The answer is yes. A Weber carburettor can operate perfectly well on E85 ethanol fuel, provided it is properly tuned.
However, it is important to distinguish between two concepts that are often confused: carburettor compatibility and carburettor tuning. A carburettor may be fully compatible with E85 from a materials standpoint, yet still require a complete recalibration for the engine to run correctly.
In other words, material compatibility never eliminates the need for proper tuning.
Are Weber Carburettors Compatible with E85?
Weber carburettors currently manufactured in Spain are built using materials that are compatible with modern fuels, including E85 ethanol fuel. The gaskets, diaphragms, needle valves, floats and other sealing components are designed to withstand fuels containing high concentrations of ethanol.
For carburettors manufactured before 2005, the carburettor body itself generally presents no issues. However, certain rubber components (gaskets, diaphragms, O-rings or needle valves, depending on the model) may have been designed before the introduction of modern ethanol-blended fuels.
When restoring or rebuilding an older Weber carburettor, it is therefore recommended to systematically replace:
- Paper gaskets
- O-rings
- Accelerator pump diaphragms
- Needle valve and seat (if necessary)
- Fuel hoses
- Brass floats (if fitted)
- Any rubber or elastomer component that comes into contact with the fuel
Once these components have been replaced, the carburettor will be fully suitable for use with today's fuels. Modern rebuild kits use materials that are compatible with unleaded petrol as well as high-ethanol fuel blends. Our range of Weber rebuild kits includes everything needed to replace these key components and ensure reliable operation.
As a result, material compatibility is rarely an issue on a properly restored or recently manufactured Weber carburettor.
Older vs Modern Weber Carburettors
Weber carburettors manufactured after 2005 are equipped with seals, diaphragms and materials specifically designed for modern ethanol-containing fuels. They are approved for use with fuels containing up to 100% ethanol (E100).
Why Can Ethanol Cause Problems?
Although the carburettor itself is compatible with ethanol, the properties of the fuel can still create issues.
Ethanol is hygroscopic, meaning it naturally absorbs moisture from the air. If a vehicle remains unused for several weeks or months, this absorbed moisture can promote corrosion and oxidation inside the carburettor.
Over time, as the fuel evaporates, it can also leave deposits that clog internal passages, jets and emulsion tubes.
Real-World Example
The photo below shows the jets and emulsion tube from a Weber 32/34 DMTL after approximately six months of storage with ethanol-blended fuel.
When the engine was restarted, it would no longer idle because corrosion and fuel deposits had partially blocked the idle circuit.
The deposits left behind after fuel evaporation are clearly visible, along with oxidation caused by the moisture absorbed by the ethanol.
Regular Use Minimises the Risk
If the vehicle is driven regularly, frequent fuel replacement generally prevents these problems from occurring.
However, if your vehicle will remain unused for several months (winter storage, restoration or long-term storage), it is recommended to:
- Completely drain the carburettor float chambers.
- Run the engine until the float chambers are empty.
- Or drain the entire fuel system before storage.
These precautions greatly reduce the risk of internal corrosion and help prevent blocked jets when the vehicle is put back into service.
Summary
Weber carburettors are fully compatible with ethanol-containing fuels, provided that older models have been fitted with ethanol-compatible components.
The main concern does not come from the carburettor itself, but rather from the effects of leaving ethanol fuel sitting in the system for extended periods. Regular use and a few simple precautions before long-term storage will help preserve the performance and reliability of your Weber carburettor for many years.
Why Does E85 Require Specific Carburettor Tuning?
The primary difference between petrol and E85 is not just their chemical composition, but also the amount of fuel required for proper combustion.
The stoichiometric air-fuel ratio is approximately:
- 14.7:1 for petrol
- 9.8:1 for E85 ethanol fuel
In other words, for the same amount of air entering the engine, E85 requires significantly more fuel.
In practice, an engine running on E85 typically requires 25 to 35% more fuel, with around 30% being the most common figure.
This is also due to ethanol's lower energy content. Per unit volume, E85 contains less energy than petrol, naturally resulting in higher fuel consumption.
Without modifying the carburettor, the engine will run too lean.
This can lead to numerous problems, including:
- Loss of power
- Hesitation during acceleration
- Unstable idle
- Difficult cold starting
- Higher combustion temperatures
- Overheating of the exhaust valves
- Increased risk of detonation or mechanical damage during prolonged high-load operation
For this reason, converting an engine to E85 is not as simple as filling the fuel tank with a different fuel. It requires a complete recalibration of the carburettor to deliver the additional fuel that E85 demands.
More Fuel Does Not Mean Jets 30% Larger
A common misconception is that if an engine needs 30% more fuel, you simply increase the main jet size by 30 points.
In reality, Weber jets are calibrated according to the diameter of the orifice, expressed in hundredths of a millimetre. For example, a 130 main jet has an orifice diameter of 1.30 mm.
However, jet flow depends on the cross-sectional area of the orifice—not its diameter.
Since the area of a circle increases with the square of its diameter, a 30% increase in fuel flow corresponds to only about a 14–15% increase in jet diameter.
This distinction is essential because it allows the correct jet size to be calculated accurately, rather than relying solely on trial and error.
In the next section, we'll explain how to quickly calculate the theoretical main jet size using a simple formula before covering all the adjustments required to successfully convert a Weber carburettor to E85 ethanol fuel.
