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Triumph Repair/Stromberg Carb Tuning Question


Hi Howard:

I hope you are doing well.

This question is in regard to tuning Stromberg carbs on a 1973 Triumph Stag. I know you do not list expertise on the Stag, but I believe these carburetors are used on many Triumph models so I'm hoping you can help.

Do you have a procedure for tuning Stromberg carbs?  You've directed me to one before for SU carbs and it works well.  I'm wondering if a different technique is required, especially since mixture is controlled by moving the jet up/down by inserting a special tool into the piston.  

Any help would be appreciated.


Hi Matt,

You are correct that I only have a limited experience with the Stag. The Strombergs should react close to the same though.

When you lift the piston of one carb a small amount you are disabling that carburetor by allowing the air to slow down over that jet. In the 4 cylinder and 6 cylinder Triumphs that cuts out 2 and 3 cylinders and even though there is a balance tube in the intake manifold, it is not enough to run the dead cylinders and thus you get the response of the engine only running on half it's cylinders.

In this case a lean condition lacks horse power so the engine dies. A rich condition allows the engine to raise in RPM due to the added air through the balance tube from the disabled carb.

You may know if the Stag has a manifold open to both carbs or does one carb mainly only feed one bank of the V-8. If that is true then the lifting of one carb piston should result in the same manner as it does on the 4's and the 6's. Thus you can use the response to adjust the jets as you would on a 4 or 6.

The adjustable jets vs adjustable needles does the exact same thing so that part is the same.

If your Strombergs have the lift pin under the lip, try it to see if you get the normal responses. If not and you don't have CO equipment you can get it close by other methods.

Since leaded fuel has been eliminated, using "Plug Readings" to adjust carbs has been made more difficult but not impossible.

Do this, confirm that the top pot has the correct level of oil in them. With the engine warmed up, Turn the adjustment screws down (rich) (possibly two full turns) until you see signs of black puffs of tail pipe smoke when the engine is reved. (count the number of turns out so that you keep both carbs the same and you know where they are at)

The black smoke is a clear sign it is too rich. (always work from rich toward lean not the other way) This method should not be done on a car with a CAT as it will cause the CAT to overheat and cause damage. Also it should not be done on a car with an 0-2 sensor. Your car should have neither.

You don't need to run it long and remove a spark plug from each bank and note the color. It should have dry flat black powder on it and even some on the center porcelain.

Black smoke and dry flat black powder on the plugs are two sure signs of over rich mixture. You may also note great quick throttle responce. Now turn the jet adjusting screws up one full turn and recheck for the smoke and plug color. You may need to do this several times to arive at a good setting. (confirm that the spark plug number is correct for the car because if someone had installed the wrong "Heat Range" plug you will get false readings. After the first change of adjustment only turn the adjusting screw 1/4 turns from then on.

Quickly you will arive at a poing that you see no more black smoke and the center of the spark plug will start to clean up a little.

I like a slightly rich mixture on my cars which will show up as no black smoke when reved up smoothly and slowly but a very small puff of black smoke if reved very quickly from a low RPM.
My plugs look dark on the center porcelain and a slight sign of black powder on the edge of the threaded part of the plug. I like a slightly rich mixture for it's added power and cooler combustion chamber temperatures.

A final test is to carry a plug wrench and find a long straight road that you can pull off an stop safely. Run down the road at a high steady cruise speed and after severl minutes at that speed turn the key off and step on the clutch so that the engine stops quickly and then you stop and pull off the road  and remove a plug on each bank and look at them. The center procelain should be from clean to just a dark tint and the outter edge should look flat black with no excess powder on it. If you rub a clean finger on it you will note a slight amount of black on your finger. A snow white center should be considered suspect of either the wrong heat range plug or you went too far lean.
Symptoms of too lean will be, surging at steady throttle, high engine temp, poor throttle responce when trying to pass a car. running on after the key is turned off.

These are very dangerous to the engine, that is why you must start on the rich side and work toward lean not the other way.


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


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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