A carburetter is a mechanical device that meters fuel into an engine's incoming airstream by exploiting the pressure drop through a venturi. A well-tuned automotive carburetter holds the air-fuel ratio within roughly ±5% of stoichiometric (14.7:1 for gasoline) across idle to wide-open throttle. It exists because gasoline engines need a combustible vapour mixture, not liquid fuel, delivered in proportion to air mass flow. You see them on everything from a Stihl MS 250 chainsaw to a 1967 Holley 4150-equipped small-block Chevy.
Carburetter Interactive Calculator
Vary airflow, target air-fuel ratio, venturi pressure drop, discharge coefficient, and fuel density to size the main jet and see the metering signal.
Equation Used
The calculator uses the orifice equation for the carburetter main jet. Air mass flow and target AFR set the required fuel mass flow, while venturi pressure drop, fuel density, and discharge coefficient determine the jet area and equivalent round bore.
- Steady main-circuit fuel flow through a round sharp-edged jet.
- Gasoline is treated as incompressible liquid fuel.
- Venturi pressure drop is the available metering signal at the jet.
- Airflow is supplied as mass flow and target AFR is by mass.
The Carburetter in Action
Air rushes into the carburetter through a venturi — a calibrated throat that narrows then widens. As velocity climbs through the narrow section, static pressure drops (Bernoulli's principle), and that pressure drop pulls fuel up out of the float bowl through the main jet, an emulsion tube, and out the discharge nozzle into the airstream. The fuel atomises, mixes, and heads into the cylinders as a combustible charge. The float bowl itself is a constant-head reservoir — a brass or plastic float closes a needle valve when fuel reaches the right level, holding the fuel surface within about 1-2 mm of the discharge nozzle tip so the metering signal stays consistent.
A single venturi can't cover the whole operating range, so a real carburetter stacks multiple circuits. The idle circuit pulls fuel from a separate passage downstream of an almost-closed throttle butterfly, where manifold vacuum is high but venturi signal is nearly zero. The main circuit takes over once airflow is strong enough to generate venturi depression — typically above 1,200-1,500 RPM on a small engine. An accelerator pump squirts a shot of raw fuel during sudden throttle openings to cover the lag before the main circuit catches up, and the choke enriches the mixture for cold starts by restricting air upstream.
Get the tolerances wrong and the engine tells you immediately. A main jet 2 sizes too small (say 0.040 in instead of 0.045 in on a Holley) leans the mixture under load, and you'll hear pinging and feel a flat spot above 3,500 RPM. Float level 3 mm too high floods the bowl, fuel weeps from the discharge nozzle at idle, and the engine four-strokes — a wet, uneven exhaust note — until it fouls a plug. A torn accelerator pump diaphragm gives a hesitation off idle that no jet change will fix. The emulsion tube holes plug with varnish on engines stored with ethanol fuel, and the mixture goes rich and erratic in the cruise range.
Key Components
- Venturi: The calibrated throat that creates the pressure-drop signal driving fuel flow. Throat diameter sizes airflow capacity — a 28 mm Mikuni VM28 suits a 250 cc single, while a 1.75 in primary venturi on a Holley 4150 fits a 350 cu in V8. Smaller venturi = stronger signal but lower top-end airflow.
- Main Jet: A precisely drilled brass orifice that meters fuel at wide-open throttle. Sized in 1/100 mm or thousandths of an inch — a Keihin #100 main flows about 5% more than a #95. Bore tolerance must hold ±0.01 mm or the air-fuel ratio drifts noticeably between identical-looking jets.
- Float and Needle Valve: Maintains a constant fuel level in the bowl. The brass or Nitrophyl float rises with fuel level and closes a tapered needle against a seat when the level is correct. Float height set 1.5 mm too high causes flooding; 1.5 mm too low causes lean stumble in corners or under braking on a motorcycle.
- Throttle Butterfly: A flat disc on a shaft that controls airflow through the carburetter bore. At idle it sits 2-5° open, exposing the idle discharge port to manifold vacuum. Worn shaft bushings let air leak past at idle and cause a stubborn lean idle that resists mixture-screw adjustment.
