The joy of carbs

I guess it’s hard now to imagine a world where electronics didn’t exist, where electricity itself was still a new technology. But that was exactly the world of 1885, when Karl Benz devised what is now considered the first carburettor. He patented it a year later – the same year that he demonstrated what is generally (if grudgingly, in France) accepted as the world’s first car – the Benz Patent-Motorwagen. Intriguingly, this first car did not use Benz’ own invention. Instead it relied on a bunch of fibres sucking liquid fuel up and letting it evaporate naturally before being sucked into the combustion chamber – a decidedly rough-and-ready method of fuelling an engine, but typical of the technology used in the various Lenoir, Otto, Brayton and other engines that preceded Benz’. These engines used various means of getting fuel into the cylinder by evaporation – none of which worked very well.

But Benz, as always, stood on the shoulders of the giants who had gone before him when he hit on the idea of the carburettor. In 1738, a full 150 years before his invention, a brilliant Swiss mathematician figured out the phenomenon that still bears his name – Bernoulli’s principle. I’ll resist my penchant for inserting a mathematical formula here, as it can be described in plain English – it states that an increase in the speed of a fluid occurs simultaneously with a decrease in the pressure of that fluid. In other words, make water, or air, flow faster and the pressure drops. A few decades later, an Italian with the equally mellifluous name of Venturi applied Bernoulli’s principle to the specific case of fluids slowing through a constriction – think of a wasp-waisted or Coke-bottle shaped pipe. Venturi showed that Bernoulli’s principle means that the pressure drops at the narrowest point or ‘waist’ of such a gadget – now known as a venturi (having lost the capital V, sadly).

It’s worth remembering here that Bernoulli and Venturi lived and died in a world of candlelight and oil lamps. When they stepped out of their laboratories at night to take a breather from all that hard thinking and even harder maths, they stepped into a world that was darker than we can imagine. They would never see an electric light, never mind a motor car. Yet the work of these gentlemen of science is what led to the aerofoil, the rear diffuser, ground effect… and the carburettor.

Because, with a few exceptions, most carbs use the venturi effect. Intake air is ducted through a venturi in the body of the carb. If you drill a small hole – or ‘jet’ – in the waist of the venturi, and feed that hole with a suitable supply of petrol, the pressure drop or suction predicted by Dottori Bernoulli e Venturi will draw the fuel into the airstream, conveniently atomising it in the process. You now have an air-fuel mix – which, if you have been foresighted enough to rig the whole shebang on the correct side of your engine, will provide the required bang in the familiar suck-squeeze-bang-blow routine that brought us all here in the first place.

Better still, the more air you flow through, usually by opening a simple flap or ‘butterfly valve’ attached to your right toe by a pedal and a cunning arrangement of rods, levers and cables, the more suction is created. Even more amazingly, if you get the dimensions of the intakes, venturi and jets just right, the engine will run at roughly the correct ratio of air to fuel right through its rev and throttle opening ranges. As usual, some folks were not happy with the power delivered by a single-venturi setup and dreamt up what came to be known as the ‘twin-choke’ carburettor, with two venturis in parallel and a bridge-piece to balance them. Twin-choke carbs allow more air (and hence fuel) to flow, and could be tuned to run smoothly at low revs, yet feed the hungry beast enough of the good stuff at high rpm. Clever stuff indeed.

But their cleverness alone does not quite explain the attraction of carburettors. I can think of no other component, with the possible exception of the supercharger, that has quite the romance of a carburettor. Only a big belt-driven supercharger sticking loudly and proudly through the hood of an American muscle car can approach the sheer fetish-value of a bank of big, thirsty carburettors lined up alongside a willing cylinder head. Not the turbocharger, hidden as it often is away in swirls of discreet ducting, nor even the ‘hold my pint’ menace of an absurdly large diameter exhaust outlet on some JDM sled. There is just something about carburettors that speaks to many of us.

Possibly it’s the sheer mechanical nature of them – the complexity of their throttle valves, the precision of their mess of springs, diaphragms, jets, needles, floats and seals. They hold the same appeal as mechanical watches – a quartz movement may be better in every objective way, but there is something about the escapements and balance wheels of a wind-up watch that appeals to us in the same way as the f-stop adjustment on a camera lens or (so I’m told) the clackety-clack action of a rifle bolt.

Maybe it’s their tactility – there are very few things in life that are more pointlessly satisfying than to reach your hand (carefully, mind) into the engine bay of a vehicle with a saucy set of carbs, twist the throttle linkage with your fingers and hear the greedy hiss of the air being sucked in and the revs rise eagerly, before letting the butterflies snap closed again and the engine settle down to a frustrated idle. Don’t ask me why – just try it and tell me I’m wrong.

And, of course, we could spend a whole other article debating the greatest carbs of all time. Their very names speak to our souls – SU, Weber, Stromberg, Holley, Dellorto: say it out loud, it’s like a prose poem to petrol. Maybe it was the big old updraft SUs that fed the greatest engine of them all (the Rolls-Royce Merlin, obvs) and whose negative-g foibles were solved by one Tilly Shilling. US readers will now be shouting ‘it’s the Holley 4-barrel!’ at their screens and they have a good point – they’ve powered millions of snuffling, crackling and rumbling V8s. Or maybe the three (or sometimes six) downdraft Webers that conducted the orchestra of music and power that is the Ferrari Colombo V12? Valid arguments can be made for them all.

But my personal favourites are the Mikuni carbs that have been used on most Japanese motorcycles since the late 1940s through to the 2000s. Even on big ‘litre’ bikes, they are tiny – each unit smaller than your fist. Stripping down and rebuilding a set of Mikunis is tricky, fiddly work, fraught with pitfalls like stripped threads on soft brass screws and puncturing delicate diaphragms and seals, but deeply fulfilling despite, or because of the above. Balancing them to flow the right amount of the magic mixture across four little 250cc cylinders involves Captain Nemo-esque tubular pressure gauges filled with mercury, endless patience and a bit of that annoying attribute called ‘feel’. If you do it right, and the engine fires up cleanly, warms up without bitching and then screams all the way to its terrifying redline, you will feel like all your birthdays have come at once.

So, over to you, gentlefolk of Ti. Breathe in deeply, exhale and release your inner nerd – you are in what I believe the kids these days call a safe space. What’s your favourite carburettor?