How to Calculate 2-Stroke Fuel Ratios

How to Calculate 2-Stroke Fuel Ratios

If you are reading this article, chances are that you are trying to figure out the best fuel ratio for your two-stroke engine.

Unlike 4-cycle motorcycles, 2-stroke engines are simpler in design and use fuel and oil mixtures to lubricate the movable parts. Motorcycle manufacturers typically indicate in the manual what 2-stroke fuel ratio to use on their machine.

Here are some tips to help you properly mix the fuel and oil and keep your engine in top performance.

But first, let’s clarify…

How Does a 2-Stroke Engine Work?

two stroke engine with spark plug on top

Two-stroke engines are a type of engine where the power cycle is performed in two movements, whereas a four-stroke engine completes it with four-piston strokes. Two-stroke engines have fewer moving parts, making them suitable for small gas-powered machines, where weight is a concern.

In a two-stroke engine, the compression and combustion occur as one stroke in the crankcase, as the induction and exhaust processes are the second stroke. So having oil in the crankcase would be impossible as it would get burned up without lubricating.

Therefore, gasoline is pre-mixed with oil in ratios to enable piston and bearing lubrication as the engine is working.

What Is 2-Stroke Fuel Ratio?

Two-stroke engines are common in small petrol-powered machines such as handheld garden power tools like chainsaws, outboard motors for boats, motorcycles, and small model airplanes. However, fuel ratios vary depending on the machine and engine capacity.

pouring gasoline into a measuring pitcher

While there are various 2-stroke engine types available, most traditional 2-stroke motorcycles use pre-mix fuel to lubricate the engine.

On the other hand, some motorcycles such as the Yamaha PW80 and some snowmobiles use an automatic lubrication system, known as auto-lube, to apply lubricant more effectively than pre-mix.

Auto-lube motorcycles feature a separate oil tank and oil is automatically mixed with the fuel before being fed into the engine. While this system is more efficient and easier for the user, the oil tank requires frequent refilling.

Do 4-Stroke Engine Motorcycles Mix Fuel?

In contrast, a four-stroke engine performs the four strokes separately and features different reservoirs for oil and fuel, and pre-mixing is not necessary.

The engine uses various methods to distribute the oil on the various movable parts. One of these is via a pressurized pump to lubricate the bearings. The splash lubrication method covers the crankshaft with oil which gets splashed on parts such as cam lobes, wrist pins, and cylinder walls.

What Is the 2-Stroke Fuel Ratio for Motorcycles?

The ratio of petrol to oil while pre-mixing for a two-stroke engine is what is referred to as the 2-stroke fuel ratio. Motorcycle manufacturers typically indicate what ratio their motorbikes use and it ranges between 24:1 and 50:1 ratios.

A 24:1 ratio indicates more oil in the fuel mix and is suitable for high RPM motorcycles such as race bikes.

Trail dirt bikes and Open Class bikes and 250cc motorcycles usually run at 40:1 and 50:1 ratios while some motocross bikes such as the KTM 50 SX run 60:1.

Tips on How to Mix 2-Stroke Fuel for Motorbikes

Four-stroke motorcycles generally use regular gasoline, which is unsuitable for two-stroke engines. We recommend using unleaded petrol with little or no ethanol to prevent clogging up the carburetor.

Even if your motorcycle is fuel-injected, it will still have a high compression ratio that requires regularly using high octane gasoline.

Ethanol is an additive added to gasoline to enable more efficient combustion. Generally, most of the fuel you get contains some ethanol, sometimes up to 10%. But, some gas stations offer ethanol-free, high octane gas at a premium price.

While it does not increase the performance or fuel efficiency of your motorcycle, high octane gas can reduce maintenance costs and prevent the engine from knocking.

Choosing the Right 2-Stroke Engine Oil

In the same way, adding the proper 2-stroke engine oil will ensure that your engine remains in top performance.

There are various types of engine oils available in the market. They include vegetable or castor oil, petroleum or mineral oil, petroleum, and synthetic oil mixtures, and 100% synthetic oils. Let’s look at them a little bit more below.

Vegetable oils, such as BeNOL Racing 2T Pre-Mix Castor Oil, offer excellent lubrication abilities and help your engine dissipate heat much more effectively. On the other hand, vegetable oils don’t mix with fuel very well. Also, when used in high ratios, it can alter the octane levels of the gas, resulting in reduced RPM and inefficient combustion.

