Tuning your carbs

Need help figuring out which Skinner Union (SU) needles you need to keep your car running right in all throttle conditions? A few years back, a genius over at Teglerizer.com developed a handy charting and searching tool for just that purpose. There’s also a version based on javascript. Give them both a try and let us know how helpful it was!

Once you have completed the pre-tune check list and corrected all problems you are ready to tackle the intricacies of tuning and synchronizing the multiple carburetor setup on you MGB or TR4. How difficult is this? Not difficult at all. Even a first-time novice should be able to precisely tune carburetors in 15 minutes or less — only if you have completed the pre-tune check list and corrected all problems.

If you are adjusting an existing carburetor or set

  • If the engine is running reasonably well you may skip these first steps.
    If you are having problems or if the car has sat for a long time first set
    the carbs to a base line. With a felt-tipped marker, mark a line where the
    dashpots (piston chamber) meets the carb body. This will be used to reinstall
    the dashpots. Remove the dashpots and pistons. With a soft cloth and some
    carburetor cleaner clean the deposits from the inside of the piston chamber
    and the both the small and large diameters of the piston. Clean the steel
    rod in the center of the piston and the bore inside the piston chamber. DO
    NOT USE ANY ABRASIVE such as sand paper – never sand blast or glass bead!
    Liberally coat the steel center rod with WD40 or similar light oil. The
    piston and side wall of the chamber should be clean and dry. Be careful to
    avoid bending or damaging the needle – avoid removing it, they bend easily
    and often stick in the piston.
  • Raise the jet until the top of the jet is even with the bridge. Back
    out the idle adjusting screw until it is clear of its stop, turn it back
    until it just touches the stop and then 1.5 turns. Replace the piston,
    spring and dashpot being sure to put the same piston back into the dashpot
    that it came out of – do one at a time to be safe.
  • Raise the piston using the lift pin on the underside of the dashpot
    mounting flange, when released, the piston should drop cleanly to the bridge
    with a distinctive “clank.” If the piston does not drop cleanly you may
    need to center the jet. If the piston drops
    cleanly lower the jet adjusting nut 2 complete turns. You now have a baseline
    adjustment and are ready to tune your carburetors.

After you have a baseline or if installing new carburetors

  • Bring the engine to normal operating temperature
  • Loosen the screws on the interconnecting linkage – each carburetor
    should operate independently of the other
  • disconnect the choke cable or any automatic enrichment device – loosen
    the cable stop nut or accelerator linkage
  • Using a tachometer, adjust the idle speed to factory specifications.
    Make your adjustment incrementally, turn the adjusting screw 1/4 turn at
    a time, adjusting each one equally.
  • Raise the jet adjusting nut, on one carburetor at a time, until the
    engine RPM just begins to drop. Depending on where your base line settings
    were you may first notice a rise in RPM before it begins to fall. Lower
    the jet adjusting nut until the RPM begins to rise and then 1/6 turn (one
    flat).
  • Readjust idle to manufacturers specifications.
  • Test fuel mixtures – using the lift pin on the underside of the dashpot
    mounting flange, press up until you feel contact with the piston and then
    lift slightly (specs are 1/32 of an inch – good luck). If the engine RPM
    increases when the pin is lifted, and remains elevated, your mixture is rich
    – raise the mixture adjusting nut. If the engine RPM drops when the pin
    is lifted your mixture is too lean, lower the mixture adjusting nut. If
    the engine RPM initially raises as the pin is lifted, and then settles back
    to the original RPM or very slightly above, your mixture is right.
  • Readjust idle speed if necessary. To synchronize the carburetors use
    a length of fuel hose. Hold one end of the hose just in front of the carburetor
    inlet – not so that it obstructs air flow – and listen to the ‘hiss’ of the
    air as it enters the carb. You will easily notice any difference in airflow
    by the intensity of the ‘hiss’. Alternatively reduce the idle speed of the
    ‘fast’ carb and increase the idle speed of the ‘slow’ carb until both produce
    the same ‘hiss’. Evenly adjust both idle speed screws to reach the desired
    engine idle speed. Recheck fuel mixture, and carburetor synchronization.
  • Adjust the pegs on the ends of the interconnecting linkage until there
    is approximately 1/16 inch clearance (free play) before the peg contacts
    the slot on the carburetor tab. Tighten all screws on the interconnecting
    linkage. Tighten the cable stop, accelerator linkage and choke cables.
  • Pull the choke until the jets are just at the point of moving but have
    not moved. Screw both fast idle screws in until they just touch the fast
    idle cam. Alternately turn each fast idle screw until the idle speed is
    increased to 1,000 RPM with the engine warm. Push the choke back in – put
    on your air filters – close the hood – You are done!

Go back to the Pre-tuning checklist.
Problems? Go to Troubleshooting your carburetors.