A carburetor meters fuel into the engine’s air stream with a venturi: the air speeds up as the passage narrows, its pressure drops, and the lower pressure pushes fuel up through a small jet. Because the pressure drop varies with the square of the air speed, halving the throat diameter quadruples the speed and raises the suction sixteenfold, so the throat size sets how the engine breathes at the top of its speed.
Part of Internal Combustion
How a venturi carburetor works, how to size its throat and jet from the engine’s air flow, and how it must change for alcohol.
A Venturi Turns Air Speed Into A Pressure Drop
Air flowing through a narrowing passage speeds up, and the faster air has lower pressure. The pressure drop is ΔP = ½ × ρ × v², with ρ the air density and v the speed in the throat (Bernoulli’s relation, a recalled result). A jet, a small fuel passage in the throat, connects to a fuel bowl at atmospheric pressure, so the pressure difference pushes fuel out into the air stream, where it breaks into droplets and vaporises as it flows toward the cylinder.
Four parts do the work. The float chamber keeps the fuel level just below the jet exit, so it cannot flood or run dry. The jet sets the fuel flow for a given suction. The throttle is a flap below the venturi that limits air, and so power. The choke restricts air for starting, giving a richer mixture when cold. The fuel and air ratio for burning is fixed by chemistry, not by the carburetor, which only tries to hold it. For ethanol it must change: the corpus gives the chemically correct mixture as 14.7 kg of air per kg of gasoline and 9.0 per kg of ethanol in Carburetor & Injection Modifications.
What You Need
- A salvaged carburetor of the right size, or the metal and drills to make a simple one.
- Drills in small steps, or ready-made larger jets, to adjust the fuel flow.
- A float and needle valve that seat cleanly.
- Fuel line, fittings and seals that suit the fuel; see Fuel Line Material Compatibility.
- A way to read the engine, such as plug colour and exhaust.
Method: Sizing A Venturi From The Engine’s Air Flow
Step 1: Find the air the engine breathes. For a four-stroke, displacement × rpm ÷ 2 is the volume per minute at best; the real figure is less.
Step 2: Choose a throat area and compute the speed. v = air flow ÷ throat area.
Step 3: Compute the suction. ΔP = ½ × ρ × v², using ρ of about 1.2 kg/m³, a recalled figure.
Step 4: Check the fuel can rise. The pressure drop must lift the fuel from the bowl to the jet exit through the jet; a very small ΔP at low speed is why a separate idle circuit is needed.
Step 5: Set the jet by trial. Enlarge a jet in steps and read the plugs and the exhaust, as the corpus does in its alcohol conversion article.
Worked Example: A Small Engine And A Halved Throat
Take a 200 cc single-cylinder four-stroke at 3,000 rpm, assumed values, with a 15 mm throat.
- Air per minute, at best: 0.2 L × 3,000 ÷ 2 = 300 L = 0.3 m³, so 0.005 m³/s.
- Throat area: π ÷ 4 × 0.015² = 1.767 × 10⁻⁴ m².
- Speed: 0.005 ÷ 1.767 × 10⁻⁴ = 28.3 m/s.
- Suction: ½ × 1.2 × 28.3² = 480 Pa. As a column of petrol of about 0.74 kg per litre (the density the corpus assumes), this lifts 480 ÷ (740 × 9.81) = 0.066 m, 66 mm.
Now halve the throat to 7.5 mm.
- Area falls to a quarter, so the speed is 4 × 28.3 = 113 m/s.
- Suction: ½ × 1.2 × 113² = 7,685 Pa, which is 16 times the first.
So a small throat gives strong suction and good fuel metering at low speed but restricts the air at high speed, and a large throat does the opposite. The throat is a compromise, and a carburetor from a bigger or smaller engine is the wrong one. The air figure is an upper bound; real engines breathe less.
Changing A Carburetor For Ethanol
Ethanol needs more fuel for the same air. The corpus works it out as 1.53 times the volume and a jet about 26% larger in diameter than for gasoline, and recommends going up about 10% at a time and reading the engine. The carrier says to enlarge the main jet by 30 to 40%, which the corpus article notes is below its arithmetic; its number is the one to start the search from. The jet is a small drilled hole, so a mistake is permanent; keep the old jet.
Common Mistakes
- Fitting a carburetor from a much larger engine. The speed in the throat is too low for the fuel to be drawn properly.
- Drilling the jet to its final size in one go. If it is too rich or lean, there is no way back.
- Leaving rubber seals and floats that ethanol attacks. They swell and leak.
- Mounting the bowl above the intake. Fuel then drips into the engine.
Safety
A carburetor is a fuel source beside a hot engine
A leak or an overflow puts liquid fuel on a hot manifold, and ethanol burns with an almost invisible flame, per Fire Safety. A lean engine can backfire through the carburetor into the engine bay. Keep sand or an extinguisher to hand, check lines for leaks at every start, and never prime or pour fuel into a hot engine.
What’s Next
- Learn next: Magneto Ignition — lighting the mixture the carburetor delivers
- Combine with: Fuel Systems — the rest of the delivery path