John Simister: Don't be afraid to get your hands oily

If a bulb blows in some new cars, the car has to be hooked up to a diagnostic system after the bulb is replaced so the fault can be cleared in the car's computer. (Tip: sometimes just disconnecting the battery and leaving it for a couple of hours before reconnecting, can also clear the fault code. I fixed a flashing odometer display in my Fiat 500 Twinair by exactly this method, and the car also went a bit better afterwards. But on your head be it.)

Classics tend not to have fault codes, which didn't really enter the car-scape until the mid-1980s along with digital engine management. They don't talk to you by digital text, as is the modern way for cars and people alike. They talk to you in real time, by sound and vision. Many and subtle are the vocalisations of a classic car's engine but listen to it and it will tell you a lot.

Nowadays, ever fewer people have the skills or, crucially, the confidence to tweak an engine's settings. They leave it to an expert, or they put up with an engine which doesn't 'feel right' without knowing why. But there's nothing much to be scared of, especially if you make it a rule to remember, or mark, your starting point so you can return to it if you wish you'd never started. 

The obvious things to adjust in an old, fully-analogue engine with no electronics are, in this order, the gap between the contact-breaker points, the ignition timing, the idle speed and the fuel/air mixture. If you have electronic ignition, perhaps a conversion from the original points, then that's one less thing to adjust.

It's important to have the spark plugs sparking at the right time before you adjust the carburettors. That's because ignition timing is an 'absolute' setting, with two marks lining up, whereas fuel/air mixture adjustment is something you do until the engine is responding correctly, and if the timing is wrong that correct response might elude you. And that mixture adjustment is the really satisfying part of the fiddling I mentioned at the start.

So, how can you tell if your engine's a bit off-song? If it's actually worn out, there will probably be smoke, rattles or knocks to tell you. Otherwise, it's a matter of sound and feel, and comparing that sound and feel with how a similar example of your make and model sounds and feels will help establish any differences.

Many are the combinations of nuances, but, broadly speaking, if the engine feels harsh, won't rev willingly and, in extreme cases, suffers from pinking (a metallic rat-a-tat) under load, the ignition timing is too advanced. Retarded timing, conversely, brings sluggishness across the speed range and a soft throttle response.

Uncertain idle and a hesitation when you try to accelerate? The fuel/air mixture is too weak. Smelly exhaust, hefty fuel consumption and a rhythmic cycling of idle speed ('hunting')? The mixture is too rich.

In variable-jet carburettors such as SUs and Strombergs, adjusting the mixture at idle speed will affect the mixture across the engine's whole operating range, albeit to a diminishing extent as the speed and load go up. In fixed-jet carbs – Solex, Weber, Dell'Orto, Zenith and the like – the mixture adjustment affects only the idle and the transition into enough speed and torque to get the car moving, and – to a degree – the crispness of a snap throttle opening. If there are response problems beyond that stage, as there were with my Singer, then the carburettor probably has the wrong jets and venturi.

So, your correctly-ignition-timed, mechanically healthy engine is idling, not very well. You can hear it occasionally stumbling, or the revs are wandering around a bit, or the exhaust note is uneven. So you cautiously screw the mixture control in a little (fixed-jet) or upwards (variable-jet), and listen to what happens. The engine may run worse, in which case the mixture was already too lean, or better because it was too rich.

You might come upon an obvious sweet spot as you screw the adjuster in this direction, a combination of the highest speed and the smoothest engine note, but if you don't you can unscrew the adjuster and see if the sweet spot can be found in that direction. It should be in there somewhere unless something in the carburettor is worn out. If your engine has more than one carburettor, the same method still applies – but you need to be sure that both/all the throttles are opening exactly the same amount.

That's what balancing the carburettors achieves, a job shrouded in mystique but actually quite straightforward. Basically, with idle speed adjusters screwed right out, make sure all the throttles are shut, then screw in the idle adjusters until they just touch their levers, and thereafter turn each exactly the same amount until you have the idle speed you want.

As you alter the mixture and idle controls you will hear and feel the engine respond, and you will sense when it's running sweetly, smoothly, happily. That, though, might not be the end of the story. The carburettors in my Singer had in an earlier life been used on a larger-engined Singer Roadster whose engine's sucking abilities were different from those of my Le Mans. The late guru of pre-war Singers, Ian Blackburn, warned of a 'massive flat spot' if using these Solex 30 FAIs in Roadster spec on a Le Mans, and he was right. It idled beautifully, but the only way actually to get it moving was to pull the choke right out.

So I followed his recommendations: 100 main jets instead of 95 (richer), 200 air-correction jets instead of 180 (to atomise the extra fuel), 21mm venturis instead of 22mm (slightly smaller in diameter but a significant reduction in cross-sectional area, to raise the fuel/air mixture speed as it's sucked into the engine and so suck harder on the main jets). Result: an engine which pulls with vigour and without hesitation, as crisply as you like.

Carburettors. What intriguing things they are. And really not terrifying at all.