Small Engines (Lawn Mowers, etc.): Oil in carb, crankcase pressure, vent tube

Question
I recently did a tune up of my B&S 3.5 classic lawnmower.I took the carb off to clean all the gunk that had accumulated inside. Since then the engine is blowing alot of blue smoke.I checked the carb and it is filled with oil. Now i have check the oil level and it is not too high. I do notice however that there is oil leaking at the gasket by the sparkplug. Is this the cause or is it irrelevant. I would appreciate any help you can give me.  

Answer
If the oil level isn't excessive and you haven't tipped the mower incorrectly (to clean underneath it), there are a couple of other things that can cause what you describe.  If the vent tube from the crankcase is plugged, there will be excessive crankcase pressure that will push extra oil past the valve guides.  If the valve guides are worn badly, they can be allowing extra oil past them and cause this problem.  Hope this helps.