old trick. worth repeating
if a carb isnt seating properly and is sucking in air, sometimes it is hard to find.... even dealers miss stuff too. Here is something that has worked for me in the past, on cars and bikes.
Take a can of WD40 and with the spray around the carb stubs between engine and carb, do this with engine idling. listen very carefully.... usually if there is a leak, the revs will increase by a couple of hundred RPM, or the engine will flutter and gag.
try not to spray WD40 on your plugs n leads while the engine is running as it is flammable and if they are shorting out (another thing that can cause those symptoms you describe) you can soon find you have a WD40 flamethrower! Just use a little caution and commonsense and you will be fine.
Hope this helps!