Part 2 – How to Calculate Weber Carburettor Jet Sizes for Superethanol E85

The main modification required when converting to Superethanol E85 is to increase the amount of fuel delivered by the carburettor.

A common misconception is that you simply need to increase the jet size by 30%, since the engine consumes approximately 30% more fuel.

In reality, this reasoning is incorrect.

Weber jets are calibrated according to the diameter of their orifice, expressed in hundredths of a millimetre. For example, a main jet marked 130 has an orifice diameter of 1.30 mm.

However, the flow rate of a jet is proportional to the surface area of its opening, not its diameter.

To obtain approximately 30% more fuel flow, the diameter only needs to be increased by around 14 to 15%.

The following formula provides an excellent starting point:

E85 Jet = Petrol Jet × 1.145

This formula gives a calibration very close to what is required on the majority of naturally aspirated engines running on Superethanol E85.

Main Jet Conversion Table

This table provides an excellent starting point.

The final calibration will always depend on the engine specification:

• engine capacity;
• venturi diameter;
• camshaft;
• compression ratio;
• exhaust system;
• altitude;
• intended use (road, competition, rally, circuit);
• actual ethanol content of the fuel (E70 to E85 depending on the season).

The mixture should ideally be checked using a wideband lambda (AFR) sensor.

Good to Know: Why Doesn’t a Weber 130 Jet Become a 160 Jet When Running E85?

A common misunderstanding when converting a Weber carburettor to Superethanol E85 is to assume that, because the engine requires around 30% more fuel, the jets should also be increased by 30%.

This method is incorrect.

The numbering of Weber jets does not represent fuel flow. It represents the diameter of the jet orifice in hundredths of a millimetre.

Therefore:

• a 130 jet has a diameter of 1.30 mm;
• a 150 jet has a diameter of 1.50 mm.

Jet flow depends on the cross-sectional area available for fuel passage. Since this area increases with the square of the diameter, a relatively small increase in diameter already creates a significant increase in flow.

Example:

• petrol jet: 130 (1.30 mm diameter);
• required flow increase: approximately 30%;
• theoretical required diameter: approximately 1.49 mm;
• equivalent Weber jet: 150.

You should therefore not directly replace a 130 jet with a 160 jet, because this represents an increase in diameter of around 23%, resulting in a fuel flow increase far greater than required for E85.

The correct method is to apply the formula:

E85 Jet = Petrol Jet × 1.145

This approach provides a consistent baseline before final engine tuning.

Essential Adjustments and Modifications

Replacing the main jets is only one part of the conversion.

To achieve smooth carburation throughout the entire operating range, several carburettor circuits must be checked.

1. Main Jets

The main jets determine mixture richness when the main circuit is operating.

Their diameter should be increased by approximately 14 to 15%, according to the formula above.

They form the foundation of any E85 conversion.

2. Idle Jets

The idle circuit supplies fuel not only at idle, but also throughout the progression phase until the main circuit becomes active.

It is therefore essential to adapt it for E85.

Increasing the idle jet diameter by 10 to 20% is generally a good starting point.

A correctly calibrated idle circuit provides:

• stable idle speed;
• smooth progression;
• crisp throttle response at small throttle openings;
• improved drivability.

3. Air Correctors

Air correctors influence mixture richness at high engine speeds.

Because E85 requires more fuel, it is generally recommended to reduce their size by 10 to 20 points in order to limit mixture weakening as engine speed increases.

This value remains indicative and must be confirmed during testing.

4. Emulsion Tubes

Emulsion tubes control how air mixes with fuel inside the emulsion well.

In many cases, the original tubes provide excellent results with E85.

If the transition between the different carburettor circuits is not satisfactory, it may be useful to test alternative references (F2, F9, F11, F16, depending on carburettor model and engine specification).

The choice of emulsion tubes is an optimisation adjustment rather than a modification that is always required.

5. Needle Valve, Seat and Float Level

Since the engine consumes approximately 30% more fuel, the fuel supply system must be capable of maintaining a constant float bowl level.

Depending on engine power output, it may be necessary to install a larger needle valve:

• 200;
• 225;
• 250.

The float adjustment and float level should also be checked.

An insufficient float level causes the mixture to become lean at high RPM, even when the jets are correctly sized.

6. Accelerator Pump

The accelerator pump circuit plays an essential role during rapid throttle openings.

Depending on engine configuration, it may be necessary to:

• install larger accelerator pump jets;
• increase the injected fuel volume;
• adjust the discharge valve.

These adjustments help eliminate hesitation and flat spots during acceleration.

7. Fuel Supply System

As fuel consumption increases by approximately 30%, the entire fuel system should also be checked.

The main points to inspect are:

• fuel pump capacity;
• fuel pressure;
• fuel line diameter;
• fuel filter;
• compatibility of hoses with ethanol.

Insufficient fuel supply will quickly limit engine performance, regardless of the jet sizes fitted.

8. Ignition Timing Adjustment

Thanks to its high octane rating (generally between 104 and 108 RON, depending on season), Superethanol E85 can tolerate more ignition advance than petrol.

Depending on engine configuration, it is often possible to increase ignition timing by 2 to 6 degrees, or even more on some highly modified engines.

The final setting depends mainly on:

• compression ratio;
• combustion chamber design;
• camshaft;
• fuel quality;
• operating conditions.

As with carburation, the best setting is the one achieved through measurement and testing, not by applying a universal value.

Good to Know: Why Changing Only the Main Jets Is Not Enough

A common mistake when converting a Weber carburettor to Superethanol E85 is to replace only the main jets and consider the conversion complete.

Although the main jets represent the most significant modification, they only control part of the carburettor’s operation.

A Weber carburettor works through several complementary circuits:

• the idle circuit, which affects idle quality and low-throttle progression;
• the main circuit, which operates mainly at medium and high loads;
• the air correctors, which determine mixture behaviour at high RPM;
• the emulsion tubes, which control fuel and air mixing within the main circuit;
• the accelerator pump, which compensates for rapid throttle openings.

With E85, each circuit must be capable of supplying more fuel.

An engine fitted only with larger main jets may suffer from:

• unstable idle;
• hesitation during acceleration;
• flat spots when opening the throttle;
• correct mixture at full load but an overly lean mixture during everyday driving;
• poor transition between carburettor circuits.

A successful conversion therefore requires achieving an overall carburation balance, not simply enriching the engine.

The recommended procedure is:

1. Establish a main jet baseline using the ×1.145 formula;
2. Adjust the idle circuit;
3. Check air correctors and emulsion tubes;
4. Verify float level and fuel supply;
5. Optimise ignition timing;
6. Validate the final setting using AFR measurements.

E85 is therefore not simply a different fuel: it requires a complete recalibration of the engine’s fuel and ignition system.