Read Adjusting%20Carbs.pdf text version

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The trick to tuning Skinners Union (SU) or Zenith Stromberg (ZS) carbs is to understand that there are two things you need to get right: the air flow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently. SU carbs were used on the early Triumphs, while ZS carbs were used on the later ones. Either type of carb can be found on TR4s. The design of each is quite similar; thus adjustment is performed in the same manner for either type. If you would like to read more detailed information about how your SU or ZS carbs work, there are excellent Haynes manuals for each of these carburetors. 6SHFLDO 7RROV

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You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the air flow balanced between the two carbs. This costs about $20 and is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge. For the fuel mixture, one helpful device is called the Gunson ColorTune (maybe ColourTune, as it's a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. It costs about $40, and while it's not absolutely essential, it makes life so much easier that it's worth the cost.

If you don't have a Gunson, the standard directions are included here for determining correct mixture (step 4 of the Adjusting Mixture procedure).

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For some ZS carbs, you will need a special mixture adjusting tool. This is a special two-part wrench that is inserted through the top of the carburetor to adjust the metering needle while preventing the air valve from rotating within the carburetor.

7HUPLQRORJ\ To tune SU or ZS carbs, first locate the following components: Throttle linkage nuts. These are the things that connect the throttle linkage (the bar connected to your foot through whatever means your car uses, cables or rods) to the carburetors' throttle levers.

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Throttle stop screws. These set the idle speed for each carb, and are located typically behind the dashpot, near the connection of the throttle linkage. Mixture adjusting nut. On early SU carbs, this is the lower of the two nuts at the very bottom of the carburetor. Later SU carburetors of the HIF type have integral

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float chambers, on which the mixture is adjusted by turning a screw. Some ZS carbs do not have adjustable mixture. ZS carbs that do allow mixture adjustment either You'll need to experiment (and I explain how) to see which way makes this richer and which way makes it leaner. Lifting pins. (SU carbs only) These are little wobbly metal pins under the dashpot. When you push up on the pin, it raises the piston in the dashpot. Find these; they're crucial if you don't have a Colortune. If you don't have them, can't find them, or have a ZS carb, you can raise the piston with a flat-bladed screwdriver pushed down the throat of the carb and twisted or levered to lift the piston. The bridge. This is the part inside the carburetor, where the gas jet opens into the airstream. You'll see a needle inside the jet, and the jet itself should be a few fractions of an inch down from the bridge itself. The jet is the brass tube that sits in the center of the bridge, with a tapered needle poking down into it. The choke linkage nuts. Comparable to the throttle linkage nuts (and usually the same size), but on the linkage that goes between the choke cable and the mixture adjustment mechanism. They make sure that both carbs are enriched when you pull on the choke. Your car may have no choke linkage, but a split choke cable which connects to each carb, in which you can forget about these...

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Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (points gap, timing, spark plug gap, new condenser, etc.) first. All of these things can affect the setting of the carbs, which should be adjusted last, if at all! After being properly set, the carbs should rarely need further adjustment. If you've got a timing light and a dwell meter, you can verify the ignition components independent of the way the car is running. When it's warm, shut the motor off and remove the air filters. Of course, it helps if the carbs are in good mechanical condition as well. But you can consider a rebuild once you have gotten things working first!

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If your car has multiple carburetors, the air flow needs to be balanced amongst all carbs before the mixture is adjusted. If you have only one carb on your car, you can proceed directly to mixture adjustment! %HJLQ E\ EDODQFLQJ WKH DLU IORZ 7R GR WKLV ILUVW ORRVHQ WKH WKURWWOH OLQNDJH QXWV /HDYH WKHP FRQQHFWHG MXVW ORRVHQ WKHP KDOI D WXUQ RU VR %DFN RXW WKH WKURWWOH VWRS VFUHZV WLOO \RX FDQ VHH WKDW WKH\ DUH MXVW WRXFKLQJ WKH WKURWWOH VWRS 7KHQ RSHQ HDFK FDUEXUHWRU WKDW LV ORZHU WKH WKURWWOH VWRS VFUHZ WXUQV RI WKH WKURWWOH VWRS VFUHZ DQG VWDUW WKH HQJLQH ,W ZLOO SUREDEO\ LGOH DW DERXW 530 GRQ W ZRUU\

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