Originally Posted by Seeker

a little oil in the air box is not unusual often caused by wear on the rings/bore, it gets there by the engine breather. The worn rings cause the crankcase to pressurise and the crankcase breather, which alleviates the pressure, is fed into the airbox.

Backfire? either the mixture is wrong or the plug isn't firing every time and when it does fire it ignites the unburnt fuel in the exhaust. Were you running it without the airfilter (this would cause a weak mixture). How old are the plugs - what do they look like: wet or sooty or light brownish-white (should be the latter).