It is already cut in half. Start the engine, open the throttle or close the choke, and watch the air and fuel inside. Drag to turn it, scroll or pinch to zoom.
Try itStart the engine, then open the throttle. Blue dots are air, amber dots are fuel.
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Scroll to take it apart
Three castings, two paths
Keep scrolling: the top lifts off, the base drops away, then the whole thing is sawn in half.
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Step 1Lift off the air hornThe top cover carries the choke, the float and the needle valve.
Step 2Drop the throttle bodyThe base holds the throttle plate and the idle screw.
Step 3Saw it in halfNow you can see the venturi, the fuel bowl and every passage.
Meet the parts
Twelve parts, three castings
Tap any part of the cutaway or pick a name below. The top casting feeds and meters the fuel, the middle one mixes it with air, and the bottom one lets the engine have it.
Hover or tap the cutaway. Amber rings mark every part.
Parts diagram
Two paths through one block
Numbers 1 to 6 follow the air from the top of the air horn to the engine. Numbers 7 to 17 follow the fuel from the hose to the spray.
Where the air comes from
The engine takes a breath
An engine is an air pump. As the piston slides down with the intake valve open, it leaves low pressure behind it, and the air outside pushes in to fill the space.
That rush of air, down through the carburetor and into the cylinder, is the only thing that powers a carburetor. There is no pump and no electronics. The harder the engine breathes, the faster the air moves and the more fuel it pulls along.
Intake. Piston down, intake valve open. Air and fuel are pulled in.
Compression. Both valves shut. The piston squeezes the mixture.
Power. The spark lights it, and the hot gas drives the piston down.
Exhaust. The exhaust valve opens and the piston pushes the burnt gas out.
The intake manifold carries the mixture from the carburetor to the cylinder.
The blue ring marks the fastest air and the lowest pressure, right at the nozzle.
The venturi
Squeeze the air, drop the pressure
Halfway down, the bore narrows into a smooth throat called the venturi. The same air has to get through a smaller hole, so it speeds up, and fast air presses less.
A small brass nozzle opens right in the throat. The float bowl next door is vented to the outside air, and that higher pressure pushes fuel up the nozzle and out into the rushing air, where it breaks into a fine mist.
0air speed in the throat, m/s
0pressure drop, mbar
Nofuel from the nozzle
–air to fuel, by weight
The float bowl
A tank that tops itself up
For the venturi to pull the right amount of fuel, the fuel must always sit at the same height: 12 mm below the nozzle tip in this carburetor. Holding that level is the float bowl’s only job.
A hollow brass float rides on the fuel. When the engine uses some, the float sinks a little and lets the needle drop off its seat. Fuel flows in from the tank until the float rises and pushes the needle shut again.
Fuel arrives along the top, drops past the needle valve and fills the bowl under the float.
12.0fuel below the nozzle, mm
Shutneedle valve
0fuel in, mL a minute
0fuel out, mL a minute
Every drop for the main circuit passes through the brass main jet at the bottom of the bowl.
Jets and mixture
A tiny hole sets the recipe
Petrol burns best with about 14.7 grams of air for every gram of fuel. The carburetor gets close to that recipe with holes drilled to a fraction of a millimetre, called jets.
Swap the main jet and you change what the whole engine is fed. Try a few sizes below, then drive the carburetor up a mountain, where the air is thinner but the jet still passes just as much fuel.
14.7air to fuel
100%air density
Tanspark plug tip
Smoothengine
The helpers
Four circuits, four jobs
The venturi on its own only works well in the middle of the range. Helper circuits take over for idling, for sudden acceleration and for cold starts.
The idle circuit
More to know
Kinds, history and care
Six kinds of carburetor
UpdraftAir flows upward through it. Early cars and tractors used them, and many small aircraft engines still hang one underneath.
DowndraftAir flows down, like the one on this page. The usual car carburetor from the 1930s to the 1980s.
SidedraftAir flows sideways. Popular on motorcycles and sports cars, often with one carburetor for every cylinder.
Constant velocityA piston lifted by suction makes the venturi grow as the engine needs more air, and draws a tapered needle out of the jet.
Two and four barrelTwo or four venturis side by side. The first ones handle gentle driving, and the others open for full power.
DiaphragmNo float at all. A flexible diaphragm meters the fuel, so chainsaws and leaf blowers run at any angle.
From a puff of vapour to a spray
Giovanni Battista Venturi describes how a fluid speeds up and loses pressure through a narrowing.
Karl Benz’s first car uses a surface carburetor: air passes over a pan of fuel and picks up vapour.
Donát Bánki and János Csonka in Hungary, and Wilhelm Maybach in Germany, patent float carburetors with a spray nozzle.
Herbert Skinner patents the SU carburetor, whose venturi changes size as the engine needs more air.
Engineer Beatrice Shilling’s small brass restrictor stops Spitfire and Hurricane engines cutting out in dives.
Four-barrel carburetors feed the big American V8s.
Electronic fuel injection takes over in new cars.
Still at work in lawn mowers, chainsaws, small motorcycles, generators, light aircraft and classic cars.
Keeping one happy
Don’t leave petrol in it for months. Stale fuel leaves a varnish that blocks the tiny jets, so drain the bowl or add stabiliser before storing it.
Clean jets with carburetor cleaner and compressed air, not wire. A scratched jet passes more fuel.
Keep the air filter clean. A clogged filter works like a half-closed choke and makes the mixture rich.
Look for air leaks at the base gasket and the manifold. Air that sneaks in there was never measured, so the engine runs lean.
Set the idle speed and the idle mixture with the engine fully warm.
Open the choke as soon as the engine will run without it.
Things worth knowing
Ice on a mild dayThe pressure drop and the evaporating fuel can chill the venturi by 20 °C or more. In damp air, ice forms even on a mild day, which is why pilots have a “carb heat” knob.
Miss Shilling’s orificeFloat carburetors lose their fuel under negative g. In 1941 a simple brass washer with a hole kept fighter engines running when pilots pushed into a dive.
Keep it levelA float bowl needs gravity to hold its level, so tip it far and it floods or runs dry. A diaphragm carburetor doesn’t care which way up it is.
14.7 is chemistryThat ratio is just enough oxygen to burn every bit of the petrol. A little richer gives the most power, a little leaner the best economy.
What SU stands forSkinner Union, after the brothers who invented it. Classic Minis and MGs breathe through SUs.
No computer neededThe whole thing is controlled by air pressure, gravity and the size of a few holes. It keeps working when every electronic part has failed.
Every Mechanism
