We investigated the system Mg1−xAlxB2, in which the heterovalent substitution of Al for Mg changes both the band filling (different valence) and the bandwidth (lattice contraction due to different ionic radii). The result is the disappearance of superconductivity at about x=0.5. Contrary to earlier findings, our carefully annealed powder samples show no evidence of phase separation. We determined the μSR lineshape in a flux line lattice. We obtain a nearly linear dependence of the second moment of the flux lattice field distribution upon doping, indicating a strong reduction of the super carrier density with increasing x. Muon diffusion is detected by zero field measurements. It persists down to the lowest temperatures also at x=0.4. The doping dependent activation energies are reported.