I'm pretty sure there aren't seals in the shaft bores. I think it's just a tight machined fit. I've never pulled a throttle shaft out though to see for sure. The one I had that was bad whistled under load. Putting grease around the throttle shafts made the noise go away until the grease sucked through. That one was really loose. If I get a chance I'll dig out one of my parts carbs and see if they have shaft seals.
Wow, whistling?! I guess LC motors aren’t as sensitive to minor air leaks. Most bikes have a carb for each cylinder and control air/fuel mixtures for engines that are 1/10 to 1/20 the size of the 2F tractor engine. When I'm working on a bike that's running like crap I get excited when I find a vacuum leak, it's usually a pretty easy fix.
Looks like it is a machined fit, none of the carb diagrams showed anything resembling a seal.