Regarding carbs on leopard engines, we’ve never in our testing seen carbs matched better to specific engines. We test all our carbs and if we change the engine we keep our fastest carb on always. The fastest carb from our experience is the fastest carb.
We race tag senior so I can’t speak to the jr. stuff but I imagine the same principle would apply. However I would also imagine a junior carb is set up different than a senior carb. Couldn’t tell you for sure.
On rebuilds, it also seems to matter how high you are revving your engine. If you are revving it consistently higher you might not get the 8-10 hour life but if lower you may actually get longer. We’ve had a couple engines go on us around 7 hours and it has been the thrust washer not the piston.
These are my non engine builder prospectives. Hope this helps.