Carburetor Bore Sizing (Jennings) Calculator

What is The Physics of CarbBoreSizing?

The technical foundation and mathematics behind the CarbBoreSizingCalc tool.

Mathematical Foundation

Gordon Jennings Carburetor Sizing Formula

Bore_{mm} = K \times \sqrt{C \times RPM}

Bore_{mm}

= Target internal diameter of the carburetor's main venturi restriction (mm).

K

= Volumetric Efficiency (VE) constant. K=0.65 for mild trails, K=0.82 for high performance, K=0.90 for extreme GP racing.

C

= Displacement of a SINGLE cylinder, converted strictly into Liters.

RPM

= The exact target engine speed where peak horsepower is desired.

Laws & Principles

Vacuum vs Volume (The Fatal Tuning Trap): Inexperienced builders assume a bigger carburetor instantly makes more power because it allows 'more volume.' This is catastrophically wrong. Carburetors do not run on volume; they run on Bernoullis Vacuum Velocity. If a 125cc cylinder sucks air through a massive 40mm pipe, the air speed drops so low that it physically cannot produce enough vacuum to suck liquid gasoline vertically up the main jet tube. The engine leans out and violently stalls.
The Venturi Bottleneck Demand: A carburetor must be mathematically sized as a deliberate 'bottleneck.' By forcing engine displacement to violently squeeze through a perfectly calculated choke point (Bore), the air is forced to accelerate to supersonic speeds over the needle jet, ripping the gasoline into a highly atomized explosive mist.

Step-by-Step Example Walkthrough

" A tuner is building a nasty fast 125cc 2-stroke motocross bike targeting peak power at 10,500 RPM. They want a high-performance VE profile. "

1. Select Volumetric Efficiency Constant: High Performance dictates K = 0.82.
2. Convert cc to Liters: 125cc / 1,000 = 0.125 Liters.
3. Calculate Displacement-Speed Load: 0.125 L * 10,500 RPM = 1,312.5.
4. Calculate Airflow Core: Square root of 1,312.5 = 36.228.
5. Apply Velocity Demand Metric: 36.228 * 0.82 (K-Constant) = 29.7 mm.

Final Result: The engine demands a strict 29.7mm bore limit. The builder should purchase a standard 30mm carburetor to maintain violent jet-siphoning vacuum; slapping on an aftermarket 36mm carb will absolutely destroy the engine's throttle response.

Quick Answer: How do you size a carburetor for maximum horsepower?

The ideal carburetor bore size is calculated using Gordon Jennings' empirical formula: Bore (mm) = K × √(Displacement in Liters × Peak RPM), where K is the Volumetric Efficiency constant (0.65 to 0.90). Crucially, a carburetor must act as a strict venturi bottleneck. If you oversize the bore to chase "more volume," the air velocity through the venturi throat plummets, Bernoulli vacuum drops, and the carburetor physically loses the ability to siphon gasoline out of the float bowl, resulting in a severe bog or engine stall. Bigger is absolutely NOT better.

Target VE Constant (K) Reference Cheat Sheet

Selecting the correct volumetric efficiency coefficient is the critical first step in determining your ideal carburetor bore. A higher K-value implies the engine has aggressive porting and race cams capable of moving air violently.

Engine Build Type	K Constant	Typical Characteristics
Stock Utility / Lawn Equipment	0.60 – 0.65	Bone stock Predator 212cc, GX390, mild Briggs flatheads. Low RPM governed limit (~3600 RPM).
Mild Trail / Commuter	0.70 – 0.75	Mild stage 1 kits, ungoverned utility engines (5000 RPM), stock mini-bikes, small CC scooters.
High Performance Moto	0.82	Performance 2-strokes, big valve heads, aggressive cams, racing exhaust pipes. High mid-range punch.
Extreme GP Race / Shifter Kart	0.90	Maximum radical porting, high RPM screamer (10k+ RPM), perfectly expansion-chamber tuned 2-strokes.

Pro Tips & Common Sizing Mistakes

Do This

✓Round down to the nearest available carburetor size. If the math calculates a 31.4mm required bore, buy a 30mm carburetor, not a 32mm. The slightly smaller bore maintains high-velocity air speed, ensuring crisp, instant throttle response off the bottom end.
✓Account for total intake tract flow. A mathematically perfect carburetor size restricts nothing if the intake manifold, cylinder head port, and valves are smaller than the carb bore. The intake tract must taper perfectly smoothly from the carb bellmouth down to the valve seat.

Avoid This

✗Never over-carb a stock block. Throwing a massive 30mm Mikuni clone on a bone-stock 212cc engine with standard valves is a fatal error. The engine cannot physically move enough air to create venturi vacuum, leading to uncontrollable rich/lean bogs.
✗Don't ignore the pilot jet circuit. When changing carburetor bore sizes, the vacuum signal changes drastically across the entire throttle sweep. A larger bore drops vacuum pressure at low rpm, almost always requiring a noticeably larger pilot jet to compensate.

Frequently Asked Questions

Why does my engine bog down when I hit the throttle with a bigger carburetor?

This is classic "bog." By installing a carburetor with too large of a bore, the air velocity inside the venturi throat drops too low. According to Bernoulli's principle, low air speed equals low vacuum pressure. If the vacuum pressure drops, the carb literally lacks the physical negative pull required to suck heavy liquid gasoline up out of the main jet tube. The engine gasps pure air, goes violently lean, and dies.

Should I buy a round slide or flat slide carburetor?

Flat slide carburetors (like Mikuni TM or Keihin FCR series) generally offer far superior throttle response because the flat guillotine blade creates less turbulence directly behind the slide in the venturi bore when compared to a massive, bulbous round slide blocking the port. For racing applications, the flat slide is vastly preferred, while round slides are generally cheaper and highly reliable for standard trail/utility use.

What does the K-constant represent in the Jennings formula?

The K Constant represents Volumetric Efficiency (VE). This is a physical modifier representing how aggressively your engine moves air. A stock utility motor with small valves and mild camshafts struggles to pull air efficiently, earning a low K value (0.65). A high-revving motocross 2-stroke with a massive tuned-pipe and huge reed valves moves air violently, earning a high K value (0.85+).

Do I calculate bore size for twin or 4-cylinder engines based on total displacement?

No. If each cylinder has its own individual dedicated carburetor (e.g., standard multi-carb motorcycle setups), you calculate the bore using the displacement of just a single cylinder. For example, a 1000cc 4-cylinder engine uses 250cc logic in the Jennings formula for each of its four carburetors.

Carburetor Bore Sizing (Jennings)