I was going to suggest some chewing gum!
I'm not sure we're going in the right direction here...most motors will run without backpressure. Maybe not the best, but should run. There actually is some back pressure from the header and the expansion chamber.
You mentioned that the choke is making it leaner...and I think it's the opposite - doesn't the choke limit the air?
Also - nobody really mentions this, but the carb is "keyed"...it should slide all the way in (the needle assembly) flush with the carb top - if it's higher, and you can see it, it's not in right, but be aware that it will drop in flush in 2 positions...If it's in correctly your throttle will move full range, and if it's wrong, it still sits well in the carb, but you get minimal throw to move the throttle (on the handlebars). If that happens, you're pretty much set with an open throttle, which will **** off a lot of engines when trying to start them. If you look down into the carb (with the assembly out) you'll see a little pimple on the cylinder wall...The part where you insert the cable into the brass part - that channel should also be where the little "pimple" is.
Jeez, I hope this is clear and I don't sound like an idiot!
Go back to basics when in doubt: set the needle to factory (second slot from the end), reseat the throttle assembly, put in a brand new plug - not a clean, used one, and go from there.