• Make sure that the engine valve clearance is absolutely right (engine MUST be very cold for those measurements)
• make sure that the ignition timing is perfectly right
• make sure you have the right spark plugs with the correct gap. Do NOT assume new plugs come with the correct gap setting.
• look with great care for evidence of damaged gaskets or damaged rubber mount. If you find damage, fix the problem
• make sure that the throttle lever allows full return of the carb throttle to resting position once the accelerator pedal is not pressed. The lever is adjustable.
• on the carbs close all four idling screws by turning them down fully (GENTLY otherwise you ruin their seats and you will need a new carb!)
• reopen all of them by two full turns
• connect the vacuum measuring equipment to the two holes in the intake manifold. It is impossible to tune the carbs by ear or without the right equipment.
• check that the carb balance screw (NOT the main idling screw) is about half way through. You must not confuse identification of these screws.
• check that the main idling screw controlling both carbs is turned in (about half way)
• engine MUST now be at 80 degree temperature otherwise good tuning is impossible
• once the engine is warm, adjust the balance screw to obtain ABSOLUTELY identical (not just similar) vacuum pressure measured at the two holes in the intake manifold with engine at about 1000 revs.
• Adjust the main idling screw to drop revs to 800. Don't rely on the car rev counter for reading revs. Use a proper rev meter hooked up to the distributor.
• Starting from the idling screw for cylinder #1, undo it slowly to see if the revs go up. If they do, leave this screw in position of max revs and turn down the revs to about 800 with the main idling screw. If the revs did not go up, turn down the cylinder #1 idling screw until the revs drop, then back it up a bit. If nothing changes, leave the screw at the initial setting and move to another cylinder. Always blip the throttle after any adjustment to clear the engine from any unburnt petrol.
• Repeat this procedure with the other 3 cylinders, always ensuring that the vacuum reading does not change.

Tuning the carbs takes a lot of time and testing. Carbs are made with delicate alloy and brass which are easily damaged by too much force with screwdrivers, spanners etc. If, after following exactly this procedure you still have a problem, then you are advised to open the carbs, clean them systematically and refit them after replacing any dubious component.

- Andrea -