Confessions of a part-time girl

What really happens when you press the Loud Pedal, and what to do if it doesn't
7:57 a.m. -- 2008-09-06

So the thing about the Solex carburettor, right, is its apparent complexity. It features a mass of linkages, springs, valves and - offering a bit of potential hilarity - 'diaphragms'. Over the years, successive models have added more bells and whistles to the basic "air intake tube that gets petrol dribbled into it", each no doubt offering an improvement but at the same time making it more and more of a nightmare to the stranded motorist.

What you have to realise is that you're not looking at a mysterious, complicated machine deliberately designed by a team of eggheads. They're not even proper eggheads; they were from Paris, and we all know that French engineers should have stuck to souffl�.

Stop thinking of it as a baffling assemblage of metal and rubber; it's an air vent and a water pistol. Well, if want to use up today's ration of pedantry right here and now, it's a "petrol pistol". The principle is the same.

You want to go faster, you open the air vent, allowing the engine to breathe more easily and therefore work harder. It's a lot like enfinblue cycling up a hill. If she's annoying you because she's going too fast (don't you just hate healthy people?) simply give her a snorkel to breathe through. If she's still going too fast, add a flap to control the rate at which she can draw air into her lungs.

This choking off of the air supply is precisely why it's called a 'throttle'. Nasty verb!

Now unfortunately, engines don't just run on air. They also like petrol* but that is supplied by an astonishingly primitive means. The carburettor has a little metal pipe sticking out into the middle of the airflow, rather like a drinking straw. It constantly dribbles a little bit of petrol... and the harder the engine sucks air, the more petrol gets drawn out of the pipe. It's kind of burpy but it works.

Now, the water pistol part. Most people, when they put their foot down, want to shift. They don't want a gradual speeding up, once the engine realises it's able to breathe more easily... they want to move now! For this reason, Solex added a water pistol, pointed right down the engine's throat. And connected the air vent to the trigger of the water pistol. When you open the air vent, you give the engine a squirt of neat petrol. "Yum!" it responds, and bursts into life.

With enfinblue, you would achieve the same effect with a water pistol filled with espresso. You would, of course, need to construct some kind of linkage between the device that controls the air vent and the trigger of the water pistol, to ensure that the squirt of espresso arrives at just the right time.

And that's effectively all there is to it. I mean, there's nothing mysterious.

A classic failure mode is that the regular dribble of petrol ceases, due to a bit of s__t in the petrol. (The word is silt, actually. You have a dirty mind.) You've simply got a blockage in that tiny 'drinking straw'. Trouble is, each time you open the throttle in an effort to make the engine work, the 'water pistol' delivers a squirt of petrol... which is exactly what the engine needs. "Rowr!" it exclaims, in a businesslike manner... and then lets you down. You're in a ludicrous situation where you have to frantically pump the gas pedal every couple of seconds, or the engine dies. I know people who have tried to drive like that.

The solution to the problem lies in a simple bit of disassembly, after which you should be able to clear the blockage by blowing in the offending tube, against the flow.

If blowing through little tube was going to mess up my lipstick, I'd probably use the pump I keep for blowing up tyres.

If the disassembly job threatened to mess up my manicure, I'd call the Royal Automobile Club. (Not that I've worn false nails in a very long time, but you get the idea.)

Today's entry was inspired by fifi, who said "I have a feeling that you could make even car repair interesting!" (See how wrong you were? Let that be a lesson to you.)

Warning: the knowledge imparted here is completely out of date now, because even the humblest cars have had fuel injection for fifteen years. (Which works more like an array of miniature super-soakers. Complicated, expensive and impossible to fix by the roadside, but kinder to the planet.) So no more carburettors, and fewer opportunities for me to act the knowledgeable hero to damsels in distress. This neatly sums up the trouble with the modern world: it's more efficient, but it's much harder to help people.

* Some engines run on a substance called 'diesel'. This is like drinking cider instead of beer, and must be considered separately at some levels. Not part of today's lesson!

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