That's exactly what is the problem, it's too lean. A trick that will help you is when you are tuning the motor on the ground lift the plane up and hold it with the engine pointing straight up for 30-35 seconds with the motor running full open. Make sure you lift with both hands and that you have a firm grip on the plane. If your motor is tuned properly the motor will continue to run at full speed. If it sags off or dies then you have it too lean and need to richen it up.

Hope this helps

Ken