I have a 21" 6.5/190CC self-propelled Troy Bilt lawn mower. After mowing for about 15 minutes it starts leaking oil. I can't tell where it is coming from. Please advise.

Sounds like this leak is not in the upper half of the engine so it must be much lower. I'm sure you checked the drain plug but recheck it again. Spray the underside and topside of the engine with a solvent and clean off and dry. Remove the blade and start the ngine. Have someone hold down the handle all the way to the grounds so you can inspect with a light. If it is coming out of the main shaft you have a bad seal. Not too expensive if you have the knowhow to repair yourself. If you must take it in to a pro it may be better to buy a new unit instaed.

If you go to purchase an oil seal for the bottom side of the engine, you will also have to obtain a sump pan gasket. That way you can do the job correctly.