Diagnosis of hard-starting Honda Helix scooter (CN250)

I'm quasi-literate in mechanical stuff, but have failed for longer than I'd like to admit to diagnose why my scooter is hard-starting, especially when cold. If there're any old hands out there who know their ways around small, carbureted engine, you might be of assistance. I cleaned the carb/jets twice, replaced intake boot, replaced the battery (it passes load test), replaced the starter motor, replaced fuel filter, tested fuel pump. It starts easily if I jump it off my running truck; starter motor sounds weak when the scooter is hard-starting. There appears to still be a slight leak around the intake (running engine sputters/stalls when I spray starter fluid at the boot).