Jerry Thurston column.
Have you ever wondered why or how manufacturers came up with the various ratios of fuel to oil for two-stroke motorcycles?
Consider… It’s a red-hot Sunday in 1956 and Trevor, a burly bricklayer, is going to see his girlfriend. Trev’s wringing the neck of his second-hand James Captain, buzzing through the lanes in a cloud of blue smoke. This morning – like he does every Sunday morning – he has quarter-filled the milk bottle he keeps in the shed for the purpose with ordinary Castrol and bunged it into the tank, along with a gallon of two-star petrol; all good, and he’s set for another week of riding.
Eager to see his sweetheart, he’s thrashing the poor little James, but it’s going to survive to take him to work on Monday and on subsequentMondays. And that’s because James’ (well, Villiers’) designers have thought about the Trevor scenario, and suggested that smoky, quarterpint to gallon 16:1 fuel to oil ratio. Note that it is based on the UK fluid measuring system… pints and gallons. Continentalmanufacturers were suggesting 20 to 1, their ratio being based on easy calculations with the metric system; 50ml of oil in a litre of fuel, 250ml in five litres etc.
So the famous 16:1 ratio for UK bikes and 20:1 for continental ones is as arbitrary as that – simply quantity that is easy to measure and puts enough oil into the fuel to take into account the hottest day of the year, the poorest quality oil available and a big rider giving the poor little engine hell.
To remove the vagaries of measuring, amechanical injection system had to be the way forward. Scott’s use of a pilgrim pump went half way there, but without any connection to the throttle, it had to be set on the generous side, consequently over-oiling at low throttle openings and barely supplying enough when the rider was giving it a fistful at medium revs, up a long hill.
It was the Japanese that took the idea forward with their throttle controlled direct oil injection. Taking the simple, entirely engine speed dependent system, and adding to it a rack opening system connected to the twist grip, it thus supplied more oil when the throttle is wide open and less when cruising. Easy.
The problem with these factory set pumps is their dependence on the right type of oil used; fill the tank with the correct stuff and they’ll run forever, but the wrong lubricant could be calamitous. It can be seen that the makers were terrified that somebody would fill the tank with sticky car oil as a cheap option, as next
It was the Japanese that took the idea forward with their throttle controlleddirect oil injection.
to the tank were stern entreaties to only use specified products. Even using the right oils, they had to be set slightly on the over-oily side to take into account the most extreme circumstances that might bemet. It is little wonder, therefore, that the two-stroke wasn’t capable of meeting the strictest modern emission standards and so were gradually legislated away.
In the future, the two-strokemay make a comeback, largely due to liquid cooling keeping cylinder temperatures within strict parameters, alliedwith sophisticated engine control units for cleaner emissions. Indeed, KTM have builtamodern twostroke off-roader featuring fuel-injection directly into the transfer ports for tractability and extraordinary (when compared to a usually greedy two-stroke) fuel consumption. The ECU is constantly taking into account engine temperature, revs, throttle position and load, adjusting not only the fuelling but also the oil supply – on a cool day with the engine just skimming along, it’ll be cutting the oil/fuel ratio as far back as 100:1; wind it open up a hill and it’ll increase the amount of oil accordingly, to maybe 50:1.
For those of us that are used to 16:1/20:1 the idea of cutting back the lubricant to that level is perhaps scary, but, these days, with modern synthetic, high film strength two-stroke specific oils though, I think we can consider the old 16:1 ratio more or less redundant. Although I’m still mixing at near that ratio with a castorbased oil for 18,000rpm kart motors, on everything else, I’m taking advantage of the modern formulations and pulling the oil/fuel ratio back to clean it up. Because I don’t have the temperature control that’s offered by liquid cooling, I cannot cut back too far, so I have settled on 40:1 for my air-cooled trials engine and 30:1 for a hard-pressed enduro motor.
I always use the best synthetic oils available, costing about £20 a litre, this compared to the very cheapest 2T oils at about £5 a litre, which, to be honest, I wouldn’t be buying even tomix at 16:1!
Why have I cut back? I can literally see I have cleaner emissions (the smoke disappears after it is warmed up) and there is almost no carbon build-up, plus I think I amgetting a cleaner, crisper throttle response, and maybe even a tad more power. Should you do similar? As ever, it’s your decision.