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SU HS2 Carbs on my Mk3
Automotive > SU Tuning

This isn't the definitive guide, but it works for me. Itís a combination of several different pieces of literature and also personal experience.

Before you tune your carbs you need to make sure that points, plug gaps, ignition leads, timing, valve clearances and the general fuel and exhaust system (i.e. leaks) are in good condition. Otherwise itís a head ache to tune them.

For spark plugs I would recommend nothing else but NGK in your Triumph Spitfire... BPR6ES Copper Core are probably the best... but BP6E & BP6ES are fine too.

NGK Technical Information Pages - lots of technical information on spark plugs
NGK UK Technical Information Pages - not as detailed as above - but with good examples of plug colour and simple to understand information

Check that your needles are correct and also that theyíre properly in the pistons. Note: Piston's work exclusively with their carb body - do not mix them up!

Make sure you have the correct springs and also that thereís some oil in the dashpots. If you are unsure of what oil to use, just use engine oil until you know any different - or can judge what to use. A thinner oil will richen the mixture on acceleration.

  1. Go for a hearty drive to get the car to operating temperature.

    Go on! Drive it like you stole it!

  2. Balancing the amount of air that is being taken in by each carb.

    Take off the air filters.

    Loosen the clamps (1) on the throttle linkage so that you can adjust each carb's throttle separately.

    Turn each throttle screw (2) the same amount until the car idles at around 800rpm. Use the cross on the screw as a guide – I always turn them so that they end up at 1/8 increments. As a rule of thumb, for both the throttle screws and mixture screws (which we’ll get to later) – when looking from above – clockwise is to increase, anti-clockwise is to decrease.

    Next you need to adjust each screw individually until they are both letting in the same amount of air – you need some sort of manometer for this - either use one you have bought (follow the instructions) or make your own using a piece of windscreen washer pipe - put one end in the carb the other in your ear – listen to the hiss – constantly comparing the “note” of the hiss for each carb.

    If the engine idles quicker after balancing, turn the screws anti-clockwise by the same amount.

    Before moving on to the next step, rev the engine up to about 2-3000rpm for a couple of seconds. You should repeat this every 2-3 minutes to keep the engine warm. As the throttle linkage clamps have been loosened the throttle pedal won't work, so push evenly on the throttle actuators (arrows) with a finger on each.

  3. Adjusting the mixture.

    You will be using each carb’s mixture adjustment nut (4) to change the amount of fuel that the carb is mixing with the air. You need to determine whether or not the carb is rich or weak – and adjust it accordingly. To do this you must use the lifting pin (3) located underneath each carb.

    Push in one pin at a time and listen to the engine speed. If the speed rises then the mixture is too rich and you should turn the adjustment nut anti-clockwise whilst looking from the top. If the engine speed drops then the mixture is too weak and you should turn the adjustment nut clockwise to richen it. When the mixture is set correctly the engine will momentarily rise and then remain the same. Turn the nut one flat at a time until you get a feel for how much to turn the nut. Again - I like to turn them one flat at a time - so then its easy to figure out how much to turn them next time.
    If your lifting pins are seized, use a screwdriver to carefully lift up the piston. Make sure you only lift it up about 1 or 2mm at most. Obviously making sure you lift each one up the same amount.

    You now need to go back to step 2 and check the air flow of each carb – it is likely that it will have changed. If it has then adjust the throttle screws and then check the mixture again.

    Now do up the throttle linkages – making sure there is a slight clearance so that the throttle cable isn’t pulling on the linkage the whole time.

  4. Once you think everything is done, go for another drive.

    Then remove the spark plugs and take a look at the colour.

    I'm not sure what's up with the one on the far right, but the rest look as they should. The mixture has ended up overall slightly too rich because the air filters lessening the amount of air entering the carb - however an anti-clockwise turn on the mixture adjustment nut should cure that.

  5. Now all you need to do is adjust the choke and you're done...

    (I might write that up at a later date - but I think Haynes covers it sufficiently)

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