Small Engines (Lawn Mowers, etc.): b&s engine smoking, john deer lawn mower, leak gas

Question
I have two riding mowers that have similar problems.
One engine is on a small John deer lawn mower and i don't know the engine size, it's used and has been converted to pull start ( I hate )it ran good the first couple times and now it smokes bad if i get it to start and it seems to pour out from around the lower spark plug location and oil drips out from somewhere and the carb seems to leak gas and maybe oil?
What is wrong and what can i do about it.

@nd mower is a sentenal tractor type that mowes good but after a while i start smeeling something burning like a plastic smell and then it will smoke bad out of the muffler and if i turn off the blades ,it goes away after a couple seconds.
Whats wrong, and what do i do about this.

Answer
On the JD check the oil level and see if the oil smells like gasoline.  

On the 2nd mower, I would lower the deck as far as possible and then get on the ground and check for the belt rubbing on something.  A flashlight will help and washing the mower to remove grease and grime may help you find the problem.  Check to see if any of the pulleys are made of plastic; you may have a bearing going bad.  Look for signs of melting or wear.

Let me know what you find.
Eric