- Idle Circuit: A separate fuel passage with its own pilot jet, idle air bleed, and mixture screw. Active below about 1,500 RPM where venturi signal is too weak to lift fuel through the main jet. A clogged 0.35 mm pilot jet on a Mikuni motorcycle carb is the single most common cause of lean idle and hard starting after winter storage.
- Accelerator Pump: A diaphragm or piston pump linked to the throttle that squirts a calibrated shot of fuel into the venturi during rapid throttle openings. Shot volume is typically 0.5-1.5 cc per stroke. Covers the millisecond-scale lag between throttle opening and main-circuit response.
- Choke: A second butterfly upstream of the venturi that restricts air to enrich the mixture for cold starts. Manual on small engines, thermostatic bimetal-spring operated on most automotive carburetters from the 1960s onward.
- Emulsion Tube: A vertical tube with cross-drilled bleed holes between the main jet and discharge nozzle. Air entering the bleed holes pre-mixes with fuel to form an emulsion that atomises better when it exits into the venturi. Hole pattern shapes the mid-range fuel curve.
Who Uses the Carburetter
Carburetters dominated gasoline engine fuel delivery from the 1890s through the 1990s and still rule the small-engine, motorsport, and vintage worlds today. They survive because they're cheap, mechanically self-contained, need no electrical power, and tolerate field repair with hand tools. You'll find them anywhere a builder values simplicity over the 1-2% fuel-economy edge electronic injection delivers, or anywhere certification rules — like vintage racing — mandate period-correct fuel systems.
- Outdoor Power Equipment: Walbro WT-series diaphragm carburetters on Stihl MS 250 and Husqvarna 450 chainsaws, where the engine operates in any orientation and a float bowl would flood.
- Motorcycles (vintage and small-displacement): Mikuni VM and BS-series and Keihin CV carburetters on Honda CB350, Suzuki DR650, and pre-2007 Royal Enfield Bullet 500.
- Motorsport (vintage and stock classes): Holley 4150 and 4160 four-barrel carburetters on NHRA Stock Eliminator and NASCAR Cup cars through the 2011 season, before EFI became mandatory.
- Light Aviation: Marvel-Schebler MA-3 and MA-4 carburetters on Continental O-200 and Lycoming O-235 engines powering the Cessna 150 and Piper Cherokee 140.
- Marine (outboard and inboard): Keihin and Mikuni carburetters on pre-2010 Yamaha and Mercury 2-stroke outboards, and Holley Marine 4-barrels on inboard MerCruiser 350 engines.
- Industrial and Standby Power: Zenith and Walbro carburetters on Briggs & Stratton Vanguard and Kohler Command engines used in pressure washers, generators, and welder-generator combos.
- Classic Automotive Restoration: Rebuilt SU HS6 and Stromberg 175 CD carburetters on Triumph TR6, MGB, and Jaguar XJ6 restorations.
The Formula Behind the Carburetter
The core sizing equation links venturi dimensions and pressure drop to fuel flow through the main jet. It tells you what jet size to start with and how the metering signal scales with airflow. At the low end of the typical operating range — small engines pulling under 30 g/s of air — venturi depression is barely 2-3 kPa and the idle circuit, not the main jet, controls mixture. At the high end on a tuned V8 pulling 400 g/s through 1.75 in venturis, depression reaches 8-10 kPa and the main jet is the dominant metering element. The sweet spot for main-jet sizing sits in the 60-90% throttle range where the main circuit is fully active but signal hasn't yet flattened against discharge nozzle choke.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁfuel | Fuel mass flow rate through the main jet | kg/s | lb/s |
| Cd | Discharge coefficient of the jet (typically 0.7-0.85 for a sharp-edged brass jet) | dimensionless | dimensionless |
| Ajet | Cross-sectional area of the main jet bore | m² | in² |
| ρfuel | Fuel density (≈ 740 kg/m³ for gasoline at 20°C) | kg/m³ | lb/ft³ |
| ΔPventuri | Pressure drop between the float bowl vent and the venturi throat | Pa | psi |
Worked Example: Carburetter in a Holley 4150 on a 350 cu in V8
You're jetting a Holley 4150 600 cfm carburetter on a mild 350 cu in Chevy small-block running at 5,500 RPM at the dyno. The engine pulls 350 g/s of air at peak power, you have 1.375 in primary venturis, and the stock 0.067 in (1.70 mm) main jet is reading 13.2:1 air-fuel on the wideband — slightly rich. You want 12.8:1 (a touch richer than stoichiometric for best power) and you need to know what jet size, what venturi depression, and what fuel flow that implies across the operating range.
