Hi Ron, thanks for the info.
Based on everything I've read, I'm convinced that I have some sort of an air leak - I changed the fuel and it made no difference. And if it runs ok by manipulating the choke, that must mean my fuel/air mixture is too lean, like you say. SO... what the heck's the problem? I've put the carb back on exactly how it was and everything seems nice and tight like before. The only thing I'm worried about is that there was a sort of ripping "seal?" "gasket?" -not sure of the exact word. It goes between the carb and the manifold where the carb attaches to the bike - it's a small piece of material, almost like cardboard, that was ripping - however, I set it in there exactly how it was and it seemed ok. Could this be the problem, a TINY amount of air getting in there? Seems unlikely, but who knows?
But shouldn't a bike run fine without an air filter (I know it's a bad idea, but in theory it should run, right?).
Hmmmm.. I don't know, I'm stumped. I wish I never took the bloody thing apart!