E10, E5, E85: which fuel for which motorcycle

On the forecourt the choice looks simple: standard unleaded, super unleaded, or E85 on the rare pump that has it. In the workshop the question looks different, because what matters isn't the octane rating but the ethanol content and what it does to the materials in the fuel system.

Here's the sorting, machine by machine.

What's actually in each fuel

• Standard unleaded (95 octane): sold as E10, so up to 10 % ethanol. It's the default grade on almost every forecourt.
• Super unleaded (97/98 octane): sold as E5, up to 5 % ethanol. Compared with standard, you're buying two things at once — a higher octane rating and less ethanol.
• E85 (superethanol): 65 to 85 % ethanol depending on the season.

The two properties of ethanol that matter on a bike:

1. It's hygroscopic: it draws moisture out of the air. Past a certain threshold the water separates out of the blend and settles at the bottom of the float bowl or the tank — that's phase separation. The engine then draws in ethanol-laden water, which corrodes.
2. It's a solvent: it attacks certain elastomers and certain alloys — older rubber fuel lines, carburettor seals, floats, some internal tank coatings, and aluminium and zamak over long periods.

The octane rating: the wrong argument

The octane rating measures resistance to pre-ignition, not the quality or the energy content of the fuel. An engine designed for 95 gains nothing at all from 98. No extra power, no better economy, no mechanical benefit.

Three cases where super unleaded is justified:

• The manufacturer requires it (minimum rating shown on the filler flap or in the manual): high compression ratios, sportsbikes, tuned engines.
• The machine pinks on standard under load — a sign you need to check the ignition, the fuelling and the state of the combustion chamber anyway.
• Long-term storage: super is supplied as E5, so it carries less ethanol than E10, and that is the point that justifies the extra cost.

Which bike on which fuel

The E10 compatibility lists published by the authorities and the manufacturers remain the reference. If in doubt, the model's manual gives the maximum permissible ethanol content.

Older machines: the real subject

On a bike built before the 1990s, three parts are exposed:

• Rubber fuel lines that aren't ethanol-compatible harden, crack and eventually swell from the inside, shedding particles that block the jets. Replacing them with hose marked as ethanol-compatible (rated for oxygenated fuels) costs a few pounds and settles the matter.
• Carburettor seals, diaphragms and floats. An accelerator-pump diaphragm under attack hardens and stops metering. Modern rebuild kits use compatible materials.
• The tank. An old internal coating can lift off in sheets. On a bare steel tank, the water ethanol pulls in accelerates corrosion — that's the source of the rust that shows up after a winter laid up.

For those machines the practical rule: run on E5 — super unleaded, store it with a stabiliser, and swap hoses and seals for ethanol-compatible parts at the first rebuild.

Two-stroke engines

A special case, as true for old bikes as for garden machinery. Ethanol slightly shifts the air/fuel ratio and upsets how evenly the oil mixes into the petrol, particularly with certain oils. The risk is a locally weak mixture and a hot spot.

Tool manufacturers overwhelmingly recommend the fuel with the least ethanol available, and for seasonal machines, ready-mixed alkylate fuel — no ethanol, virtually no benzene, stable for several years. The price per litre is high, but on the five litres a year a household chainsaw drinks, it works out cheaper than a carburettor.

E85 in a motorcycle

Short answer: no, not straight from the pump. In practice the question barely arises here, because E85 is hardly distributed on UK forecourts — it comes up mostly for riders filling up abroad, or looking at a conversion.

Running on E85 needs roughly 30 % more fuel flow for the same mixture, which means injectors and a map to suit, compatible materials throughout the system, and specific cold-start management. Car conversion boxes don't transpose to a bike as they are, and an undeclared conversion creates a type-approval and insurance problem.

On a fuel-injected bike, a tank of E85 by mistake means a very weak mixture, erratic running and, if you push on, a thermal risk. What to do: don't start it, drain the tank and the fuel rail.

Storage: where it all happens

Modern petrol ages fast. The light fractions evaporate, gums form, and the ethanol pulls in moisture.

• Beyond a month without riding: add a fuel stabiliser.
• Beyond three months: full tank (less air, less condensation) plus stabiliser — or the opposite, tank and float bowls drained on an older machine. Both schools have a case; the worst option is a half tank left as it is.
• On a carburettor bike: shut the fuel tap and run the engine until it dies to empty the bowls. That's what saves you gummed-up jets in spring.
• On 2-stroke garden kit: drain it, or switch to alkylate fuel.

Spring is also when a lot of bikes go in for their annual MOT — and the ones that turn up on last autumn's fuel are the ones that won't start on the test lane.

The symptom that should ring a bell

Hard starting after a lay-up, an unstable idle, a flat spot on the throttle: on a carburettor machine, in the vast majority of cases it's neither the ignition nor the compression, but stale fuel that has left deposits in the jets, or water sitting in the bottom of the bowl.

Before you take anything apart, check how old the fuel is. Draining the system and putting fresh fuel in settles a good share of cases — and saves a pointless strip-down. For the rest, describing the symptom, the model and the year to the technical assistant sorts fuelling, ignition and compression into the right order.