Daniel Hopkins posted this on FaceAche group 'British Motorcycles Pre '87'.
Motor fuel supplied to the public has traditionally been manufactured by extracting the hydrocarbon molecule octane from crude oil by fractional distillation (‘cracking’). Pure octane is a flammable straw coloured liquid. It is less dense than water (0.7gm/ml) and has a flash point of 13 degrees centigrade which is sufficiently high to make it safe to handle. It has been used as the fuel of choice for spark ignition internal combustion engines since the end of the 19th century.
Octane burns most efficiently if mixed with atmosperic air at a ratio of 14:1, the so-called ‘stochiometric mixture’. The purpose of a carburettor is to acheive this mix at a range of operating speeds.
All pump fuels are a mixture of chemicals. For the fractional distillation process to be economic, there is an inevitable overlap with molecules with larger and smaller numbers of carbon atoms, certainly C6 throught to C12. This variability had led to fuel being described by a number, for example ’87 Octane’. The number refers to the pressure at which the fuel will self ignite without a spark. The higher the number, the higher the pressure. Since fuel is compressed in an internal combustion engine, it follows that a high compression motor will require a higher octane number to operate safely. High octane numbers have no effect on power output.
Recently there has been pressure on governments and fuel suppliers to alter the mix of pump fuels for a variey of reasons connected to cost and environmental issues.
The most common change has been to mandate the addition of alcohol (ethanol and some methanol) to the fuel mixture. Typically this has been between 5% and 10% by volume. Ethanol is a naturally ocurring organic solvent with a flash point of 12.7 degrees centigrade. It is less dense than water at 0.79gm/ml. By comparing the numbers for octane, it is easy to see why ethanol is attractive to fuel suppliers in that it’s storage and handling characteristics are broadly similar.
The physical and chemical effects of ethanol.
Ethanol is hygroscopic (absorbs water directly from the atmoshphere) and readily evaporates if stored in an open container. ‘Open containers’ include many motorcycle petrol tanks and all carburettors.
To quote Christensen & McCormick, “Loss of fuel quality by weathering occurs in conjunction with water uptake resulting in undesirable property changes prior to the onset of phase separation. Water uptake of an ethanol blend during storage in a humid environment can result in phase separation; however, if fuel is stored long enough for phase separation to occur the fuel was likely also unfit for purpose because of reduction in vapor pressure and other undesirable changes to fuel properties before phase separation was observed.”
What this means is that if you leave your fuel in an open container it will attract water which will then separate out and cause corrosion. At the same time, the fuel volatility (ability to start the engine) will rapidly degrade. We’ve all heard the term ‘stale petrol’ but this is much worse.
Christensen goes on to say “Fuel tank manufacturers recommend emptying the tank if equipment is to be stored as long as 3 months. These experiments have shown that this practice is needed to prevent loss of fuel quality from hydrocarbon weathering as well as to safeguard against phase separation due to water uptake.”
To summarise; Motor fuel containing ethanol stored in open containers will
rapidly become less volatile
attract water which will collect in the tank and carburettor
The chemical effects of ethanol fuel mixtures will vary depending on the percentage mix. Ethanol is a smaller molecule with an oxygen atom included so it’s burning characteristics in air are different. In a study in 2014 it was found that the “Low stoichiometric ratio of ethanol causes the stoichiometric ratio to decrease with increasing ethanol in fuel. In cases where amount of fuel sprayed in the fuel system by an adjustment is not increased, ethanol increase causes fuel mixture to become leaner.”
Additionally, it was found that the “High octane number and heat of evaporation of ethanol also improve knock resistance of engine. Using this feature, which increases the knock resistance, in some studies where the ignition advance and compression ratio of the spark ignition (SI) engines are increased, significant improvements in performance values such as efficiency and power have been achieved.”
In another study it was found that “In comparison to the pure gasoline, adding ethanol increased sensitivity to the spark energy. The good burn showed a fast flame growth and larger flame radius. While the poor combustion revealed a long-narrow flame shape and inferior development of the flame radius. Also, the E85 and E100 demonstrated a moderate natural luminosity or nearly transparent-like appearance.”
To summarise; The burning characteristics of ethanol mixtures will
Require more fuel for the same power output, the stochiometric ratio changes
Increase anti-knock qualities but only if the fuel is fresh
Increase sensitivity to ignition system efficiency and timing accuracy
The characterstics of classic motorcycles:
For the purposes of this article I shall not be considering any motorcycle fitted with fuel injection.
Although engine technology has generally improved over time, it has proceeded in fits and starts for commercial or historic reasons. It is convenient to divide machines into those manufactured
Before 1945 (1)
1945 to 1965 (2)
1966 to 1980 (3)
After 1980 (4)
(1) Motorcycles manufactured in this period were generally configured to run on fuel with an octane number of less than 60. (Considerable fuel improvements were made for aviation but this was not made available to the public) Many ignition systems were self-generating (magneto) and carburettors tend to deliver a rich mixture. The majority of ignition systems in this period are manually controlled. Modern fuel generally works well in such machines because it’s specification is so much better than the fuel available when they were manufactured.
