Small Engines (Lawn Mowers, etc.): Craftsman mower wont start, self propelled mower, heavy oil

Question
I have a craftsman 5.5 hp self propelled mower which will not start. I checked the flywheel key for shear and it is O.K. The mower will back fire every now and then thru the carb and I notice something like dark heavy oil coming out of the muffler. I don't know if this is engine oil or maybe gas with a lot of carbon in it. I am hoping it is not a major engine problem

I took down the carb and cleaned it and replaced the gaskets just to be sure it wasn't yhe problem.

can you help?

Answer
Hi Allen,

If it doesn't start, check for spark at the plug.  Then determine if is getting the proper amount of fuel.  Is the plug wet from too much fuel or will the engine run when fuel or starting fluid is squirted into the carb intake?
The backfiring sounds like a valve problem.  Check the compression.  Does it burn oil or smoke?

Let me know what you find.
Michael