Small Engine Troubleshooting Problems? Ask a Mechanic Now.
The most common place these leak from are the crankcase. make sure all bolts are tightened on the crankcase and cylinder. I have repaired several of these for this same problem
See diagram #76. Make sure these bolts are tightened down. Usually the oil leaks from this area. http://www.m-and-d.com/pdfs/MTD/TB425CS_41ADT42C063pl.pdf
You never stated what model it was. I was using a basic model. I am a Certified TB Tech and Dealer.I will OPT out though.