Mineral oils, such as Bel-Ray 2T Mineral Engine Oil, are a cheap option and are typically derived from low-quality crude oil and blended with additives to improve combustion and lubricating ability. Petroleum oils with some synthetic additives are of better quality while being affordable.

These engine oils are suitable for motorcycles with long intervals between engine rebuilds. However, fully synthetic 2-stroke engine oils, such as Motul 800 2T Factory Line, are the best for motorcycles. They are made to have the most efficient burning, giving out almost zero emissions and high lubricating ability.

Premium quality synthetic oil mixtures enable you to run lean 100:1 fuel ratios without wearing out the engine parts. It is especially true for 2T trials bikes where high oil to fuel ratios can easily gum up the engine due to hard revving.

However, fully synthetic oils typically blend a small percentage of castor oil or high-quality base crude oils for high lubrication results. Additionally, certified engine oils with JASO FA, JASO FB, or JASO FC specifications are suitable for modern 2-stroke motorcycles.

How to Read 2-Stroke Fuel Ratio Charts

Once you have the fuel and engine oil in the required ratios, the next step is to mix them in a green jerrycan. There are many online pre-mix calculators that one can use to determine the correct ratio, depending on how much gas you want to use. However, this simple chart shows you how to calculate manually, and it’s an excellent resource you can refer to whenever you’re stuck.

Mixing Ratio 1 Gallon Gas 2 Gallons Gas 3 Gallons Gas
16:1 8oz 16oz 24oz
20:1 6.4oz 12.8oz 19.2oz
32:1 4oz 8oz 12oz
40:1 3.2oz 6.4oz 9.6oz
50:1 2.6oz 5.2oz 7.8oz
100:1 1.3oz 2.6oz 5.2oz

You can see from the chart that as you increase the amount of fuel, the oil required goes up in proportion. For instance, suppose your bike has a 40:1 fuel ratio, and you have 2.5 gallons of gas to fuel.

Look at the chart and see how much oil you’d need for 1 gallon of gasoline, which is 3.2 ounces. Then multiply the 2.5 gallons with 3.2 ounces to get 8 ounces of oil. Therefore, for 2.5 gallons of fuel, you would need 8 ounces of 2-stroke engine oil for my mixture.

Always ensure that your oil mixes evenly with the gas by shaking the jerrican vigorously before pouring it into your gas tank. Also, we recommend only using two-stroke motorcycle engine oil for mixing to prevent getting your engine damaged.

Once your gas is mixed, ensure that you use all of it because it doesn’t keep well. Using a stale gas and oil mixture may cause blockage to some parts of your engine, making it perform less efficiently.

How Long Can You Keep a 2-Stroke Oil and Fuel Pre-Mix?

You can keep pre-mixed fuel for up to two months with proper storage, and it also helps to use fuel stabilizers. Two-stroke engine oil typically has a 2-year shelf life once opened.

Additionally, if you are using unsealed oil, it is vital to ensure that it has not been exposed to air, moisture, extreme heat, or drastic temperature fluctuation as it can alter its viscosity and lubricating ability.

On the other hand, using fuel only with a two-stroke engine will quickly result in engine damage because the moving parts will not get lubricated. Also, using fuel with a high ethanol content will cause the fuel and oil mixture to separate as it absorbs moisture over time.

Final Thoughts

Two-stroke motorcycles run on a mixture of gas and oil for the engine to perform efficiently. The premix is typically blended in fuel ratios depending on the type of machine. While most motorcycles can run on 32:1 and 40:1 fuel to oil ratios, several factors determine what two-stroke fuel ratio is ideal for your motorbike.

However, if manual blending isn’t your thing, you have the option of using commercially made premix fuel for convenience. Whatever you decide, ensure you use good quality fuel and oil to keep your motorcycle engine in top performance.

Avatar for Joshua D. Mattie

Joshua D. Mattie

My motorbike addiction began with 50cc at 5 years old. I rode motocross as a teenager & into my 20's when I worked as a mechanic. This helped me to see the light—sportbikes & cruisers became a passion. Now I'm building BikersRights to be the #1 resource for everything on 2 wheels!

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