The Quiet Art of S.U. Synchronisation: Balancing Twin Carbs Without the Voodoo

A pair of SU carburettors out of balance will still start. That is the problem. The car runs, so you live with a slightly lumpy idle, a flat spot on a cold morning and a throttle that feels as if one carb is doing more work than the other. Then winter arrives, the battery gets tired, and the whole thing becomes a ritual of coaxing and throttle blipping.

Balancing SU carbs is not black magic. It is a mechanical ballet: set the stage, listen, measure, adjust and repeat. The tools are simple, the patience is not optional, and the reward is an E-Type that pulls cleanly from low revs and settles into a steady idle.

Why SUs Drift Out of Step

SUs are beautifully simple, but they are still mechanical. Throttle shafts wear, linkages develop play, return springs soften and dashpot oil disappears. A car that sat through the winter of 2025/26 may also be breathing stale fuel and a slightly sticky piston. None of this is a crisis. It just means the carbs need to be treated as a pair, not two separate devices.

Before you touch a screw, make sure the ignition system is healthy. A misfire can feel exactly like an out-of-balance carburettor. Check the plugs, points or electronic ignition, fuel pressure and valve clearances. Balance is the last job in a tune-up, not the first.

Tools That Earn Their Place

You do not need a wall of expensive equipment. A decent screwdriver that fits the idle and mixture screws properly is essential. So is a spanner for the linkage clamps. Add a vacuum gauge or a Carbalancer, a length of hose to use as a listening tube, and the correct SU dashpot oil.

A Gunson Carbalancer is a familiar sight in UK garages, and it does the job well. A Colortune can help with mixture, but it will not balance the carbs for you. The most useful tool is still your ear, followed by the discipline to make one adjustment at a time.

Set the Stage Before You Listen

Get the engine fully warm. Fit clean air filters or the standard intake arrangement. Check that the pistons move freely and that both dashpots have the same oil level. Too much oil makes the piston lazy; too little makes the mixture weak under acceleration.

Look at the throttle linkage. With the engine off, open and close the throttle slowly. Both carburettors should start to move at the same point. If one opens before the other, sort out the linkage before you chase the idle screws.

The Listening Pass

With the engine idling, use a length of hose to listen at each intake. You are listening to the hiss of air, not the exhaust note. Adjust the idle speed screw on the weaker-sounding carb a tiny amount at a time. Blip the throttle, let the idle settle and listen again. Small movements. The SU is sensitive, and a quarter turn can be a big change.

If the engine is hunting or the idle is too high, bring both idle screws down together. Do not set one carb beautifully and leave the other doing its own thing.

The Vacuum Gauge Verdict

Place the Carbalancer over one intake, note the reading, then move it to the other. Adjust the idle screw until the readings match. On many E-Type installations, you will need to loosen the clamp between the two carburettors so each can be set independently, then re-clamp the linkage and check that both still move together.

This is where patience matters. Adjust, blip, settle, read. Repeat until the gauge agrees with your ear. If the readings will not settle, look for a vacuum leak at the intake gaskets or a worn throttle shaft. No amount of twiddling will fix a mechanical air leak.

Common Mistakes

• Adjusting mixture before balance. Balance first, then mixture.
• Over-oiling the dashpots. A little is enough.
• Mixing up the idle screw and the mixture screw. Know which is which before you turn anything.
• Ignoring worn throttle shafts. They cause uneven running that no gauge will cure.
• Setting the idle too low. A slightly higher idle is kinder to the charging system and the gearbox.
• Revving the engine with the air cleaner off and calling it a road test.

The Road Test

Once the gauge and your ear agree, take the car out. A balanced pair should pull cleanly from about 1,500 rpm in a higher gear, settle to a steady idle at traffic lights and start without drama the next cold morning. Check the plugs after a few miles. Both should tell the same story.

Recheck the balance after 100 miles or so. New linkages settle, and a carburettor that was perfect in the garage can drift once it has done some real work. That is not failure. It is the ballet continuing.

Balancing SUs is not about forcing the car into submission. It is about listening to what the engine is telling you and making small, measured corrections. Do that, and the E-Type rewards you with the kind of smooth, torquey progress that made these cars so loved in the first place.