Just a comment on model airplane style carbs, from someone who's used them but not built them (I fly control line, so I mostly take them off and replace them with venturies):
First, the air bleed type looks to me to be really easy to build, If you can make even a 1-cylinder 4-stroke work, you can build an air-bleed carb with one hand tied behind your back. There's lots of little fiddly bits, but as long as you can make the barrel a good fit into the carb body, you'll be fine.
Second, the world has gone over to two-needle carbs. These achieve the same effect as an air bleed carburetor, but instead of an air bleed, they use a main needle that regulates fuel feed at high speed, and an idle needle that regulates fuel feed at low throttle.
In both cases I'm a bit vague as to how and when the changeover is made -- I suspect that getting the midrange working really well is going to be a challenge with either one of these carbs. I also know that for model airplane usage, you need to adjust the idle to be a bit rich, so that when you do nail the throttle the engine doesn't just stall. If you throttle down and adjust the air bleed for maximum RPM at idle, you're guaranteed a stall when you try to accelerate. You have to juggle the idle speed & mixture to get a sustained idle (keeping the glow plug warm) and still get acceleration with no, or at least minimal, sag when you punch it.
They generally work pretty well pulling fuel from a fuel pickup that's 3" to 6" away from the engine depending on the size of the tank, although this is done in part by undercarbureting a bit so you have a lot of vacuum to pull fuel.
Brian, I dunno if you're already using this style of carburetor, so if I'm filling you in on stuff you already know -- sorry!