Two-Stroke Fuel Ratios: Pre-Mix Myth vs. Reality

Mar. 05, 2015 By Rick Sieman

Let’s see … so your bike is running on the rich side, so you put less oil in the gas to lean it out, right? Wrong.  Or maybe your bike is running a bit too lean, so you figure that if you put more oil in the gas that should take care of the problem.  Wrong again.

Many dirt bikers are mixing their gas at ratios as high as 75 to l or even 100 to 1 with the new generation oils in the belief that their bike will put out the most horsepower at a higher ratio. Riders who foul plugs all the time are putting less oil in their gas/oil mix in the belief that the oil is fouling the plugs, and many racers are trying to solve “too rich, too lean” problems by changing the gas/oil mix instead of the jetting.

There are a few good reasons to run a fuel/oil mix at ultra-thin ratios in a two-stroke. High ratios such as 100 to 1 are usually environmental reasons, such as for outboard motors. The exhaust of an outboard motor goes directly into the water, and environmentalists are worried about the oil in the mix polluting the lakes and rivers.

There’s a myth that the less oil you use in your gas, the more horsepower you get. Conversely, many dirt riders actually forget to put any oil whatsoever. We know of one guy who forgot to mix oil into his gas and actually rode it for two hours without seizing it. All the bearings were ruined and the piston was worn out, but it didn’t seize!

Actually, you can get more horsepower out of a two-stroke engine with enough extra oil in the gas, because the oil provides a better ring seal and, therefore, more compression. People think that gas burns more efficiently with less oil, and therefore you get more performance. It almost makes sense if you look at that one statement alone.

The seal of the piston is critical. If you remove the lubricants from the gas, the viscosity of the mixture becomes lighter and more prone to vaporization. With a lean mixture there is less oil to seal the rings. The sealing of the rings has more to do with the performance of the engine than the possibility of having better-burning gas with an ultra-lean gas/oil ratio.

The old fashioned two-stroke oil that was on the market years ago was designed to run at 20:1, and it was basically petroleum with a few (very few) additives.  Then, when high-performance oils came along they cost more to make and sold for a higher price. They got into these high mixing ratios in order to justify the higher prices. 

If you do foul plugs, it is more than likely caused by poor jetting, not a bit too much oil. If you get your bike jetted correctly, have a fresh plug and a strong ignition system, you won’t foul plugs.

When the motor is idling, or at lower RPM, that’s when the machine has a greater chance of fouling a plug.  Minibikes and 125s have even less chance of fouling plugs, because they are ridden at such high RPM. Because of the ultra-high RPM, the load on a given part is much higher on a 125 than on an Open bike.

Plugs should not foul at richer ratios if you are using high-quality oil in the mix. High-quality oils will have a good detergent/dispersing package that holds down the contaminants which produce plug fouling. A typical example: you go from a 50:1 ratio to a 20:1 ratio. Your engine will now run leaner, and you’ll have to make jetting changes. You’ll need bigger (in number) jets because the oil molecules are thicker and the flow rate (the amount coming through the jet) is less. Aha! The volume of fuel has changed. The oil takes up some volume that the gas used to occupy, so your engine is getting less gas and needs to be richened up.

So which ratios should two-stroke gas/oil be mixed? A properly jetted engine will run better, last longer and develop more power at a lower oil ratio than at a higher one. But what is the proper amount, and how do you know a quality oil from a bad one?

The ratio a rider should use in his two-stroke will depend on the size of the machine and the type of riding being done. An 80cc racer will require much more oil in the mix than a 500cc play bike. The best bet is to consult the owner’s manual and follow the advice of the engineers who designed the motorcycle.

As for which oil to buy, that depends on the type of riding being done. Someone who races will require a higher-quality oil for its superior ingredients and properties than someone who only play rides and doesn’t put a lot of strain on his engine. A good, high-quality oil will cost more than a poor-quality oil, because of the higher cost of ingredients, such as synthetic diesters and ash-less detergent dispersing packages. Quality ingredients cost more money, and that makes the quality oils more expensive.

Our advice is to buy a quality oil and run it at a moderate ratio. We’ve used 32:1 for many years. In race bikes that are ridden hard, we might go a trifle richer at say … 28:1. For a trail bike, 40:1 would be the way to go, assuming that you used a quality oil. If you own a mild-mannered bike, consider a 50:1 ratio.

