The oily/fuely yuk that collects in your airbox is normal, it's WHY you have drain tubes in it. It happens from many sources, collected moisture from the air, stray unburned/partially burned fuel vapours, oil vapour from the crankcase breather etc which all condenses in the bottom of the airbox.
The drain tubes come standard with "squirrel condoms" on them to stop unfiltered air entering the engine. If you're one of those people who can't help but tweak the throttle when you start it cold you're going to get backfires from the carbs. That will not only blow off the squirrel condoms, but potentially blow up your airbox too.
