Adjust your SU carb

Your classic car just isn’t running quite right. The engine is assembled correctly and the ignition system sparks at the right time, but the performance still feels a bit flat. Or maybe there’s a bout of dyspepsia when you try to accelerate, or the tickover is rough and maybe smelly. Time, perhaps, for an exploratory tweak of the carburettor.

This micro-guide concentrates on variable-jet, constant-vacuum carburettors such as SU or Stromberg CD. Fixed-jet units – Weber, Dell’Orto, Solex, Zenith or suchlike – are very different beasts, to be covered another time.

Don’t be afraid of adjusting and experimenting. Typically you’ll be turning the brass nut (SU) or screw (Stromberg) at the base of the jet assembly, and you’ll be adjusting the idle speed via a screw on the throttle linkage. Provided you count the number of ‘flats’ you’ve turned an adjuster, you can always return to the starting point.

Start by checking the dashpot has enough oil in it to let the carburettor piston’s damping system work properly. Next, get the engine fully warmed up.

Now, check the fuel/air mixture by lifting the carburettor’s piston about a millimetre and holding it there. If the engine slows, the mixture is too weak. If it speeds up, too rich. If it stays the same, or speeds up just slightly and quickly returns, then it’s set correctly. Some SUs have a built-in lifting pin for this purpose. Otherwise, you’ll need to remove the air filter so you can lift the piston with a slender screwdriver or similar.

If the mixture seems too rich, screw the brass jet adjuster upwards, a flat at a time, and listen for a change in the engine note. You are weakening the mixture by raising the jet and reducing the size of the hole into which the tapered metering needle fits, and beyond a certain point the engine note will stumble. Stop there, go back two flats, do the piston-lift test, repeat as needed.

If the mixture seems too weak, screw the adjuster downwards. The engine note becomes smoother and the revs rise; continue until the revs drop again or the engine starts to ‘hunt’ rhythmically, then turn the nut back up to the sweet spot of smoothness, which is likely to be around a flat or two richer than the start of the weak-mixture stumbling. Again, check and fine-tune with the piston-lift test.

Reset the idle speed if it has changed, refit the air filter if necessary and go for a drive. Better? We hope so.