I really don't know how go-cart engines work, but given that they are two stroke, it almost doesn't matter what the fuel is, the inefficiencies are all, "built-in" as it were. Maybe at go-carting forums, they have more specific information. Although it is probably useful to keep the engine as maintained as possible: proper spark plug maintenance, clean fuel tank and lines, clean, so far as possible, the combustion chamber. But whether its better to have high octane fuel, which is slower burning, or lower octane fuel, which ignites faster, I don't know. I also don't know if modern automobile gasoline, blended with ethanol is helpful for two-stroke engine or not -- although I don't know where you're going to find gasoline without ethanol, or other oxygenate, anyway. Let us know what go-cart forums suggest.