Fuel Pump Picture
Wet Fouled Plug
Fuel and Oil mixture for dirt bikes explained
The Facts About Fuel : Oil Mixtures

Everyone who owns a 2 stroke engine knows that they need to burn oil in the fuel. However, the amount of oil to add, per litre of petrol, is the subject of much debate and confusion. In this article, I'll attempt to clear up some common misconceptions about fuel to oil ratios.

As far as 2 strokes go, there are actually 2 fuel ratios to deal with, the fuel to oil mixture and the fuel to air ratio. These two are closely related to each other, and both need to be correct for maximum performance and reliability.
Most people look at the amount of smoke coming out of a 2 stroke exhaust and make a judgement on whether the bike is running Rich or Lean. The general idea is the more smoke the bike is making, the richer it's running. As strange as it sounds, the opposite is usually true.

The oil in the fuel is used for lubrication of the moving parts in the engine and without enough oil the engine will soon have a catastrophic failure. Bike manufacturers go to a great deal of trouble to calculate the correct amount of oil required for their engines, and that information is readily available on their websites or in the owners manual. The ratios can vary wildly, with my 2007 KX250 requiring a 32:1 mixture (32 litres of petrol to 1 litre of oil), yet a friends 2007 KTM250SX uses an amazing 60:1. The best advice you can get is to follow the manufacturers recommendations EXACTLY, despite what other people may say, and if you read on, you'll find out why.

The main reasons people use a higher fuel : oil mixture are generally - trying to increase performance from a perceived 'too-rich' mixture, trying to reduce plug fouling from the same condition and cost of the oil itself (my friend with the KTM uses almost half the oil I do per litre of petrol).
The picture on the right is a wet fouled plug, which is a common sight for most 2 stroke dirt bike riders. This type of plug fouling is caused by the engine running too rich, which most people incorrectly believe to be too much oil in the fuel. It's actually caused by an incorrect fuel to air mixture, meaning that the engine is getting more petrol than it can burn for the amount of air it's getting. An adjustment of the carby or replacement needle is required to fix this problem.
Usually in this situation, people tend to reduce the amount of oil in the fuel, in an attempt to 'lean-out' the engine. This actually increases the amount of petrol getting into the engine, as less space is used up by oil and adds to the over rich state, leading to further wet fouled plugs. As the situation gets worse, the owner may further reduce the oil again and again, not only worsening the plug fouling but possibly starving the engine of oil and leading to a major failure.

As you can tell, reducing the amount of oil to make the bike perform better has exactly the same result, actually making the bike richer and richer as you attempt to lean it out. Performance suffers and catastrophic failures can result. If your bike is actually running too rich, make the necessary carby adjustments to fix the problem.

On the flip side of the coin, some owners actually increase the amount of oil in the fuel, believing that they are protecting the engine by providing more oil to it. This is often seen where manufacturers specify very high ratios like 60:1 (60 litres of petrol per 1 litre of oil) and the user gets concerned about the small amounts of oil being used. Surely more oil can't hurt the engine? Wrong.

In extreme cases, massive amounts of oil can cause carbon fouling of the plugs, but the real danger lies in the fuel to air ratio. As the amount of oil in the fuel increases, the amount of petrol getting into the engine decreases, as the oil is taking up space. As the amount of air is unchanged, the bike will run leaner than it's supposed to. A lean running engine will run hotter than a properly tuned engine and major engine damage can quickly result. It is safer to run a slightly rich setting than gambling on a lean running engine.

So as you can tell by now, attempting to lean out or richen an engine by adjusting the fuel to oil ratio is fraught with danger and makes little sense. A properly tuned engine shouldn't foul plugs so there's no need to alter fuel to oil ratios either.

As for the cost of the oil itself (which can be rather expensive), reducing your oil consumption by enough to make a noticable difference to your daily riding costs will probably quickly lead to a major and VERY expensive engine rebuild.

Follow the manufacturers recommendations, make sure your fuel to air mixture is correct and you will get maximum performance and maximum life from your engine.
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This page is about correct jetting, of you're looking for fuel-oil ratios, visit our free mixture chart page