'Lean' means not enough gas in the fuel/air mixture.
If an engine runs lean it overheats and if it's very lean it gets so hot...it's burns a hole through the piston crown and you lose all compression.
The size of the main jet is stamped into it, for your bike they are in increments of 5, for example only...If you find you main jet is #120 the next sizes bigger are #125 and #130.
Air leaks can cause lean running as well as a MJ too small, splits/cracks in the inlet manifold...loose screws...broken gaskets...leaking crank seal (left side) are all possibilities of potential air leaks, it's no use jetting bigger than the makers original spec to compensate for leanness caused by an air leak!