One of the things you should do is run high-octane gas with any two-stroke mix.  When all of the two-strokes (the old days) were developed, they all used Castrol petroleum oil at a 20:1 ratio and found that 92-octane gas had the octane reduced to 72 with presence of that much oil.  Modern oils won’t affect the fuel quite as much, but if you started with 86- or 87-octane regular fuel, you can see where you’ll end with a very low octane mix. You could end up with a “pinging” bike.

Race gas? You don’t need it in your two-stroke unless you’re a pro or expert, and most expert level riders are on the new-generation four-strokes.

Use only two-stroke engine oil in two-stroke engines. Do not use automotive oil like SAE 10W-30W, or the like. Two-stroke engines burn oil and are designed to do this, and they require the proper oil in the gasoline.

Mix the gasoline and oil thoroughly. One method is to take your gas and oil can to the gas station and mix right there at the pump. Fill the gas can about 1/3 full and then add the proper amount of oil, then fill the container. The gasoline pumping rapidly out of the nozzle mixes the oil and gas together quite well.

Shake the gas can vigorously before filling your gas tank. The oil must be suspended evenly in the mix, so the engine gets lubricated evenly. If the oil is not mixed thoroughly, the engine starves for lubrication, and the spark plug gets oil stuck on it.
Gasoline is also important. Head for your manual for types of gasoline and octane rating your engine requires. Some older engines require leaded gasoline. Most of the newer engines run on leaded or unleaded.

Once gasoline is mixed, use it. Don’t buy 10 gallons of gasoline and use five gallons. Gasoline allowed to sit gets stale and gummy. This gummy stuff sticks to carburetor parts and air passages, which eventually will restrict airflow, thus changing the air-gasoline mixture.

All the major manufacturers produce two-stroke racing engines in their off-road motorcycles. Virtually all of them recommended 20:1 or 24:1 mix ratios.

1. The higher the engine RPM, the more oil they ran in the fuel. (e.g.: a 125cc machine that routinely lived in the 10,000 - 13,500 RPM range ran 20:1 or 24:1. The 250cc engines that ran between 6,500 and 9,000 RPM ran 32:1 or 40:1, and the Open Class machines (251cc and up by AMA, but they were all 400+cc engines, usually 465s, 490s, or 500cc) ran 50:1.

2. Additionally, Husqvarna did some testing in the mid ‘70s that was very interesting. They put three identical stock engines on a dyno and ran them for several days at varying RPM and load conditions. Then both motors were torn down and inspected. The engine running Castor-based oil had the least wear, followed by the synthetic oil, and finally the engine running standard two-cycle oil.

3. A second test they performed was to run synthetic in two identical engines and one was run at 24:1, the other was run at 50:1 The engine that ran 24:1 had less piston skirt wear, and less rod bearing wear, but had the same main roller bearing wear as the engine run at 50:1.

Ounces of Oil to Gallons of Gas
Ratio Gas to Oil 1 Gallon Gas 2 Gallon Gas 3 Gallon Gas 4 Gallon Gas 5 Gallon Gas
16 to 1 8 Oz Oil 16 Oz Oil 24 Oz Oil 32 Oz Oil 40 Oz Oil
20 to 1 6.4 Oz Oil 12.8 Oz Oil 19.2 Oz Oil 25.6 Oz Oil 32 Oz Oil
32 to 1 4 Oz Oil 8 Oz Oil 12 Oz Oil 16 Oz Oil 20 Oz Oil
40 to 1 3.2 Oz Oil 6.4 Oz Oil 9.6 Oz Oil 12.8 Oz Oil 16 Oz Oil
50 to 1 2.56 Oz Oil 5.12 Oz Oil 7.68 Oz Oil 10.24 Oz Oil 12.8 Oz Oil
100 to 1 1.28 Oz Oil 2.56 Oz Oil 3.84 Oz Oil 5.12 Oz Oil 6.4 Oz Oil

To Use: Pick your ratio, pick the amount of gas you want to use and follow the columns and rows to the correct amount of oil. For 1 gallon at 20 to 1, follow the 1 gallon column down to the 20 to 1 row and you’ll find 6.4 Oz of oil. Newsletter
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