We often get customers that ask us what type of gasoline they should use in their scooter. GY6 engine manufacturers (the common 139QMB or 157QMJ 4-stroke 50cc and 150cc scooters manufactured in China and Taiwan) recommend the use of 90+ octane fuel. We wanted to stop and explain why.
In a common internal combustion engine (which your scooter is), a mixture of gasoline (fuel) and air enter the combustion chamber. When your engine’s piston reaches the top of its stroke, the air/fuel mixture is compressed at the top of your combustion chamber. Your spark plug “fires” (sparks) and ignites the air/fuel mixture causing it to explode and push the piston back down to the bottom of its stroke, thereby creating power.
Octane is a measure of the pressure (compression in engine terms) at which this air/fuel mixture will spontaneously combust. Gasoline with a lower octane rating will spontaneously combust at lower engine compression; where gasoline with a higher octane rating will spontaneously combust at a higher engine compression.
The small 50cc and 150cc single cylinder engines in many GY6 scooters have fairly high compression. Using a lower octane gasoline will cause the air/fuel mixture in your combustion chamber to combust spontaneously at a point before the piston reaches top dead center and your spark plug fires. This “pre-combustion” is often heard as “engine knock,” where a first explosion occurs due to air/fuel spontaneously detonating under pressure, rather than combustion occurring when your spark plug fires. This pre-combustion causes engines to run inefficiently, and can damage to your engine over time.
The second reason to use higher octane gasoline, is that they are often “premium” fuels, which mean they have detergents in them to help clean your fuel delivery system. The carburetor jets in the 50cc and 150cc scooters are very small (and notoriously clog leading to poor engine performance). The extra detergents in premium gasoline helps keep those small jets clean and clear so that the engine operates like it should.
As such, most scooter manufactures recommend the use of a 90+ octane premium gasoline. While slightly more expensive, a 1 to 2 gallon fill-up common for most scooters keeps the overall price difference to a few cents.
Happy and Safe Riding!