ASK THE MXPERTS: FUEL-INJECTED OR AIR-INJECTED TWO-STROKES

Note the lack of any fuel lines on the Husqvarna two-stroke throttle body.

Dear MXperts,
What is the difference between a four-stroke EFI throttle body and a two-stroke EFI throttle body? I saw a fuel-injected KTM 300XC-TPI two-stroke at my local dealership. I looked closely at the throttle body, and it looked the same as the one on my KTM 450SXF. Why couldn’t KTM, Husky and TM just use the existing throttle body instead of reinventing the wheel?

Look again. You will not see any fuel lines going to the throttle body of a KTM, Husky or TM fuel-injected two-stroke. On a four-stroke, the fuel is injected at the throttle body into the air stream coming through the throttle body’s venturi. The air and injected fuel form a combustible mixture that ignites when compressed in the combustion chamber of a four-stroke. Two-stroke fuel injection only uses the throttle body to meter the amount of air into the crankcases. The fuel isn’t injected into the engine until the air in the crankcase is pumped into the transfer ports. On a fuel-injected two-stroke, the air comes through the throttle body, but the fuel bypasses the throttle body and goes straight to the transfer ports, where the air and fuel are misted on their way to the combustion chamber. Obviously, two-strokes need to have oil mixed in the gasoline, but a fuel-injected two-stroke doesn’t pre-mix the gas/oil mixture in a gas can; instead, the oil is injected in the air path with an Autolube-style oiling that controls the mixture ratio via the ECU.

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