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
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
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.
This page is about correct jetting, of you're looking for fuel-oil ratios, visit our free mixture chart