I have an old John Deer riding mower, 11HP engine (Model# 52707,Type 0228-01, Code 8110211).It starts great and runs fine, but after mowing grass with it for a little while it start to smoke from the bottom of the engine and starts leaking oil.Then it starts to loose power. Is the main oil seal worn and/or the bearings need to be replaced.

Yes, Sounds Like The Oil Seal Is Bad, But It Should Leak Even While In Storage To A Certain Extent, Jokingly Adding, It Will Quit Leaking When You Run Out Of Oil, Just A Little Laugh. If You Are Able To Do So This Is Not That Hard To Fix, Remove Mower Deck, Remove Pto Pulley From Bottom Of Engine, Remove Four Nuts And Bolts, Disconnect The Wiring Harnesses, Remove The Sump Pan( Bottom Casing Of Engine) Remove Seal, Clean A Way All Gasket And Gunk From Sump Pan, Replace Sump Pan Gasket, Seal And Reassemble, Add Fresh Oil And Mow Away.
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Thanks, Mowerman2468