A pair of balanced carburettors could make the world of difference to your engine. John Simister explains all
Does your classic car have twin carburettors? Is the idle lumpy and the pick-up ragged? Does the engine feel a touch rougher than it should?
If so, the throttles may not be opening exactly simultaneously. Which means the carburettors need to be balanced. Balancing is thought of as a black art best left to the experts, but it isn’t difficult. Forget rubber tubes and listening to sucking sounds: here’s the simple way. Start by unscrewing the idle-speed adjusters of both carbs so their tips are clear of their stops. When the carbs are balanced, both butterflies will now be completely shut.
How we achieve this depends on how the carbs’ butterflies are linked to each other. If there’s a clamp between their spindles, loosen it, hold each butterfly shut and retighten the clamp. Now screw each idle adjuster back in until it just touches the carburettor body and is on the point of opening its butterfly. Next, screw each in one turn to open the throttles enough for the engine, with luck, to idle. If it does, set to the right idle speed by adjusting both idle screws by the same amount. If it doesn’t, turn each idle screw a little more until it does.
Another system has a single idle-speed screw and a balancing screw altering the position of one carb’s spindle relative to the other. Adjust the idle at one carb, and the other follows.
Again the principle is that both butterflies should be fully shut with the idle adjuster screwed right out. You can discover that point by placing something on the end of each spindle – an adjustable spanner works well – that allows you to monitor each spindle’s movement as you turn the balancing screw between them.
Turn it one way and one spanner moves as its butterfly opens. Turn it the other way, and the other moves. At the mid-point between the start of each spindle’s movement, the carbs are balanced and you can screw the idle adjuster back in as required. Job done, and welcome to a new world of throttle-crispness.
Words: John Simister