As well as checking the shear pins make sure the 1st stage is feeding into the 2 nd stage, I have seen blowers where the 1st stage was turning in and out from the 2nd stage.
With regards the 2nd stage impeller and the gap, I have on my previous Allied 72 inch rear mounted blower added combine belt to reduce the housing clearance, I have it on my walk behind blower and that made a dramatic difference but it does depend on the initial gap. Even if you fit it tight it will wear in quite quickly, added benefits that it sweeps out any gravel that might get stuck.