A successful conversion of a Weber carburettor to Superethanol E85 is not limited to replacing the jets. Once the carburation has been adapted, it is also important to understand the specific operating characteristics of this fuel.
E85 offers many advantages, particularly excellent resistance to knock and interesting potential for tuned engines. However, some characteristics require special attention, especially during cold starts and extended periods of storage.
Cold Starting with E85
Cold starting is often the main difference noticed after converting to E85.
Ethanol has lower volatility than petrol. At low temperatures, it evaporates less easily, making the formation of the air/fuel mixture more difficult during starting.
This is why an engine running exclusively on E85 may require additional enrichment during the first few seconds of operation.
Depending on the vehicle and the Weber carburettor configuration, it may be necessary to adjust:
- the choke setting;
- the cold-start system;
- the fast idle setting;
- the starting procedure.
On some engines equipped with a manual choke, a few additional steps may be required:
- pull the choke further when starting;
- allow the engine speed to rise for a few seconds;
- gradually reduce the enrichment as the engine warms up.
This behaviour is normal and does not necessarily indicate that the carburettor is incorrectly tuned.
Why Does E85 Composition Vary Depending on the Season?
The Superethanol sold at fuel stations does not always have exactly the same composition.
Depending on the time of year, it may contain a varying proportion of petrol in order to improve:
- fuel volatility;
- cold-weather starting;
- winter operation.
In summer, the fuel may contain a higher proportion of ethanol.
In winter, the petrol content is generally increased to make starting easier.
This variation explains why an engine perfectly tuned for E85 may sometimes require minor seasonal adjustments.
Can You Occasionally Use E85 with an Unmodified Weber Carburettor?
In some cases, yes.
Many carburetted engines can accept blends containing a moderate proportion of E85 mixed with petrol without difficulty.
Mixtures in the region of:
- 10%;
- 20%;
- sometimes 30% E85,
can generally be used without major modifications, depending on the engine’s original mixture setting.
However, this tolerance depends on many factors:
- original carburettor calibration;
- overall engine condition;
- outside temperature;
- vehicle usage.
On the other hand, running pure E85 with a carburettor calibrated for SP95 or SP98 petrol is generally not recommended.
The mixture will become too lean, especially under heavy load.
Risks of Running Too Lean on E85
An excessively lean mixture can cause:
- loss of power;
- higher combustion temperatures;
- overheating of exhaust valves;
- intake backfires;
- reduced performance;
- in extreme cases, mechanical damage.
The risk is particularly significant on older or heavily used engines, as they generally do not have the safety margins of modern engines.
An AFR measurement allows you to quickly confirm whether the calibration is suitable.
E85 on a Tuned Engine
Superethanol E85 is particularly interesting for sporting or modified engines.
Its high octane rating, generally higher than conventional petrol, provides increased resistance to detonation.
This can allow:
- higher compression ratios;
- increased ignition advance;
- reduced risk of knock;
- improved thermal efficiency.
Ethanol also provides an interesting internal cooling effect.
As it evaporates in the intake tract, it absorbs a significant amount of heat, which can help reduce the temperature of the incoming mixture.
This advantage explains why alcohol-based fuels have been widely used in motorsport for many years.
Limitations of E85 on a Tuned Engine
Despite its advantages, E85 also introduces some limitations.
The main ones are:
Increased Fuel Consumption
The engine will generally consume approximately 20 to 35% more fuel.
This increase is normal and directly related to ethanol’s lower energy content compared with petrol.
More Demanding Tuning
E85 offers significant potential, but it requires precise calibration.
A high-performance engine will often require more detailed adjustment of:
- carburation;
- ignition timing;
- engine temperature;
- starting strategy.
Reduced Range
Higher fuel consumption means reduced driving range with the same fuel tank capacity.
Precautions During Extended Storage
Ethanol is hygroscopic: it naturally absorbs moisture from the air.
When a vehicle fitted with a Weber carburettor is stored for a long period, it is recommended to take certain precautions:
- avoid leaving E85 sitting in the carburettor for several months;
- run the engine regularly;
- check the condition of fuel hoses and seals;
- use fuel stabiliser during long-term storage.
For classic cars used occasionally, some owners prefer to finish the season with a fuel mixture containing more petrol before a long storage period.
Real-World Example
The photo below shows jets and an emulsion tube from a Weber 32/34 DMTL after approximately six months of storage with ethanol-containing fuel.
When the engine was restarted, it would no longer hold a stable idle due to deposits and corrosion partially blocking the idle circuit.
The residues left behind after fuel evaporation can clearly be seen, along with oxidation marks caused by moisture absorbed by the ethanol.
Common Mistakes During an E85 Conversion
Several mistakes occur regularly:
Installing Jets That Are Far Too Large
Increasing jet size excessively does not guarantee better operation.
An overly rich mixture causes:
- loss of power;
- excessive fuel consumption;
- carbon deposits;
- poor throttle response.
Ignoring the Idle Circuit
An engine may appear to run correctly at full load while still being unpleasant to drive if the progression circuit has not been adapted.
Forgetting the Ignition System
E85 often allows ignition optimisation, but a factory ignition setting does not always take advantage of its full potential.
Not Measuring Mixture Richness
“Tuning by ear” quickly reaches its limits.
A wideband lambda sensor remains the most reliable tool for validating an E85 conversion.
A Successful E85 Conversion: The Correct Method
The recommended approach is:
- Check the mechanical compatibility of the carburettor and fuel system.
- Install ethanol-compatible components where required.
- Increase the main jets according to the calculation formula.
- Adapt the idle circuit and check progression.
- Check air correctors and emulsion tubes.
- Verify float level and fuel delivery capacity.
- Optimise ignition timing.
- Carry out testing using AFR measurements.
This method allows you to achieve an engine that is reliable, enjoyable to drive, and capable of fully exploiting the advantages of Superethanol E85.
