ASK THE MXPERTS: LIFE WITH A TWO-STROKE STARTS WITH A RATIO-RITE

PRE-MIX PRIMER FOR A FIRST TIME SMOKER DAD

Dear MXperts,
I know nothing about motorcycles, but I want to buy my 14-year-old son the Yamaha YZ125 that he has been begging for on his upcoming 15th birthday in four months. My neighbor, who doesn’t know anything about motorcycles either, told me that I can’t just pour gasoline into a YZ125 and that I have to pre-mix it with oil. Please tell me everything I need to know about “pre-mixing” gasoline.

Although there are fuel-injected two-strokes, even they require the engine’s oil to be pre-mixed into the gasoline. Some off-road models have oil tanks and automatically squirt oil into the fuel via the engine’s transfer ports. These are called Transfer Port Injected (TPI) engines and it is mostly used on off-road enduro bikes. KTM, GasGas and Husqvarna’s fuel-injected motocross bikes, do not have an oil tank, nor do they inject oil into the the transfer ports to mix it with gasoline, instead they inject pre-mixed gas and oil via the bike’s throttle body directly into the combustion chamber. These are called Throttle Body Injected (TBI) engines.

The key difference between TPI and TBI injection, is that a TPI engine does everything for the rider (short of putting the two-stroke oil in the bike’s oil tank), while the TBI engine needs to have the gasoline and two-stroke oil pre-mixed in a separate gas can, which is then poured into the bike’s fuel tank. For a motocross racer, TBI is better than TPI—even with the added hassle of having to measure the correct amount of oil needed for every gallon of gas. Why? By adding the oil to the gas in a fuel can, the rider can control the ratio of oil-to-gas, while with a TPI system, the bike’s electronic control unit (ECU) selects the ratio—and the rider has no control over how or when the oil is injected. When you pre-mix the amount of oil your machine calls for in a gas can, you bypass the computer. It is a simpler solution than relying on oil tanks, oil pumps and oil injection systems. However, Yamaha YZ125’s are not fuel-injected—they require the oil to be pre-mixed in a gas can at the chosen ratio of oil to gas. Here are the basics.

(1) Pre-mix oil is not the same as the engine oil you pour into your car’s crankcase. It is specially formulated to work in high-performance two-stroke engines. There are a lot of quality brands of pre-mix oil. MXA runs Maxima K2 pre-mix oil.

(2) Three things dictate oil usage: first, oil quality; second, oil quantity; third, intended usage. A Yamaha YZ125 needs a quality oil because in most cases you are only going to mix approximately 3 ounces of oil per gallon of gasoline. Engine usage has to be taken into consideration because the same size engine that is used for off-road riding could run less oil than if it is used for motocross racing.

(3) What is the best ratio for motocross? MXA believes that the best ratio for 125cc, 250cc and 300cc two-stroke race bikes is 40:1 (40 parts gasoline to 1 part of oil), but the best source can be found in your bike’s owner’s manual. The chart below shows the pre-mix ratios on the left, the gallons of gas across the top and how many ounces of oil are needed per gallon. For example, if you wanted to run a 40:1 ratio in five gallons of gas, you would add 16 ounces of oil (that is 3.20 ounces per gallon of gas). 

(4) How should you mix your gas and oil together? The only safe way to mix your oil and gas is to measure the proper amount of oil into a calibrated measuring cup. The most popular premix measuring tool is the Ratio-Rite. It is sold in every motorcycle shop in America for a few bucks. The Ratio-Rite has calibrations that tell you how many cc or ounces of oil you need for each prescribed amount of gasoline. You pour the pre-measured amount of oil from the Ratio-Rite into an empty gas can. Once you have the oil in the gas can, add half the gas you are blending (for example, 2.5 gallons if you are mixing 5 gallons of gas). Then seal the gas can and shake it vigorously. Then add the rest of your fuel.

(5) Do your best to only mix up the amount of fuel that you can use in the next two weeks. Otherwise, the aromatics of the gasoline will leach through any plastic gas can. Additionally, the plastic gas tank on your bike is also vented, and fuel in it will also lose its punch over time. Once you open a bottle of pre-mix and use some of it, put the cap back on. Pre-mix oil has a long shelf life, but the solvent contents can evaporate over a three-year period.

(6) Metal gas cans keep pre-mixed fuel at full potency longer than plastic gas cans