Given
- ṁair = 350 g/s at peak
- Target AFR = 12.8 :1
- Cd = 0.80 dimensionless
- ρfuel = 740 kg/m³
- ΔPventuri at peak = 8500 Pa
- Current jet = 0.067 in (1.70 mm)
Solution
Step 1 — at nominal peak power (5,500 RPM, 350 g/s air), compute the required fuel mass flow for 12.8:1 AFR:
Step 2 — solve the venturi equation for the required jet area at 8,500 Pa depression:
Step 3 — convert area to jet diameter:
Round to the nearest Holley jet — a #69 jet (0.0691 in) lands you on target. That's 2 sizes up from the stock #67. At the low end of the operating range (cruise at 2,500 RPM, ~80 g/s air), venturi depression collapses to roughly 1,200 Pa and main-jet flow drops to about 4 g/s — barely active, with the power-valve and idle circuits doing most of the metering. At the high end with the engine over-revved to 6,500 RPM pulling 410 g/s, depression climbs to ~11,500 Pa and the same #69 jet flows about 32 g/s.
That low-end behaviour is why you can't tune cruise mixture with main-jet changes alone — the jet is barely flowing at part throttle. The high-end number tells you the carb is approaching its airflow ceiling: above 6,200 RPM you'll see the AFR curve flatten because the venturi signal saturates and a 750 cfm carb would start to pay back.
Result
Install a Holley #69 main jet (0. 0689 in / 1.75 mm) and you'll land at 12.8:1 AFR at peak power, where the engine makes its strongest, cleanest pull. The 4 g/s cruise number versus 27 g/s peak number shows just how little the main jet contributes at part throttle and why the power-valve circuit and idle mixture screws matter for drivability — the sweet spot for main-jet tuning sits between 60% and 90% throttle. If your wideband still reads rich after the jet swap, suspect (1) a stuck or leaking power valve dumping extra fuel below 6 in Hg manifold vacuum, (2) a float level set 2-3 mm high so fuel siphons past the discharge nozzle at moderate signal, or (3) a worn throttle shaft bushing letting air-bypass measurements go off — all three masquerade as a jetting problem and waste a Sunday at the dyno.
Carburetter vs Alternatives
Carburetters compete with electronic fuel injection (EFI) and, in the small-engine world, with diaphragm-pump carburetters versus throttle-body injection. The choice comes down to cost, fuel-economy targets, emissions requirements, and how much electrical infrastructure the application can support.
| Property | Carburetter | Port EFI | Throttle Body Injection (TBI) |
|---|---|---|---|
| Air-fuel ratio accuracy | ±5% open-loop | ±1% closed-loop with O2 sensor | ±2-3% closed-loop |
| Cost (small-engine OEM) | $15-60 | $200-500 | $120-300 |
| Cold-start behaviour | Manual or thermostatic choke, 30-60 s warm-up | Automatic, near-instant | Automatic, near-instant |
| Power requirement | Zero electrical | 30-60 W (pump + ECU + injectors) | 30-50 W |
| Fuel economy vs EFI baseline | 5-15% worse on transient cycles | Baseline | 2-5% worse than port EFI |
| Field rebuildability | Hand tools, $20-40 kit, 1 hour | Diagnostic scanner + injector flow bench | Scanner required |
| Ethanol tolerance (E10/E15 storage) | Poor — varnish plugs jets within 6 months of stale fuel | Good — sealed system | Good — sealed system |
| Tuning workflow | Jet swap + float adjust, mechanical | Laptop remap of fuel table | Laptop remap |
| Typical service life before overhaul | 500-2,000 hours on small engines | 5,000+ hours | 5,000+ hours |
Frequently Asked Questions About Carburetter
Air density falls about 3% per 1,000 ft of elevation, but the venturi pressure drop scales with air mass flow, not volume. The main jet keeps flowing roughly the same fuel mass while the engine ingests less air mass, so the AFR drifts rich by about 3% per 1,000 ft.
