ok, my $.02 and i hope i get something right just so you can enjoy your bike.
First, sounds like your front cylinder is not moving air properly (because you mentioned the diaphragm) check your whole exhaust, maybe you have a clogged cat for the front cylinder, also check for dents on the exhaust, all the way.
Second, try spraying starting fluid or carb cleaner on the front carb with the engine on, see if it makes a difference. (please be very carefull with this, don't burn yourself)
Third, have you check the pick up coil? you mentioned you had spark, and that makes me think it doesn't has anything to do with it, but check it none-the-less.
Fourth, have you checked your choke cables? if they get stuck open for some reason it will make your symptoms.
Remember something, you need 3 things for combustion, spark, fuel and air.
i have blackberry messenger and whatsapp, i can give you that info so we can chat and talk about possible faults, I'm a mechanic.
GZERO screwed with this post 05-08-2012 at 10:35 PM