Small Engines (Lawn Mowers, etc.): Lotsa oil in Air Filter, craftsman riding mower, crankcase oil

Question
Dear Expert,
My Craftsman riding mower w/ Briggs & Stratton 15.0hp engine model# 28N707 was leaking oil very bad from crankcase gasket,(otherwise it was running great) and in attempt to replace the gasket I guess I bolted it down too tight and literally busted the side of the engine so my brother gave me a smaller engine; a B&S 12.5hp, engine model# 286707 of which a B&S repair shop told me it would be adequate. It looked clean, had no signs of oil stains on or around it or other leaks that were visible. He had not run the engine for several years. I installed it. I transfered my carb,cables, gov plate, spring, choke link, gov link because my originals looked newer. The only diff is my original carb(now installed) has the anti-afterfire solenoid on the bottom of the carb where the replacement engine did not. I found no oil in the engine, none registered on dipstick, so I put in new B&S 30w oil, manual said it took 48 ounces so it has 48 in it. New spark plug. Now the problem:
I cranked and ran it for first time, a little stubborn getting going but finally did. I cut one swipe of standard small yard and it gushed oil out of the air filter housing so bad that it was pouring down on top of muffler. No sign of oil anywhere else on engine. The oil in the filter has a smell of gas as well. Needless to say it cut off and I was afraid of so much oil I don't want to try to crank it til I find out what needs to be done. Why did all this oil gush out into the air filter and why does it smell like it's got gas in it? What do I do?

Answer
 Hello D:

 The Oil in the Air Filter will Smell Of Gas. Check the Crankcase Oil for Gas Smell. If it Smells of Gas then Drain the Crankcase and Allow it to Vent for 24hrs before Refilling with Oil. This Allows any Gas Vapors in the Crankcase to Evaporate. 1- If the Engine Isnt Overfilled with Oil, Check the Crankcase Vent. If the Vent is Stuck it will Allow the Vaccum from the Carburator to Suck Oil from the Crankcase into the Carburator and Air Filter. You can View a Breakdown of this Engine at this Addy http://www.outdoordistributors.com/ and Click on the B. Then Select Briggs and Stratton. Then Select Dont have Part Numbers. Then Scroll Down to the File 28 and Click on this. Now Scroll Down to the 286700 Folder and Click on this. This will Download a Breakdown of this Engine for you to View Online. On Page 2 of 17 Diagram Number 8 is the Vent. I am here if you Require more Assistance. Hope this Helps. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John