idle you get pulses of vacuum on the piston diaphragm which makes the piston jump up and down so you need to be sure that you have oil in the top dampener to stabilize the piston movement. That damper is also the only accelerator enriching device on the carb. I like to use engine oil as opposed to some of the damper oil available because the engine oil is a little thicker and gives me a better quick throttle response.

When we got the first V-12 in our dealership all the mechanics in the shop had to play with it and one of the things we would do was to short out one plug wire and get the rest of the mechanics to listen to the engine idling and tell us if it is hitting on all 12 cylinders and most of the time you could not tell that it was missing one cylinder.

That is partly why we would not do what was called a "Tune Up" unless we did a compression test first on every car.

As far as higher CO readings then other cars I would just go by what the passing range is for that car as far as the Emission Test requires. I don't remember what ranges we seen on our equipment as we had no emission testing back then.

Glad to hear you got it to pass and I think you can reach a happy medium between what the Emission requirements are and what performs well.


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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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