A trip from sea level to a 6,000 ft pass shifts a 13:1 mixture to roughly 11.2:1 — black-smoke territory on a small engine. The fix is a smaller jet (1-2 sizes down per 3,000 ft) or, on motorcycles like an XR650 ridden over mountain passes, a leaner needle clip position.
The discharge coefficient Cd isn't fixed at 0.80. Real Holley jets vary between 0.72 and 0.86 depending on jet entry geometry, jet age, and whether the bore is sharp or has been polished by years of fuel flow. A Cd of 0.86 instead of 0.80 flows 7.5% more fuel — exactly the 0.5-AFR-point error you're seeing.
Practical fix: trust the wideband, not the calculation. Use the formula to get within one or two jet sizes, then dial in the last bit by reading the meter under load.
Below about 7,000 RPM and 600 hp, a single 750-850 cfm Holley out-flows what the engine can use and gives a stronger venturi signal at part throttle, which means crisper throttle response and better mid-range fuel control. Dual quads only pay off when peak airflow exceeds about 950 cfm and you can stage the secondaries — otherwise both carbs run with weak signal at cruise and metering gets sloppy.
The classic 1960s tunnel-ram dual-quad setup looks fantastic on a show car but typically costs 0.2-0.4 second in the quarter mile versus a properly sized single 4-barrel on the same engine.
Idle circuit is clear — that's why it idles. The high-speed circuit through the main needle is plugged with ethanol-fuel varnish. On a Walbro WT diaphragm carburetter the high-speed needle and its tiny cross-drilled passage clog before the idle passage because the high-speed circuit sits idle (no flow) for months while the engine is stored.
Pull the H-needle, run carb cleaner through the passage, blow it out with compressed air, and reinstall to the factory preset (usually 1 turn out from seated). If the bog persists, the metering diaphragm has hardened — replace the carb kit, it's a $12 part.
A pure slide carb like a Mikuni VM ties throttle position directly to venturi area, so a sudden twist of the throttle drops venturi velocity, kills the signal, and the engine stumbles until airflow catches up. A CV carb decouples the rider's throttle (a butterfly) from the slide, which rises only as fast as manifold vacuum can lift it against a diaphragm spring.
The slide self-regulates to keep venturi velocity nearly constant — hence the name. You give up some peak response but gain bulletproof part-throttle metering, which is why 1980s-onward street bikes used CV carbs and racing bikes stayed with flat-slides.
Float level is the single most underestimated tolerance in carburetter rebuilding. The discharge nozzle sits 1-2 mm above fuel level by design — that small head difference is what stops fuel siphoning out at idle and what calibrates the metering signal at part throttle. A float set 3 mm high effectively reduces that head to zero or negative, and the carb dribbles fuel whenever the engine ingests air.
Use the manufacturer spec (usually given as a measurement from float bowl rim to float top, in mm) and a vernier caliper or a setting gauge. On a Holley with sight plugs you can verify with the engine running — fuel should just wet the threads of the sight plug, not pour out and not be invisible below.
Almost always the accelerator pump, not the jets. The main circuit needs 50-150 ms to spool up after the throttle opens because the venturi has to re-establish flow and the fuel column in the main well has to accelerate. The accelerator pump bridges that gap with a calibrated raw-fuel squirt.
Check the pump shot visually — pull the air cleaner, snap the throttle, and watch for a clean fuel stream from each pump nozzle. No squirt or a weak dribble means a torn diaphragm (Holley) or a stuck pump piston (Carter). On Holleys you can also change the pump cam colour to alter shot duration — a brown cam delivers fuel later than a pink cam, which fixes a lean stumble that an enrichment-only fix can't address.
References & Further Reading
- Wikipedia contributors. Carburetor. Wikipedia
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