I added a 1.5mm shim as you discussed above and that helped. Then my starter stuck and would not release. I cut a new case gasket from thicker material and installed (2) gaskets.
Works like a charm now.
With the stock gasket mine did not want to allow the gears to mesh proper and when they would then they would not release.
I am thinking the shim did not help as much as adding thicker gasket to allow the gears to mesh much easier.
I still kick it to start when cold but once warm just a tap of the switch and magic everytime
