It could be an air leak, which could be at the rubber flange that connects the throttle body to the cylinder, then commonly develop cracks under the clamp, and another possible leak location is the crank seal. Right and left side is relative to where you are standing, so remember that the primary clutch is on the PTO (Power Take Off) side and the recoil starter is on the MAG (Magneto) side. If the injector is providing fuel and it is still lean, then look for an air leak.
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