The Carburettor: A Venturi, a Jet and a Float

Print a venturi tube: 12 mm bore narrowing smoothly to a 6 mm throat and opening gently back out, with a 2 mm side hole at the throat into which a fine tube is sealed. Lead that tube down into a cup of coloured water 30 mm below the throat — the cup is your float bowl, and the height is the level the float holds. Blow air through with the air compressor through the regulator, starting very low. At some air speed the water climbs the tube and is torn into a spray at the throat. Raise the cup and it starts at lower air speed; lower it and it needs more. That is why a carburettor needs a float: the fuel level must not change. **Water only.** Never demonstrate this with petrol: an atomised fuel spray is an explosive mixture.

**Wilhelm Maybach, 1893** — the spray-nozzle carburettor with a float chamber, developed for Daimler engines, the pattern for carburettors for most of the next century. No patent number is asserted here. Earlier 'surface' and 'wick' carburettors had simply let air pass over or through petrol, and their mixture wandered with temperature and fuel level. Fuel injection replaced the carburettor in cars from the 1980s, for precise mixture control and emissions; carburettors survive on small engines. **Honest limits.** The simple venturi law fails at very low airflow (idle) and during sudden throttle opening, so real carburettors carry extra circuits. The mixture shifts with altitude, because air density falls. And petrol vapour is dangerous: carburettors are worked on outdoors, away from flame.