(2) During the period after the War, engine development began to produce higher output per litre by a combination of increased compression and changes in valve timing - particularly ‘overlap’. Brake mean effective pressure (Bmep) starts to rise, leading to increased operating temperatures. Pre-ignition (‘knock’) was controlled by a mixture of an increase in fuel quality and the addition of Lead Tetraethyl (an extremely poisonous but very effective anti-knock additive).
Engines manufactured during this period will generally run well on modern fuel provided it is fresh. Old fuel will generally fail to start and must be mixed with fresh. Due to the lower effective energy content/volume, it is necessary to review mixture and adjust carburettors accordingly. Attention should be paid to ignition efficiency, both in terms of advance settings and spark plug grade. Ignition coils do actually degrade over time and may need replacement.
(3) This is the problem period. Manufacturers were increasingly desperate to produce ever higher Bmep values from old designs. The Japanese began to advance with 2-stroke types which easily produce high bhp/litre values. Few engines manufactured in the period are problem free and many need careful modification to run reliably on modern fuel. I’ve divided the various common types but it’s worth considering all the issues when evaluating a particular machine.
British 4 strokes
Often manufactured with little attention to tolerances. It is essential to check that pistons in twins and multis rise by the same amount. Cylinder head clearances should be identical. Many engines were contructed with ‘hemi’ heads with little or no squish band. The squish area around the edge of the piston must be 40thou (1mm). Then check that the valves don’t tangle with the piston. Under no circumstances run one of the higher compression motors on anything except fresh fuel. Make sure that the ignition is correctly adjusted and that advance curve is correct. You need to be particularly careful with later Triumph twins and the 750cc Commando.
British 2 strokes
Often supplied with very poor clearances and tolerances. 2 strokes rely entirely upon accurate squish clearance. Some machining of the cylinder head may be necessary to achieve this, particularly on BSA and Greaves machines. Most Villiers engines are effectively pre-war designs so may be treated as category (1). The majority of 2 strokes are fitted with fixed ignition systems. This may need to be retarded somewhat to compensate for the faster flame advance on modern fuel.
Japanese 2 strokes.
These engines were relatively powerful and cool running. Generally manufacturing tolerances are excellent. The problem is that they were designed to run with lead additive to control knock. It is important to pay careful attention to the squish band which is virtually absent on the earlier motors (up to 1973) leading to serious detonation problems. The only answer is to fill the heads with alloy weld and re-machine to a modern combusion chamber shape. These motors may require some re-jetting to run efficiently. Most engines from the period are fitted with the infamous metal shrouded plug cap. These should go in the bin.
Japanese 4 strokes.
Generally give little trouble provided the fuel is fresh. Most machines of this type run relatively hot. Because modern fuel is more volatile, a hot engine in traffic may lead to erratic running. Many 4 strokes from this period are fitted with double ended coils and ‘wasted spark’ ignition. These may give trouble with ethanol fuel mixtures because they are inefficient. It may be necessary to fit single ended coils. The manufacturers were under increasing pressure from the US to make engines run lean. It is worth checking the mixture and fitting larger jets if necessary.
Italian 4 strokes.
The usual hilarious mixture of wonderful design and shocking production quality means every machine needs to be carefully checked out. The large block Moto Guzzi twins are unlikely to give much trouble because it is a light car engine and thus massively under stressed in a motorcycle. In most of these engines the squish clearance is too large, so it is worth machining this until correct. All the Ducatis and most small Italian 4 strokes need careful checking to ensure that they are reliable. A lot of the smaller Dellorto carburettors were supplied in a ‘cheap’ version to the manufacturers and these often wear poorly and may require replacement. The Laverda 750cc and 1000cc models give little trouble but you will have to be particularly careful with the smaller motors.
(4) Engines produced after 1980 were designed to run on unleaded fuel which was being introduced worldwide. Machines tend to have better designed cylinder heads which generally cope well with modern fuel mixes. Because manufacturers were under increasing pressure to achieve lean-running, many such machines run hot - especially in traffic. This may lead to uneven running with an alcohol mix. Re-jetting must be considered.
The general rules:
Always use fresh fuel
Do not store fuel in motorcycles for more that 2 months
Drain fuel from tanks and carburettors into sealed cans
Pay careful attention to squish bands after 1965
Ensure ignition systems are working efficiently
Consider re-jetting to prevent weak mixture
Consider updating or replacing coils
Review the operation of magneto systems
Consider lining fuel tanks with a modern sealant e.g POR15
Clean out and remove any corrosion from carburettors.
